Codeforces Deltix Round Spring 2021 [Div.1 + Div.2]
By deltixlab, 3 years ago, translation, In English

deltix

Hi Codeforces!

We are DELTIX. Founded in 2005, DELTIX is one of the market leaders in software development for financial research and products for systematic and algorithmic trading. In 2020 DELTIX joined the EPAM family. Our mission is to turn promising ideas into breakthrough products fast.

We are experts in:

  • aggregation, storage, and processing large volumes of time-series data
  • data modeling
  • testing and deployment of quantitative models

In our team we value skills like:

  • knowledge of algorithms
  • high-performance coding
  • low latency data streams processing

Learn more about DELTIX

Throughout the year, once per quarter, we will be inviting you to join DELTIX rounds at Codeforces. Today, we are excited to welcome you to the first installment of our rounds (joined Div1 и Div2) — Deltix Round, Spring 2021 (open for everyone, rated, Div. 1 + Div. 2), that will start on May/30/2021 17:35 (Moscow time).

It is an open and rated round for both divisions.

We have prepared the following trophies for you:

  • 1st place = the most desired console of 2021 — PlayStation 5!
  • 2nd place = Nintendo Switch
  • 3rd place = Nintendo Switch Lite
  • 1-100: branded t-shirts

Another 100 t-shirts will be distributed randomly between 100 participants outside of top-100 and who have solved at least one problem and participated in rated Codeforces rounds before.

Problems have been prepared by our employees: Vladik, netman, AleXman111 and sdryapko.

We would like to say a word of appreciation:

We will offer participants 8 problems and 135 minutes to solve them. We wish everybody good luck and high ratings!

Fill out a short contact form if you are interested in internship and/or employment opportunities or would like to speak with recruiters or members of our team.

Contact Form →

UPD: The scoring distribution is 500 — 1000 — 1500 — 2250 — 2250 — 3000 — 3250 — 3250.

Thank you all for participating! (editorial)

Congratulations to the winners:
1. tourist
2. Radewoosh
3. Um_nik
4. maroonrk
5. ecnerwala
6. jiangly
7. SSRS_
8. Petr
9. scott_wu
10. Maksim1744

We would like to express our special congratulations to the top three leaders! We will try to send you your well-deserved prizes as soon as possible :) Unfortunately, we can not list the people who received 100 random T-shirts due to the fact that the search for cheaters is not completed.

  • Vote: I like it
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  • Vote: I do not like it

Comments Comments (344)
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3 years ago, # |
  Vote: I like it +174 Vote: I do not like it

So Which game are you going to play tourist on your brand new Play Station 5?

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    3 years ago, # ^ |
      Vote: I like it +86 Vote: I do not like it

    he already has 50 of them from code jam and hackercup

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    3 years ago, # ^ |
    ← Rev. 4   Vote: I like it 0 Vote: I do not like it

    I came up with same comment to post in my mind but then I also knew that this praising will be surely down there already!

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      3 years ago, # ^ |
        Vote: I like it +2 Vote: I do not like it

      I think you should reconsider your statement after the contest is over!

      And see the standing once :)

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3 years ago, # |
  Vote: I like it +171 Vote: I do not like it

Crossing my fingers that "we value skills like high-performance coding" doesn't mean "Anyone who uses java is going to have a concussion from banging their head against their desk because of how tight we made these time limits"

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    3 years ago, # ^ |
      Vote: I like it +41 Vote: I do not like it

    If you think about it, screencasts are high-performance coding.

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    3 years ago, # ^ |
      Vote: I like it -30 Vote: I do not like it

    Will it be better if time limits are according to the languages?

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      3 years ago, # ^ |
        Vote: I like it -13 Vote: I do not like it

      It will be better if we have websites according to languages!

      maymay
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      3 years ago, # ^ |
        Vote: I like it +3 Vote: I do not like it

      you are a classic example of minority being bashed by majority... xD

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    3 years ago, # ^ |
      Vote: I like it +30 Vote: I do not like it

    Well, it's a finance/trading company. Taking the phrase "make really good decisions really fast" to a completely new level is the only chance they have to be profitable. Their limits are imposed by competitors, not arbitrarily chosen. (That also gives you all the more opportunity to fail in this industry.)

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    3 years ago, # ^ |
      Vote: I like it -20 Vote: I do not like it

    Still not as tight as ur mom

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3 years ago, # |
  Vote: I like it +35 Vote: I do not like it

The more different rounds, the better! I also really hope that the tasks will be good.

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3 years ago, # |
  Vote: I like it +31 Vote: I do not like it

Nice T-shirt :)

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3 years ago, # |
  Vote: I like it +40 Vote: I do not like it

"Throughout the year, once per quarter, we will be inviting you to join DELTIX rounds at Codeforces." Wait, so we're gonna have one of these every 3ish months? :O

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    3 years ago, # ^ |
      Vote: I like it +113 Vote: I do not like it

    Yes, that's the plan! :) I hope we will delight you with our tasks

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3 years ago, # |
  Vote: I like it +5 Vote: I do not like it

I would love to have that PS5.But I won't be able to achieve that this time.Maybe 5-6 years later in some div1 + div2 I will get PS6!!!

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    3 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Buying with your money is better in than, 5-6 years :)

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      3 years ago, # ^ |
        Vote: I like it +7 Vote: I do not like it

      I am poor.Besides,winning something has a different sort of satisfaction.

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3 years ago, # |
  Vote: I like it +2 Vote: I do not like it

Best of luck to all the participants :)

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3 years ago, # |
  Vote: I like it +25 Vote: I do not like it

Warning: This round will be slightly harder, because I will be on a plane instead of donating my rating.

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3 years ago, # |
  Vote: I like it +25 Vote: I do not like it

Please make the pretests strong! That's all I ask.

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3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Woah 8 problems? This one is big.

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3 years ago, # |
  Vote: I like it +9 Vote: I do not like it

Nowadays codeforces is giving a lot of gap between two contests

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3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Nice prizes !! Hope to win one of them in the future :))

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3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

8 problems in 2 hour and 15 minutes... are we going to face some easy questions!

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    3 years ago, # ^ |
      Vote: I like it -11 Vote: I do not like it

    :)

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    3 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    maybe it's a trap

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      3 years ago, # ^ |
        Vote: I like it +3 Vote: I do not like it

      yes its_Atrap

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        3 years ago, # ^ |
          Vote: I like it -7 Vote: I do not like it

        Poor trap sir, Everytime someone(me) ends up using his handle for replying to his comments ends with hoooge upvotes. But when he is using his own handle to reply he ends up getting downvotes. Please give him deserved upvotes

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3 years ago, # |
  Vote: I like it +15 Vote: I do not like it

Me being a 'specialist' calculating my probability of winning a PS5 in a combined round. But I forgot I'm bad at probabilities and even CP (-_-)

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3 years ago, # |
  Vote: I like it -37 Vote: I do not like it
With all due respect
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3 years ago, # |
  Vote: I like it +2 Vote: I do not like it

Can I have a T-shirts from the random generator :)))

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3 years ago, # |
  Vote: I like it +91 Vote: I do not like it

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    3 years ago, # ^ |
      Vote: I like it +100 Vote: I do not like it

    I actually disagree with my meme I don't want to enjoy my new color

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3 years ago, # |
  Vote: I like it +1 Vote: I do not like it

Throughout the year, once per quarter, we will be inviting you to join DELTIX rounds at Codeforces.

I'm excited to see that! Woahh, more div.1 + div.2 means more participating for me

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3 years ago, # |
  Vote: I like it +9 Vote: I do not like it

Time for SpeedForces.

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3 years ago, # |
← Rev. 2   Vote: I like it 0 Vote: I do not like it

when you know there are playstation 5! you must be ready for the contest

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3 years ago, # |
  Vote: I like it +24 Vote: I do not like it

I feel a little envious about the top three.

QQ图片20210512215006.gif

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3 years ago, # |
  Vote: I like it +3 Vote: I do not like it

I'm very excited for this one, since it's happening after a long time. All the best everyone.

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3 years ago, # |
← Rev. 2   Vote: I like it 0 Vote: I do not like it

I already knew that the PS5 is going to be one of LGM's. :)

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3 years ago, # |
← Rev. 7   Vote: I like it 0 Vote: I do not like it

Deletdd

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3 years ago, # |
  Vote: I like it -6 Vote: I do not like it

Hope Bugaboos will be good. May random be on my favor (that doesn't mean i want t-shirts :p)

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3 years ago, # |
  Vote: I like it +7 Vote: I do not like it

how many bugaboos?

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3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

As a tester, I can assure the problem set is fun, well atleast the first half surely is. Good luck!

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    3 years ago, # ^ |
      Vote: I like it +24 Vote: I do not like it

    Thanks for confirming you weren't able to solve any of the last half problems

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    3 years ago, # ^ |
      Vote: I like it +16 Vote: I do not like it

    As an author, I want to assure that the second half is also worthy of attention :)

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    3 years ago, # ^ |
      Vote: I like it +32 Vote: I do not like it

    I didn't think it was much fun at all. The first 3 problems were pretty trivial and the rest were way too hard for me.

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3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Good Luck everyone for this round!!!

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3 years ago, # |
  Vote: I like it +23 Vote: I do not like it

We have four purple questionsetters!

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    3 years ago, # ^ |
      Vote: I like it +33 Vote: I do not like it

    no :)

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      3 years ago, # ^ |
        Vote: I like it +4 Vote: I do not like it

      Why the downvotes and no? Is the usage of question not allowed?

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        3 years ago, # ^ |
          Vote: I like it -6 Vote: I do not like it

        Sorry, I didn't really understand your question. If you think that I put a minus on your comment, then this is not true)

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3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Where is Anton when you need him?

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    3 years ago, # ^ |
      Vote: I like it +117 Vote: I do not like it

    Killed by 313 people yesterday :'(

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      3 years ago, # ^ |
        Vote: I like it +8 Vote: I do not like it

      That's kind of like what I meant. Wonder why I got downvotes...

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3 years ago, # |
  Vote: I like it +40 Vote: I do not like it

The winner of the contest will be probably have no time to play PS5 otherwise he wouldn't be the winner in first place :D

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3 years ago, # |
  Vote: I like it +18 Vote: I do not like it

what about scoring distribution? Also as a contributor i want some testing.

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3 years ago, # |
← Rev. 3   Vote: I like it +10 Vote: I do not like it

:)

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3 years ago, # |
  Vote: I like it +5 Vote: I do not like it

everyone talking about prizes, I'm thinking to become candidate master asap.

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    3 years ago, # ^ |
      Vote: I like it +6 Vote: I do not like it

    bro, you improved a lot after the 17th or 18th contest that you gave. Can you tell what specifically did you do or how did you practice in that period

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3 years ago, # |
  Vote: I like it +3 Vote: I do not like it

is it impossible to improve in codeforces without watching anime ?

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3 years ago, # |
  Vote: I like it +2 Vote: I do not like it

hoping this will not be a div 1.5 round !

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3 years ago, # |
  Vote: I like it +9 Vote: I do not like it

Score distribution looks sus

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    3 years ago, # ^ |
      Vote: I like it +21 Vote: I do not like it

    Seems like it'll be a contest of who can get D and E for me...

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3 years ago, # |
← Rev. 2   Vote: I like it 0 Vote: I do not like it

Can someone clarify that will it be rated for Div. 2 participants or not?

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3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

is it going to be a difficult or moderate one looking at the scoring distributions

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3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Codeforces is broken on my PC but it's working fine on mobile. Is it only happening to me?

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3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

It will be my first contest as a specialist. Hoping for the best.

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3 years ago, # |
  Vote: I like it +1 Vote: I do not like it

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3 years ago, # |
  Vote: I like it -32 Vote: I do not like it

deltix round == bad round

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3 years ago, # |
  Vote: I like it +26 Vote: I do not like it

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3 years ago, # |
  Vote: I like it +7 Vote: I do not like it

Looking at the standings, it seems Tourist is in urgent need of another PS5 ! Oh by the way, is there going to be Bugatorial for this round?

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3 years ago, # |
  Vote: I like it +4 Vote: I do not like it

What's wrong with B??

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    3 years ago, # ^ |
    ← Rev. 2   Vote: I like it +4 Vote: I do not like it

    Observation.. For any two pair you need to perform 1 1 2 1 1 2 or 2 1 1 2 1 1 or 1 2 1 2 1 2 or any valid formation ;)

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      3 years ago, # ^ |
        Vote: I like it +1 Vote: I do not like it

      1 2 1 1 2 1 works also...Indeed a good observation problem

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      3 years ago, # ^ |
        Vote: I like it +1 Vote: I do not like it

      Yeah... I got the hint just after I expressed my frustation here. It was my problem I read the question hastly. I think the basic step is how to swap two numbers without third variable logic.

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    3 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Just upvoting you because your name is Luffy
    \m/_(>_<)_\m/

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3 years ago, # |
  Vote: I like it +7 Vote: I do not like it

And i thought the contest will be easy since it has 8 questions.LOL!!!!!!!!!

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3 years ago, # |
  Vote: I like it +18 Vote: I do not like it

I really wish an easier problem was in place of D. Instead of writing this comment, I would have been solving a problem that was actually in my league. Contest would have been more enjoyable :(

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3 years ago, # |
  Vote: I like it +1 Vote: I do not like it

Out of > 18 000 registered participants only 8 700 submitted anything after 90 minutes.

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3 years ago, # |
  Vote: I like it +20 Vote: I do not like it

speedforces

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3 years ago, # |
  Vote: I like it +1 Vote: I do not like it

D is too tough to even try

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    3 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    The fact that you were able to solve A, B, C in this contest indicates that you already punched way above your weight.

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      3 years ago, # ^ |
        Vote: I like it -36 Vote: I do not like it

      I am expert lol, I am fucking up some recent contest hence I am giving from this fake id

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3 years ago, # |
  Vote: I like it -20 Vote: I do not like it

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3 years ago, # |
← Rev. 2   Vote: I like it +166 Vote: I do not like it

statement of C is shit

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3 years ago, # |
← Rev. 3   Vote: I like it 0 Vote: I do not like it

Please, help understand problem E

this bruteforce solution gives wa on 2-nd and 3-rd samples

Spoiler
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    3 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Different sequences of operations have may have different probabilities, so you can't just calculate sum and divide by their number.

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3 years ago, # |
  Vote: I like it +9 Vote: I do not like it

Problem statements should have been more clearer.(also no need of that image in each ques) :(

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3 years ago, # |
  Vote: I like it +22 Vote: I do not like it

Oh wow! Not only bugaboos were great, but also these beautiful pictures in every bugaboo! Awesome contest.

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3 years ago, # |
  Vote: I like it +30 Vote: I do not like it

how to solve the problem D ?

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    3 years ago, # ^ |
    ← Rev. 2   Vote: I like it +5 Vote: I do not like it

    You would only need to test up to 2**p = 32768 bitmasks, and I think which can be optimised.

    I have yet to pass systests however.

    Edit: Failed systests, my solution is actually O(N * 2**(2*p))

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      3 years ago, # ^ |
        Vote: I like it +3 Vote: I do not like it

      I came up with a solution, with complexity O(const * n * 2 ^ p) but it's TLE.

      how fast did you check is the mask good or no ?

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        3 years ago, # ^ |
          Vote: I like it +5 Vote: I do not like it

        You don't need n, you can maintain a map with the number of each state. As for iterating subset it is 3^p. So total complexity is O(n + 3^p)

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3 years ago, # |
  Vote: I like it +11 Vote: I do not like it

Thanks for the interesting problems!

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3 years ago, # |
← Rev. 2   Vote: I like it +140 Vote: I do not like it

Ok C is the worst problem I have ever seen. I spent about 40 minutes staring at it and I didn't understand the statement till now. How could this problem even got accepted by coordinators?

Apart from that the problems were great but this really ruined the contest for me.

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    3 years ago, # ^ |
      Vote: I like it +45 Vote: I do not like it

    I would never have understood it if not for the samples, but reading the sample explains it pretty well, I think.

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      3 years ago, # ^ |
        Vote: I like it +24 Vote: I do not like it

      I'm not sure. At the end I just implemented some greedy shit that got accepted :D

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    3 years ago, # ^ |
      Vote: I like it +16 Vote: I do not like it

    look at the image

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3 years ago, # |
  Vote: I like it +94 Vote: I do not like it

Such a lousy statement on Problem C, it's a shame. Incomprehensible what this is about. The setters work hard to develop and work out great problems, and then ruin the result with an incomprehensible statement text. Why? I do not get it.

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    3 years ago, # ^ |
      Vote: I like it +4 Vote: I do not like it

    Still don't know what we are supposed to do exactly, lol.

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    3 years ago, # ^ |
      Vote: I like it +12 Vote: I do not like it

    I have assumed the problem only by looking at the picture.

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3 years ago, # |
  Vote: I like it +8 Vote: I do not like it

wasn't able to understand what the task C meant till end of the contest ;(

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3 years ago, # |
  Vote: I like it +6 Vote: I do not like it

Pun Intended, tourist give me your old play station. :)

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3 years ago, # |
  Vote: I like it +4 Vote: I do not like it

Am I the only stupid one who write about 100 lines for C and get 6 was??? It feels terrible (especially I notice other people in the same room pass this bugaboo neat and quickly :(

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3 years ago, # |
  Vote: I like it +2 Vote: I do not like it

My strategy after struggling for hours with C..

Just relax in the forthcoming Deltix Rounds. Peace.

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3 years ago, # |
← Rev. 2   Vote: I like it 0 Vote: I do not like it

could someone provide counter case for this submission of problem c. Got 6 WA on this problem.

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3 years ago, # |
  Vote: I like it +97 Vote: I do not like it

I hate combined rounds! I was sure that I would be able to hack a few people in D because my solution was randomized and usually, there are people who still don't seed their random number generators. However, because the round was combined and the room was mostly greens or cyans, there were only two solutions in my room, one of which had good random and the other didn't seem to have random at all.

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    3 years ago, # ^ |
      Vote: I like it +13 Vote: I do not like it

    Wow, how do you do it without randomisation?

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      3 years ago, # ^ |
      ← Rev. 3   Vote: I like it +13 Vote: I do not like it

      I tried all possible subsets of (sensible) currencies (there is at most 2*p = ~30 of those) XD I implemented custom bitset and pruned execution if some subset yielded less than n/2 interested peoples. I doubt it will pass max tests, it passed pretest within 700ms though. For reference: Submission 117911694

      Edit: Systests passed in 750ms :D Hooray

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        3 years ago, # ^ |
        ← Rev. 3   Vote: I like it 0 Vote: I do not like it

        I did almost the same thing. Maybe you can continue the loop when this state's number of $$$1$$$ is less than or equal to the current answer.

        And it got TLE :(

        Then I tried to break at 2.8s, it passed again :((

        And my solution got hacked(WA) yet...Now I wonder whether there's a solution without random

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          3 years ago, # ^ |
          ← Rev. 2   Vote: I like it +3 Vote: I do not like it

          The main difference between our solutions is the presence / absence of prunning. What do I mean by that?

          Consider that you use some subset of currencies (for example currencies 100110) and you've concluded that it's impossible to satisfy at least $$$\frac{n}{2}$$$ people. This means that you don't have to evaluate all "super-masks" (supersets of currencies), for example 110110, 101110, 100111 and any other. And I see that you evaluate the satisfacion for those masks anyway. In my case, the reccurence does not "enter" any states corresponding to superset (which is hard to do with loop).

          The other difference is the computation of guests satisfaction. From what I see your logic is somewhat convoluted because of your data structure — you have bitset mapping which person likes what currency, and I have it the oposite way — which currency is liked by whom. It gives fast computation to check if enough guest will be satisfied (it allows for computing bitwise and in O(n/32)).

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            3 years ago, # ^ |
              Vote: I like it +8 Vote: I do not like it

            You are right. I totally don't consider supersets of failed masks. But I remember bitset's time factor is divide by 32 or 64 too...

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        3 years ago, # ^ |
        ← Rev. 2   Vote: I like it +24 Vote: I do not like it

        I did similar thing and got TLE on test 112!!!

        Edit : It can be saved if I get a T-shirt!

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        3 years ago, # ^ |
          Vote: I like it 0 Vote: I do not like it

        I tried to implement similar idea, but in python. Guess I had no chance

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        3 years ago, # ^ |
          Vote: I like it 0 Vote: I do not like it

        Can you explain your approach?

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          3 years ago, # ^ |
            Vote: I like it +4 Vote: I do not like it

          Yes.

          First, you start by selecting only currencies that interest at least n/2 friends. It can be shown, that there are at most $$$2p \leq 30$$$ such currencies. In my code I store their indices in vector relev (it contains only RELEVant currencies).

          Secondly, you create bitsets for your relevant currencies. In other words, you create around $$$2p$$$ bitsets of size $$$n$$$. We need those to be betsets because we need them to quickly compute bitwise and and number of ones (ie. popcount).

          Now we proceed to the reccurence. If you have some subset of first $$$idx$$$ currencies, you can either add the next one to the set or skip it and proceed with the same subset. If you added your currency, check if it satisfies at least $$$\frac{n}{2}$$$ people. If it doesn't — don't evaluate reccursive call (bitset zadowoleni, which means "satisfied" in Polish, stores the bitset of people satisfied with current currency subset; sklej method computes bitwise and). Now simply return the best of the two reccursive calls. In short, recurrence structure is as follows:

          solve(currencies subset S, int idx):
    if idx == all currencies count:
        return S
    S' = S.union(currency number idx)
    if S' is liked by at least n/2 poeple:
        solve(S', idx + 1)
    solve(S, idx + 1)
    return best of recurrence calls

          Finally you simply reconstruct the answer with all currencies based on relev vector.

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            3 years ago, # ^ |
              Vote: I like it 0 Vote: I do not like it

            What would be the time complexity, O(2**(2p)) for the bitset, and O(N) for the comparison, leading to O(2**(2p)N)? Or am I missing something?

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              3 years ago, # ^ |
                Vote: I like it +1 Vote: I do not like it

              Roughly speaking, yes. But note, that any solution can have at most $$$p$$$ currencies so we are quaranteed not to check all $$$2^{2p}$$$ sets even in the worst case, but only sets with size up to $$$p$$$. In the average case I believe it will compute much less currency subsets.

              Additionaly, comparison has a very low constant factor due to bitset ($$$O(N/32)$$$).

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                3 years ago, # ^ |
                  Vote: I like it +8 Vote: I do not like it

                Thanks, this is a nice solution, especially how it utilizes the powers of 2 to suppress the time complexity :D.

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        3 years ago, # ^ |
          Vote: I like it 0 Vote: I do not like it

        Hey Wielomian, I tried running your submission — https://codeforces.net/contest/1523/submission/117911694 but it is now TLE'ing in TEST-131. Any workaround how to proceed with this?

        My submission with the same code — https://codeforces.net/contest/1523/submission/118490042

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          3 years ago, # ^ |
          ← Rev. 2   Vote: I like it +1 Vote: I do not like it

          I hacked it back in the day (You can see the hack at Submission 117911694). It seems that the system tests were simply weak and that there was no max test with $$$2p$$$ relevant currencies (which is weird). And thus allowed for such a solution to pass. Possibly this testcase was added after the hack for upsolvers.

          To me it seems that one must incorporate the $$$dp$$$ as described in the editorial rather then bitsets to have fast update of people counts for subsets, which I don't understand, honestly.

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    3 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    How to solve D with randomisation (

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      3 years ago, # ^ |
        Vote: I like it +37 Vote: I do not like it

      Select a random person. With probability $$$\frac{1}{2}$$$ they are in the $$$\frac{n}{2}$$$ chosen in the optimal solution.

      Then you can do bitmask DP over only the bits that they like.

      Repeat 30 times for $$$2^{-30}$$$ chance to fail.

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        3 years ago, # ^ |
          Vote: I like it 0 Vote: I do not like it

        Hi, Suppose I select a person and I do all the bitmask, there are 2^p masks but in order to know how many members are there in that mask, I need to iterate all of n right ? Even with iterating only among the persons where the mask exists, I will still be needing 2^p instead of n, so the complexity becomes O(iters * (2^2p)) or (O(iters * n * 2^p) which is worse than the brute force O(n * 2^p) ). How to eliminate 'n' or another 2^p ?

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          3 years ago, # ^ |
            Vote: I like it 0 Vote: I do not like it

          You count how many times each of the possible $$$2^p$$$ bitmasks appears in $$$\mathcal{O}(n)$$$ time. Then you do SOS dp in $$$p \cdot 2^p$$$ time to count for every bitmask how many times a bitmask containing all of its bits appears.

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            3 years ago, # ^ |
            ← Rev. 5   Vote: I like it 0 Vote: I do not like it

            Actually how to count each of the possible 2^p masks appear in O(n) time ? Wont it be O(n * 2^p) ? could you pls elaborate ?

            Also could you pls elaborate on what you are doing here ?

            for (int x = 0; x < k; ++x) {`
`			for (int y = 0; y < (1 << k); ++y) {`
`				if (y & (1 << x)) cou[y ^ (1 << x)] += cou[y];`
`			}`
`		}`...
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3 years ago, # |
  Vote: I like it +1 Vote: I do not like it

I reduced problem D into a bipartite graph problem where friends would be in one side and currencies would be in another side. Now, if we select x from friends and y from currencies, we need all x's to be connected with all y's and we gonna take maximum y such that x meets sufficient conditions.
Couldn't proceed further.

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3 years ago, # |
← Rev. 2   Vote: I like it +5 Vote: I do not like it

how will you know who will be included in the list of 100 random people who will receive t-shirts?

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    3 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    +1

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      3 years ago, # ^ |
        Vote: I like it +4 Vote: I do not like it

      they wrote to me "after checking the cheaters, we will publish the script and the list" :)

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3 years ago, # |
  Vote: I like it +3 Vote: I do not like it

I'm so nervous that my D will get TLE. I just use something like bruteforce....

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3 years ago, # |
  Vote: I like it +220 Vote: I do not like it

Fine, finally a reading comprehension round.

Spoiler
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3 years ago, # |
  Vote: I like it +7 Vote: I do not like it

I made an assertion statement in C to check if the first character is 1 or not, I got a runtime error! I can't believe it has more than 3k AC.

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3 years ago, # |
  Vote: I like it +40 Vote: I do not like it

how did testers approve C , the language was pathetic , wasn't able to get what the task meant till end...

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3 years ago, # |
  Vote: I like it +1 Vote: I do not like it

Wait when you changed n upper bound to 1000 for B? I always think it should be 2000 and the requirement is 5000... i spent like 1 hour thinking about how to get better than a 3n solution but the question description :(

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3 years ago, # |
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what is wrong in my submission of C link

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3 years ago, # |
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OOf just lacked a tiny bit of time for D lul

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    3 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Tried Bruteforcing got TLE Tried Bitmasking + Bruteforcing got TLE Tried Further bitmasking + Bruteforcing contest finished

    OH NO

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3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Why the next contest is after 2 weeks (⋟﹏⋞)

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    3 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    +1. Frequency of codeforces contest has decreased compared to month back.

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3 years ago, # |
← Rev. 3   Vote: I like it +2 Vote: I do not like it

Randomised solution for D:

Randomly take a friend and assume it to be the part of the final subset now compare all other friends with this subset and use submask dp in the end to find maximum size subset. I took 25 random friends.

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3 years ago, # |
  Vote: I like it +20 Vote: I do not like it

why so bad...

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3 years ago, # |
← Rev. 3   Vote: I like it +341 Vote: I do not like it

Does problem C's initial statement form a proper problem? Before the sentence 'then the sequence of items should always remain increasing in lexicographical order' was added, I couldn't found the operations like

1

2

1.1

are invalid from the initial statement.

The announcement of the addition of previous sentence was so late that it's hard to say that the impact of this problem was little.(actually, I've use so long time to debug my code which was doing something like the previous operation)

Hope for the appropriate response.

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    3 years ago, # ^ |
      Vote: I like it +274 Vote: I do not like it

    It's not only "hard to read" or "hard to understand" but it's broken as a problem. I can't say it's enough to be unrated but should take into account whether making this round unrated or not. MikeMirzayanov Vladik

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      3 years ago, # ^ |
        Vote: I like it +262 Vote: I do not like it

      I think this mistake is enough to be unrated.

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        3 years ago, # ^ |
        ← Rev. 2   Vote: I like it +147 Vote: I do not like it

        I agree. Making such a significant change to the problem statement 40 minutes into the contest is unacceptable. This contest should be unrated.

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          3 years ago, # ^ |
            Vote: I like it +8 Vote: I do not like it

          Agree "take into account whether making this round unrated or not".

          For me , I wasted at least one hour in C . And have no time to read E.

          MikeMirzayanov,Vladik

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            3 years ago, # ^ |
              Vote: I like it 0 Vote: I do not like it

            I totally agree with you !

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              3 years ago, # ^ |
                Vote: I like it +45 Vote: I do not like it

              Did you forget to log out your clone to comment? This is just hilarious.

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    3 years ago, # ^ |
      Vote: I like it +74 Vote: I do not like it

    I agree, the problem basically changed halfway through. I don't think this should happen.

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      3 years ago, # ^ |
        Vote: I like it +99 Vote: I do not like it

      yes, the addition was not just the description. it fundamentally changed the problem.

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    3 years ago, # ^ |
      Vote: I like it +63 Vote: I do not like it

    We are sorry for this issue in the statement. None of the testers and authors noticed that the formal definition of a multi-level list was not exactly correct. The statement was corrected and an announcement was made as soon as the issue was noticed. Fortunately, most of the participants followed the common sense about what a multi-level list is and authors meant exactly that.

    By the way, I think when a high-rated participant finds something strange in a problem, like statement does not match usual definitions, or some other mistake, they should report that to the authors (in any contest, not only here). It is much more probable that there is a mistake or that you misunderstood something rather than authors deliberately made a trap without mentioning it explicitly. Reporting that will help authors and save your time.

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      3 years ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      Will the round still be rated?

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        3 years ago, # ^ |
          Vote: I like it +65 Vote: I do not like it

        Yes.

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          3 years ago, # ^ |
            Vote: I like it 0 Vote: I do not like it

          Nikolay, Is that somehow possible to retest all solutions for problem A in this contest? What I've noticed is that there are 5 lists of participants who got TL on this problem, whereas only 1 list of people passed system tests, and all of them had linear algorithm, but according to the editorial quadratic solution should pass. Thanks in advance

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          3 years ago, # ^ |
            Vote: I like it +18 Vote: I do not like it

          Lost t-shirt by this shit problem, also lots of rating.

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      3 years ago, # ^ |
        Vote: I like it +49 Vote: I do not like it

      so, you mean that the problems are still remain correctly even if it is mathematicaly wrong but can perdict it from some kind of fuzzy "common sense"?

      I don't so much care about whethere it is unrated or not, but the dealing with this issue to say "if youre high rated and found it strange, report it" is a question mark(as statement makes a sense without the order constraint, I have no idea but somewhere of my code is wrong.)

      Problems should have been established only by the problem itself. not depening on something lile "common sense".I hope the future rounds wont repeat and I can help it by reporting.

      I have one nice idea for preventing: let's totally get rid of STORIES :)

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      3 years ago, # ^ |
        Vote: I like it +17 Vote: I do not like it

      I don't understand — there are so many problems — Anton coordinated ones especially, where the "definition" is the trick to the problem. By putting the pressure on these high rated participants, you are effectively scapegoating them to do the work that authors should have done — listen to tester feedback.

      Oh, and, "none of the testers noticed..." is just shifty phrasing. Testers complained about the statement of C saying it was confusing. You know that.

      I did not take this contest, but I am equally outraged about the way this matter was handled.

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    3 years ago, # ^ |
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    I made the same mistake ,and I wasted at least one hour in it and get four WA :<

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    3 years ago, # ^ |
      Vote: I like it +30 Vote: I do not like it

    Does problem C's initial statement form a proper problem? Before I got AC using the decimal numeral system, I couldn't found the operations like

    1
    2
    3
    4
    5
    6
    7
    8
    9
    A
    B

    are invalid from the initial statement.

    It was so late that it's hard to say that the impact of this problem was little.(actually, I've use so long time to debug my code which was doing something like the previous operation)

    Hope for the appropriate response.

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3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

who else got TLE in A? :d I think for big number of m and "1111...110111....1" this kind of solution was the main reason right?

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3 years ago, # |
  Vote: I like it +19 Vote: I do not like it

Can anyone explain how to solve E?

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3 years ago, # |
  Vote: I like it +81 Vote: I do not like it

The problem statement of C is unreadable.

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3 years ago, # |
← Rev. 4   Vote: I like it -32 Vote: I do not like it

[. .

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    3 years ago, # ^ |
      Vote: I like it +1 Vote: I do not like it

    omg, that must feel bad ;)

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    3 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    omg!

    rating predictor is showing -213 OMFG

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    3 years ago, # ^ |
    ← Rev. 2   Vote: I like it 0 Vote: I do not like it

    So basically you participated but decided not to submit because didn't want to ruin you rating and as a result you did exactly that

    What irony

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      3 years ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      I didn't even wanted to participate ( the biggest reason being I was busy in upsolving today's ABC's E) I started contest after a good amount of time , But that happened . leave it , I think nothing could be done .

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3 years ago, # |
← Rev. 3   Vote: I like it 0 Vote: I do not like it

[](https://ibb.co/y4sNBRs)

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3 years ago, # |
  Vote: I like it +3 Vote: I do not like it

I resolved the bug in my solution to problem C, but only 12 seconds were left. I couldn't submit it. I wish I had 1 more minute.....This is so saddening.

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3 years ago, # |
  Vote: I like it +95 Vote: I do not like it

Didn't unterstand E correctly, FSTed in both C and D, bad day for me :(

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    3 years ago, # ^ |
      Vote: I like it +81 Vote: I do not like it

    Not your fault, this round is shit.

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    3 years ago, # ^ |
      Vote: I like it +57 Vote: I do not like it

    If you wonder how is it possible to FST in C, here's the answer:

    I stored the input in an array $$$a$$$.

    The array $$$a$$$ is an array of integers in my default source, but when I solve problem A, I changed it to char. And when doing problem C, I forgot to change it back:(

    And it passed pretests:(

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      3 years ago, # ^ |
        Vote: I like it +34 Vote: I do not like it

      Imagine writing all problems in the same file, lol.

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3 years ago, # |
  Vote: I like it +25 Vote: I do not like it

can anyone of the testers , problem setters tell what was going through their mind when they approved the language of problem C , complete nuisance it was

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    3 years ago, # ^ |
    ← Rev. 2   Vote: I like it +3 Vote: I do not like it

    You don't know the dance and says floor is tilted (naach na jane aangan tedha)

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3 years ago, # |
  Vote: I like it +22 Vote: I do not like it

Weak pretests on D!

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3 years ago, # |
  Vote: I like it +1 Vote: I do not like it

Asking for hack: my submission for D

My solution is totally wrong but I can't hack it.

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    3 years ago, # ^ |
      Vote: I like it +6 Vote: I do not like it

    It was interesting!

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      3 years ago, # ^ |
        Vote: I like it +8 Vote: I do not like it

      Your hack is amazing!

      By the way, I've written another wrong solution which passed all the test cases.

      You may hack it here :)

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3 years ago, # |
  Vote: I like it -33 Vote: I do not like it

In problem A, I think it is immoral to write the constraints as such

t <= 1E3
n <= 1E3
.
.
.
The sum of n in all testcase does not exceed 1E4

When I read the top 2 lines I would think the intended solution to be O(n), while an O(n^2) solution would also pass.

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    3 years ago, # ^ |
      Vote: I like it -8 Vote: I do not like it

    I dont't get it, why this comment is getting downvotes

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3 years ago, # |
  Vote: I like it +306 Vote: I do not like it

I think problem C is different problem before and after the problem statement is changed.

before changing the problem statement,

1 1 2 2 1 3 3

is valid sequence(because you can make a list like below:

1
1.1
1.2
2
2.1
3
1.3

),but after changing the problem statement,this is invalid sequence.

I was very sad about this change.

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    3 years ago, # ^ |
      Vote: I like it +113 Vote: I do not like it

    I've chosen the solution same as yours and get "Wrong answer on test 2."

    As allowing the "non lexicographical order" operations just extend the valid operations and the fact that "all testcases has solution", our solution have give a answer.

    but the verdict was "Wrong Answer" so I guess the output checker was checking is it lexicographical order or not.

    If the output checker was taking into consideration but forgotten to write in the statement, it's a so so FATAL MISS!!!!

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    3 years ago, # ^ |
      Vote: I like it +21 Vote: I do not like it

    not gonna lie, I saw the first version of the problem. Didn't understand it at all. I only understood it from the image. I guess the image actually has better explanation that the statement its self Lol. But yeah, that is really sad :c

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    3 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    You can provide solution with test case : 10 1 1 1 2 2 1 2 3 3 3 is it a correct test case ?

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      3 years ago, # ^ |
      ← Rev. 2   Vote: I like it +18 Vote: I do not like it

      with my (before the changing problem statement) solution (117891713 ),

      1
1.1
1.1.1
2
1.2
1.1.1.1
1.1.2
3
1.3
1.1.3

      is provided.(after changing, this sequence is invalid.)

      You can solve (before the changing) this problem by listing all items that can appear next.

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    3 years ago, # ^ |
      Vote: I like it +19 Vote: I do not like it

    I solved problems F and G after contest, and both were interesting. I also like problems D and E. Thanks for preparing this round.

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3 years ago, # |
  Vote: I like it +12 Vote: I do not like it

In problem B, I realized that I needed to do 6 operations for a pair, but I multiplied 6 by 1000, although there are only 500 pairs( So I came to a more complex solution after an hour.

Maybe someone will be interested https://codeforces.net/contest/1523/submission/117914942

In this solution I am doing 14 operations for every four of numbers. If the number of numbers is not divisible by 4, then I do 6 operations for the remaining pair.

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3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Hey, anyone else did not pass Problem A because they used Python??

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    3 years ago, # ^ |
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    Yo bro me too, python is crap

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      3 years ago, # ^ |
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      I think I will just have to stop using it. This is the second time this happened to me. I am sad cuz its just so comfortable to work with strings in python...

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        3 years ago, # ^ |
        ← Rev. 2   Vote: I like it 0 Vote: I do not like it

        I used pypy3 and passed A (although I was scared that it wouldn't!)

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      3 years ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      If anything, with very few exceptions, Python's performance is more than enough for div2 participants

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    3 years ago, # ^ |
    ← Rev. 2   Vote: I like it +3 Vote: I do not like it

    I used Python 3 for problem A, B, and C with no issues. Maybe you just had a bad implementation.

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3 years ago, # |
  Vote: I like it +14 Vote: I do not like it

What was the point on the weak pretests for A?

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    3 years ago, # ^ |
      Vote: I like it +1 Vote: I do not like it

    What is the point of having 37 pretests for D and still I fsted ?

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3 years ago, # |
  Vote: I like it +7 Vote: I do not like it

As a contestant, I want a t-shirt :v .. That T-shirt is so beautiful.. <3

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3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Operation can only be applied if the list does not contain two identical items afterwards
Can someone explain what this statement means with respect to problem C ?

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    3 years ago, # ^ |
      Vote: I like it +1 Vote: I do not like it

    It means you can't have duplicate items in the list, for example [1, 1.1, 1.1, 1.2] is not a valid nested list.

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    3 years ago, # ^ |
      Vote: I like it +3 Vote: I do not like it

    This statement on tallying it with the sample explanation left me puzzled and blank!

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3 years ago, # |
← Rev. 2   Vote: I like it +3 Vote: I do not like it

The last second I found I just use rand()%n+1 to select a person randomly on problem D. That's why I got wrong answer 5 times on pretest 36. I will never forget this trap. :(

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3 years ago, # |
  Vote: I like it +9 Vote: I do not like it

such a very good contest, many thanks for deltix !!!

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3 years ago, # |
← Rev. 2   Vote: I like it +3 Vote: I do not like it

This round was as good as the problem statement of C:)

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3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

chert

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3 years ago, # |
  Vote: I like it +116 Vote: I do not like it

I misread E that he continues turning lights on until K consecutive lights are on.

Explanation of the example matches it :(

I think it is way harder than the original problem. Is there an easy way to the problem that I understood?

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    3 years ago, # ^ |
      Vote: I like it +10 Vote: I do not like it

    I'm not sure if you wanted a subquadratic solution, but I think you can do it in $$$O(n^2)$$$ with the following lemma.

    Lemma: Let $$$X$$$ be a random variable that takes non-negative integer values. Then $$$\displaystyle E[X] = \sum_{i = 1}^{\infty} Pr[X \ge i]$$$.

    From the lemma, it is sufficient to find $$$\displaystyle \sum_{i=0}^{\infty} Pr[\text{There aren't k lights in a row after i steps}]$$$. This can be done with DP in $$$O(n^2)$$$ by finding the number of ways to pick $$$i$$$ lights from the first $$$j$$$ such that no $$$k$$$ of them are consecutive, for all $$$i,j$$$. Maybe there is some way to optimize this, idk.

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3 years ago, # |
← Rev. 2   Vote: I like it +7 Vote: I do not like it

Finally a good round after back to back bad rounds for me.I do admit even I didn't like the problem statement of C. But that table at the bottom helped me a lot. :)

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3 years ago, # |
  Vote: I like it +63 Vote: I do not like it

abc were the most rubish problems I had ever seen

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    3 years ago, # ^ |
    ← Rev. 2   Vote: I like it +3 Vote: I do not like it

    A and C was supposed to be non existent due to their bad quality. But I find B not that bad for a Div 2 A or an easy Div 2 B.

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      3 years ago, # ^ |
        Vote: I like it +10 Vote: I do not like it

      Idk but I felt C statement was not clear enough ,add that midway change,I solved a whole of different version of it until I reread statement .Maybe I had to work upon my English:(

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3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

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3 years ago, # |
  Vote: I like it +16 Vote: I do not like it

who else understood the statement for C because of the image?

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3 years ago, # |
  Vote: I like it +9 Vote: I do not like it

Well, I was about to get purple, until my D FSTed. Do you guys know how it feels when you are about to purple for the first time and you end up losing rating instead cuz of FST ! :sob: :sob: :sob:

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3 years ago, # |
  Vote: I like it +102 Vote: I do not like it

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3 years ago, # |
← Rev. 2   Vote: I like it 0 Vote: I do not like it

after triggering it all items in the list were replaced by a single number: the last number originally written in the item number. f***, after looking at the test data I realized that I solved a completely different problem for C, I thought that the list was replaced by the last number i.e the digit written on the list. Also the sample test data contained all a[i]<=9. The problem statement just ruined it man. The writer should provide a concrete example of what he wants to convey.

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3 years ago, # |
← Rev. 2   Vote: I like it +8 Vote: I do not like it

The problem C itself was not bad
But the statement was crappy and if none of the testers noticed it, they did crappy job as testers.

I don't know what exactly is tested during preparation, but clarity of the problems statements should certainly be one of such things

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3 years ago, # |
  Vote: I like it -7 Vote: I do not like it

About problem C. I read the problem sentences before the announcement and thought the answer is not always increasing in lexicographical order. So I was confused when I got the announcement. But I trust that the writers will not change the sentence in such a way that what the problem means will change and therefore I searched for what was wrong with my thought. Eventually, there prove to be nothing wrong with my thought (as there are mentions in other comments). Would I have been better not trusting the writer? I'm worried whether I can trust the writers in future contests :(

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3 years ago, # |
  Vote: I like it +17 Vote: I do not like it

What a shame, there are 15 testers while none of them notice the problem in C

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3 years ago, # |
  Vote: I like it +61 Vote: I do not like it

To not keep you waiting, the ratings updated preliminarily. In a few hours/days, I will remove cheaters and update the ratings again!

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3 years ago, # |
← Rev. 5   Vote: I like it 0 Vote: I do not like it

can anyone help in Problem A my same code got accepted in pypy while it got tle in python

my python submission https://codeforces.net/contest/1523/submission/117886063

my pypy submission https://codeforces.net/contest/1523/submission/117932947

Ps: I tried pypy after the contest

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    3 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Thats why you should consider pypy over cpython when the runtime is tight for your solution.

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      3 years ago, # ^ |
        Vote: I like it +1 Vote: I do not like it

      in Pypy runtime is 200 ms only runtime is same as others n**2 is there anything cdeforces can do for this issue?

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        3 years ago, # ^ |
          Vote: I like it 0 Vote: I do not like it

        It is not an issue. Pypy is faster than Python most of the time.

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3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Now Radewoosh will be at the top of codeforces rating !!

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3 years ago, # |
  Vote: I like it +180 Vote: I do not like it

I can already feel the breath of the wild.

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3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

After this round it is clear that why antontrygubO_o is the best coordinator on this platform.

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    3 years ago, # ^ |
    ← Rev. 2   Vote: I like it +16 Vote: I do not like it

    You didn't even solve the round. What are you talking about?

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3 years ago, # |
← Rev. 5   Vote: I like it -21 Vote: I do not like it

ok

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3 years ago, # |
  Vote: I like it -9 Vote: I do not like it

weak pretest on A!!!!

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3 years ago, # |
  Vote: I like it +19 Vote: I do not like it

Weak system test on D .

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3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Will there be a random distribution of other 100 t-shirts or will it be priority based like 50 to initial 1000s ,25 to next 1000s ,and likewise?

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    3 years ago, # ^ |
      Vote: I like it +8 Vote: I do not like it

    T-shirts will be distributed completely randomly among the participants who have fulfilled the conditions.

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      3 years ago, # ^ |
        Vote: I like it +16 Vote: I do not like it

      will the list of candidate be posted publicly or the lucky ones will get mails just curious to know?

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        3 years ago, # ^ |
          Vote: I like it 0 Vote: I do not like it

        We will post all people in a comment. And also publish the script.

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3 years ago, # |
← Rev. 2   Vote: I like it 0 Vote: I do not like it

.

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    3 years ago, # ^ |
      Vote: I like it +3 Vote: I do not like it

    If our analysis of the problem was not clear to you, then I recommend listening to the explanation of the solution from Errichto: https://youtu.be/2S19tjbu_48?t=270 . His reasoning completely coincides with ours when drawing up the problem.

    By the way, this problem could be presented formally, if we formulate it in terms of graphs. Then we would have got a tree, and the numbers denoted the ordinal number of the edge at the parent. As we understand now, in such a formulation, the task would be liked by a larger number of participants. But as the aforementioned Errichto notes, we wanted to make the task more interesting by connecting it with real life.

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      3 years ago, # ^ |
        Vote: I like it -53 Vote: I do not like it

      Nothing bad of making the problem interesting, but what you did is just make it an unreadable piece of shit. That is NOT how one should prepare a problem.

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        3 years ago, # ^ |
          Vote: I like it +32 Vote: I do not like it

        I sincerely do not understand why to show so much aggression. How would you write a statement for this task?

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          3 years ago, # ^ |
            Vote: I like it -13 Vote: I do not like it

          Then why to give this task?

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            3 years ago, # ^ |
              Vote: I like it +58 Vote: I do not like it

            As KAN mentioned above, the problem was read by a large number of testers, as well as by all authors. None of us found a problem in the formal definition of what a nested list is. If to answer your question briefly, the answer will sound like "we did not know that the task would be incomprehensible to some people because nothing indicated it." Of course, if we knew that the task will cause negative emotions, then we would not have added it to the round.

            We do not have an aim to hold a round just in order to hold a round. We wanted to make it good and memorable. Great efforts have been made to make the round a good one. There are people who did not want to be mentioned in the announcement, but put a lot of effort into preparation. For example, an illustrator and translator.

            I just want to point out that conclusions have been drawn regarding this situation and we will involve even more people in the next round so that such problems do not have a chance to exist.

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              3 years ago, # ^ |
                Vote: I like it +27 Vote: I do not like it

              None of us found a problem in the formal definition of what a nested list is.

              That's a problem. I understood it pretty fast but I chalk it up to my experience and language skills and most importantly looking at the examples. Still, I instantly concluded that the definition is terribly written — people who don't have those language skills should struggle with it. The fact that nobody realised that when testing isn't an explanation, it's a problem by itself.

              1. Start with defining an item as a sequence of numbers (with '.' as separator). The first definition of an item looks vaguely like multiplication, the info that it's a sequence is buried in the middle of the statement.
              2. Cut out the "nested" part until you're giving an example, only then explain that it's a nested list. We're dealing with a sequence of "items" (as defined before) for a formal definition and a nested list for an informal one.
              3. The part with building it by inserting items is complicated by itself, but there's no good way around it and it can be viewed as part of figuring out the solution to rephrase it as defining a tree, so I guess that's ok.
              4. There's nothing formal about "starting a list of a deeper level" and "continuing the current level", especially considering that the word "level" doesn't appear in the statement ever again, so it doesn't provide any help connecting concepts. However, if it was obvious that an item is a sequence of integers, you can state outright that the 2 cases are a sequence with $$$1$$$ appended and a sequence with the same length but the last element incremented.
              5. "the last number originally written in the item number" — You really don't see what's wrong with a phrase "number in number"? Each item is replaced by a single integer — the last element of the item (already typedef'd as a sequence). There, done.
              6. On the output, you could've at least described what's getting printed in more detail than "print a nested list". On the $$$i$$$-th line, print the $$$i$$$-th item of your list — a sequence of integers separated by '.'.

              There are probably more details but I'm sure this would've helped a lot.

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                3 years ago, # ^ |
                  Vote: I like it +9 Vote: I do not like it

                Thank you very much for the detailed message! I think that everything you wrote makes sense.

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          3 years ago, # ^ |
            Vote: I like it +36 Vote: I do not like it

          William is a huge fan of planning ahead. That is why he starts his morning routine by creating a nested list of upcoming errands.

          A nested list is a numbered list where a period indicates the outer list contains a nested sublist. For example, 

          1
1.1
1.1.1
1.1.2
2
2.1

          is a valid nested list.

          Formally, the following conditions must be met for a nested list to be valid.

          • A list and all its nested sublists must be lexicographically increasing.
          • A list can be nested inside another, if and only if, its lexicographically previous identifier exists and is nested in the same outer list.
          • No 2 list elements can have the same identifier. (Consider each row, for eg. 1.1.1 as an identifier in the list)

          Note: There may be any number of nested levels and the numbering of the lists follows base 10 (decimal system).

          Unfortunately, William has lost his original list, but he remembers the last number in each row. Help William find any valid nested list that matches the description given by William.

          Note 1:

          1
1.1.1

          is not valid as 1.1 doesn't exist.

          Note 2:

          1
1.2
1.2
3

          is not valid as it violates 2 rules

          • 1.1 does not exist, hence, 1.2 can't exist
          • 1.2 is repeated.
          • 2 is missing in the outermost list after 1, rather 3 is present.

          Hopefully, something like this could have made the task simpler to comprehend. At least I would easily get what is needed to be done if it was presented like this.

          The picture in the question is a hint to understand the question, but I don't suppose many would have bothered to look at it.

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3 years ago, # |
  Vote: I like it +515 Vote: I do not like it

Thanks for the round! I enjoyed problems F and G, and the rest of the problems are nice too.

To everyone: please be respectful towards problem authors. If you're criticizing, do it in a constructive manner. It's disappointing to see a ton of comments with nothing but aggression.

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3 years ago, # |
  Vote: I like it +41 Vote: I do not like it

I have never seen a round organizer answering each and every query of the participant which has been done by Vladik.Hats off man.It was a great contest .Ignore the idiots who are blaming you for tough language of the problem.You provided with pictures ,you did your best ignore the idiots.

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3 years ago, # |
  Vote: I like it +19 Vote: I do not like it

Dibs on tourist's previous PS.

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3 years ago, # |
  Vote: I like it -34 Vote: I do not like it

Vladik please make sure that the t-shirt reaches my address.

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3 years ago, # |
  Vote: I like it +48 Vote: I do not like it

Today I received 2 messages from the system telling that my codes for problem B (117877311) and C (117883398) matches with the codes of aerfanr for problem for B (117886611) and C(117894142).

So please note that I do not share my codes to anyone and I have been using rextester.com to run my codes, which does not automatically publish any code. The match is a coincidence which has occured due to the simplicity of the solutions, For both of the submissions the main part is only 10-12 lines and the approaches are same as in the editorial. Also check that at any point, the implementations entirely different.

MikeMirzayanov Please help me and tell me if there is anything more I can do to prove the coincidence.

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    3 years ago, # ^ |
      Vote: I like it +57 Vote: I do not like it

    Thanks, this issue has been resolved before you wrote. You can see that you both are official participants of the round.

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      3 years ago, # ^ |
        Vote: I like it +53 Vote: I do not like it

      Thank you very much.

      It's great that you care so much that you resolved it even before I wrote.

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3 years ago, # |
← Rev. 8   Vote: I like it -27 Vote: I do not like it

Who created the question. There was a mistake in 1 problem where the range can go out

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3 years ago, # |
  Vote: I like it -38 Vote: I do not like it

MikeMirzayanov,Vladik is the round declared unrated, or the ratings would be returned soon. please clarify.

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3 years ago, # |
  Vote: I like it -41 Vote: I do not like it

Ratings are back

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3 years ago, # |
  Vote: I like it +24 Vote: I do not like it

Will you publish a list of people who have won T-shirts or just let them know?

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3 years ago, # |
  Vote: I like it +12 Vote: I do not like it

The search for cheaters is completed. Waiting for the list :)

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3 years ago, # |
  Vote: I like it +13 Vote: I do not like it

For Problem C, Please anyone review my explanation.

  • We know that the sequence forms a valid list so the first element must be 1.
  • Whenever we are encountering 1 in the sequence we add it to the end of stack because all other previous depths must have encountered 1 already.
  • If we encounter any number other than 1 say x.We search from the end of stack for x-1 and replace x-1 with x.This is optimal because

    • Let's suppose the current stack is a,x-1,b,x-1,c
    Choosing the right most x-1, a,x-1,b,x -- state 1
    Choosing any x-1 other than rightmost a,x -- state 2
    It's not hard to see that state 1 gives us more choices and also includes the choices of state 2.Hence,It's fair to go with state 1.

  • Also,the sequence in stack will never repeat because

    • Suppose we got a sequence of length n .a1,a2,.......an
    In case of adding 1 It will increase the sequence length so It will change.
    In case of other than 1 It will definitely change some element of the sequence and decreases its length even though we might end up with length n again but its element will be different as It has already got changed.

P.S- I want to improve on proofs. Your review will be constructive for me.

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3 years ago, # |
  Vote: I like it -69 Vote: I do not like it

Please don't downvote me, my contribution is turning negative :(

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3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

MikeMirzayanov, Vladik/ Guys, hurry up, please, I have been updating the site for 2 days in a row to find out the list ;(

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3 years ago, # |
← Rev. 2   Vote: I like it +118 Vote: I do not like it

Congratulations to tshirts winners! You will be contacted via private messages with instructions to receive your prize.

As usual, we used the following two scripts for generating random winners, seed is the score of the winner.

get_tshirts.py
randgen.cpp
List place Contest Rank Name
1 1523 1 tourist
2 1523 2 Radewoosh
3 1523 3 Um_nik
4 1523 4 maroonrk
5 1523 5 ecnerwala
6 1523 6 jiangly
7 1523 7 SSRS_
8 1523 8 Petr
9 1523 9 scott_wu
10 1523 10 Maksim1744
11 1523 11 VivaciousAubergine
12 1523 12 qwerty787788
13 1523 13 aid
14 1523 14 kmjp
15 1523 15 yhx-12243
16 1523 16 LJC00118
17 1523 17 fedoseev.timofey
18 1523 18 duality
19 1523 19 never_giveup
20 1523 20 risujiroh
21 1523 21 frame233
22 1523 22 xay5421
23 1523 23 ksun48
24 1523 24 NotaMotuaQAQ
25 1523 25 oleh1421
26 1523 26 KK_Smiler
27 1523 27 neal
28 1523 28 Isonan
29 1523 29 JettyOller
30 1523 30 Ormlis
31 1523 31 Swistakk
32 1523 32 xtqqwq
33 1523 33 Rewinding
34 1523 34 mango_lassi
35 1523 35 Itst
36 1523 36 ainta
37 1523 37 Toxel
38 1523 38 dqa2021
39 1523 39 353cerega
40 1523 39 molamola.
41 1523 41 receed
42 1523 42 GyojunYoun
43 1523 43 sansen
44 1523 44 gamegame
45 1523 45 mnbvmar
46 1523 46 froggyzhang
47 1523 47 AutumnKite
48 1523 48 wlzhouzhuan
49 1523 49 Errichto
50 1523 50 Bredor
51 1523 51 ShimaRin
52 1523 52 Farhod
53 1523 53 Amoo_Safar
54 1523 54 saketh
55 1523 55 WeakestTopology
56 1523 56 kotamanegi
57 1523 57 MForest
58 1523 58 Anachor
59 1523 59 orzdevinwang
60 1523 60 hank55663
61 1523 61 starusc
62 1523 62 KrK
63 1523 63 uwi
64 1523 64 gmh77
65 1523 65 ilyakor
66 1523 66 20I6wudi
67 1523 67 kclee2172
68 1523 68 xpysss
69 1523 69 Rubikun
70 1523 70 AwakeAnay
71 1523 71 BigBag
72 1523 72 penguinman
73 1523 73 arzhantsev64
74 1523 74 enjapma
75 1523 75 Egor.Lifar
76 1523 76 leaf1415
77 1523 77 TeaPot
78 1523 77 RainAir
79 1523 79 Jatana
80 1523 80 tatyam
81 1523 81 LastDance
82 1523 82 yamunaku
83 1523 83 Monogon
84 1523 84 TadijaSebez
85 1523 85 gisp_zjz
86 1523 86 flashmt
87 1523 87 Mr_Wu
88 1523 88 haruki_K
89 1523 89 isaf
90 1523 90 dreamoon_love_AA
91 1523 91 -is-this-fft-
92 1523 92 dragonslayerintraining
93 1523 93 balbit
94 1523 94 vasilescu_mihai
95 1523 95 kshitij_sodani
96 1523 96 Isoeasy
97 1523 97 compute
98 1523 98 lucaperju
99 1523 99 dalt
100 1523 100 SSerxhs
114 1523 114 adamant
180 1523 180 BlueDiamond
227 1523 227 Ari
258 1523 259 knightL
290 1523 291 Grzmot
325 1523 326 Ahmadsm2005
508 1523 507 Agnimandur
552 1523 552 swapnil159
754 1523 755 bqq
913 1523 908 saptarshi1729
944 1523 940 smax
1029 1523 1031 rbrahul
1240 1523 1240 vkgainz
1287 1523 1287 Diana773
1291 1523 1287 the0dd1out
1341 1523 1335 SergiuS2003
1363 1523 1363 Abhi60
1425 1523 1424 kalp.s.vyas
1432 1523 1432 WAZXYY
1470 1523 1472 Nikhil_Kr
1547 1523 1551 aars
1622 1523 1625 BitmasKing
1731 1523 1738 AakashKr
1755 1523 1762 Mohaimin66
1822 1523 1828 Murtaza1112
1910 1523 1913 Hosea
2090 1523 2099 tdsb
2169 1523 2177 Abu_Musa_99
2177 1523 2187 samcpp
2185 1523 2196 abovhan
2192 1523 2201 Yanko_
2203 1523 2213 imagine7
2213 1523 2222 husain_box
2262 1523 2270 devansh05
2289 1523 2297 Crayon144
2409 1523 2422 YashShah
2422 1523 2431 7eZb_ElNa7o
2529 1523 2539 sam571128
2537 1523 2551 linbei
2544 1523 2556 hastidoshi36
2599 1523 2613 1459007298
2658 1523 2671 boogiep0p
2677 1523 2690 SpeedOfGangarians
2937 1523 2957 YazanAlattar
3111 1523 3131 DontLookBack
3164 1523 3182 AmanG247
3220 1523 3242 tokusakurai
3293 1523 3311 NEFU_Smith
3408 1523 3432 Beast_Within
3449 1523 3471 Ikwinder.ka.ur
3550 1523 3577 343_guilty_spark
3872 1523 3890 chronon
3981 1523 4010 MrArpit
3988 1523 4010 sgprsp
4008 1523 4040 Prostoegor239
4083 1523 4111 petertromso
4308 1523 4337 Harshit4u
4334 1523 4368 ojha_abhi
4544 1523 4573 shubham.gupta8416
4545 1523 4573 brahm
4781 1523 4820 aarya10
4803 1523 4840 coderfool
4833 1523 4875 SooN_
4873 1523 4914 AllINeed
4887 1523 4923 bekodeg
4937 1523 4976 Ceudan
5121 1523 5168 nish_ads
5135 1523 5178 shauryasinha020
5328 1523 5375 shreas
5385 1523 5442 AIM-9L
5387 1523 5442 dhruvinpatel1831
5410 1523 5466 hkmihir
5501 1523 5551 MindlessGoober
5600 1523 5649 Discardim
5630 1523 5691 coderrohan14
5662 1523 5723 VioletVal
5665 1523 5723 Sparkk
5723 1523 5782 go_ch
5760 1523 5803 tanjum
5875 1523 5944 iyangye
5938 1523 6005 adityateltia22
5947 1523 6020 abhishek2061a
5951 1523 6020 E404_Not_Found
5955 1523 6020 Blaze7
6034 1523 6112 Sunlet_orz
6111 1523 6187 Asif732
6156 1523 6225 Rajat02
6163 1523 6244 vishwas_26
6189 1523 6269 Palak_2109
6308 1523 6394 ayush_agrawal5
6340 1523 6422 learner101
6477 1523 6564 jainanshika193
6568 1523 6649 cj2021
6573 1523 6665 knuck_les
6581 1523 6665 pandey_2021
6602 1523 6691 harshant
6697 1523 6786 aman1606
6729 1523 6820 MayurK
6849 1523 6949 Prateek_Saini
7066 1523 7130 BitWiseOperator
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    3 years ago, # ^ |
      Vote: I like it +8 Vote: I do not like it

    Hi ! How can i receive my t-shirt?

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      3 years ago, # ^ |
        Vote: I like it +22 Vote: I do not like it

      You will be contacted via private messages with instructions to receive your prize.

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    3 years ago, # ^ |
      Vote: I like it +17 Vote: I do not like it

    Just my personal opinion. In general and not specially related to this contest. But, I think "You would accept the T-shirt if given" option just below Confirm Participation of contest.

    I'm sure many of Top people (say tourist for eg) would not want t-shirt. It would be waste for him and it could be really motivating and worthy for someone below him. I meant at least one person would be very happy if tourist didn't receive t-shirt.

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    3 years ago, # ^ |
    ← Rev. 2   Vote: I like it -8 Vote: I do not like it

    ohhhhhhhh~

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    3 years ago, # ^ |
      Vote: I like it -10 Vote: I do not like it

    Sorry, I'd like to ask. I'm on the list, but there's no message informing me of my T-shirt.

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    3 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    hii..! I haven't received my t-shirt yet. Could I get any update about this..

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      3 years ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      Hello! T-shirts haven't been shipped yet, but are ready to go and should reach their owners shortly.

      Unfortunately, this is not as fast a process as we would like. We had to wait for all the participants to send the size of their T-shirts and only then start making them.

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    3 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    I haven't received my t-shirts till now!

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    2 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Hi! I haven't received my t-shirt til now. Almost a year has passed. Could I get any update about it?

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      2 years ago, # ^ |
      ← Rev. 2   Vote: I like it +3 Vote: I do not like it

      Hi! Please contact Una_Shem via codeforces private messages. We sent all the T-shirts to the codeforces side, according to the latest information I have, the T-shirts for the first two rounds have already been sent.

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3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

When you send a mail that is you received tshirt Or not????

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