dario2994's blog

Codeforces Global Round 11
By dario2994, 4 years ago, In English

Hi!

On Oct/10/2020 17:50 (Moscow time) we will host Codeforces Global Round 11.

This is the fifth round of the 2020 series of Codeforces Global Rounds. The rounds are open and rated for everybody.

The prizes for this round are as follows:

  • 30 best participants get a t-shirt.
  • 20 t-shirts are randomly distributed among those with ranks between 31 and 500, inclusive.

The prizes for the 6-round series in 2020:

  • In each round top-100 participants get points according to the table.
  • The final result for each participant is equal to the sum of points he gets in the four rounds he placed the highest.
  • The best 20 participants over all series get sweatshirts and place certificates.

Thanks to XTX, which in 2020 supported the global rounds initiative!

Problems for this round are set by me. Thanks a lot to the coordinator antontrygubO_o, to the testers dacin21, Giada, H4ckOm, DimmyT, Retired_cherry, oolimry, nkamzabek, Prakash11, Tlatoani, coderz189, nvmdava, stack_overflows, dorijanlendvaj, and to MikeMirzayanov for the Codeforces and Polygon platforms.

The round will have 8 problems and will last 180 minutes.

The (unusual) scoring distribution is: 500-750-1000-1000-1500-2250-2250-4500.

Why such a scoring distribution?

I hope you will have fun solving the problems!

UPD: The round is postponed by 15 minutes because just before the round there will be a 10-minutes-long unrated testing round. Considering the recent Codeforces downtime, this is a measure to make sure that there will not be technical issues during the real round.

UPD2: There were no technical issues during the testing round, hence the real round will happen. Good luck and see you in the scoreboard!

UPD3: I hope you liked the problems, here is the editorial.

UPD4: Congratulations to the winners!

  1. Benq
  2. yosupo
  3. ksun48
  4. Um_nik
  5. ecnerwala
  6. sunset
  7. maroonrk
  8. zscoder
  9. SirShokoladina
  10. gamegame

UPD5: And congratulation to Petr who upsolved H before the editorial was posted! You made me happy!

Announcement of Codeforces Global Round 11
  • Vote: I like it
  • +1025
  • Vote: I do not like it

Comments Comments (180)
| Write comment?
»
4 years ago, # |
← Rev. 4   Vote: I like it -164 Vote: I do not like it

"The low scores for problems B-C-D-E-F-G should not suggest that they are easier than usual."
but i hope 1000 pointer C,D is not as hard as recent Div2 675 C and if they are, then honestly they deserve more points.
edit: I want atleast 500 downvotes now.

→ Reply
»
4 years ago, # |
← Rev. 2   Vote: I like it +269 Vote: I do not like it

→ Reply
»
4 years ago, # |
  Vote: I like it +98 Vote: I do not like it

as a tester, I want contribution

PS
→ Reply
  • »
    »
    4 years ago, # ^ |
      Vote: I like it -69 Vote: I do not like it

    as a participant I also want contribution.

    → Reply
  • »
    »
    4 years ago, # ^ |
      Vote: I like it -6 Vote: I do not like it

    We will give you a contribution after our solutions will not fail in the system test.

    → Reply
  • »
    »
    4 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    how did you become a tester? can you tell me? if there is a link that refers to becoming a tester of the round I would like to know thanks

    → Reply
»
4 years ago, # |
  Vote: I like it +306 Vote: I do not like it

.
→ Reply
»
4 years ago, # |
  Vote: I like it +92 Vote: I do not like it

Solving F+G+H, exactly

→ Reply
»
4 years ago, # |
← Rev. 2   Vote: I like it -132 Vote: I do not like it

[del]

→ Reply
»
4 years ago, # |
← Rev. 2   Vote: I like it -35 Vote: I do not like it

Oh please, then make score of every problems twice. What if some contestant gets C/D but not B and gets bad position due to closer score distribution. And in combined rounds, its common to get tougher B(at least for me)

→ Reply
»
4 years ago, # |
  Vote: I like it +133 Vote: I do not like it

....so basically if LGMs can solve H they will participate, else skip the round?

→ Reply
  • »
    »
    4 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    ... especially if they are not running for top positions in the 6-round series

    → Reply
  • »
    »
    4 years ago, # ^ |
      Vote: I like it +11 Vote: I do not like it

    I hope not! I hope they will participate to have fun, not (only) for the rating.

    → Reply
»
4 years ago, # |
  Vote: I like it +21 Vote: I do not like it

Note the duration -- 3 hours!

→ Reply
»
4 years ago, # |
  Vote: I like it +254 Vote: I do not like it

As a tester, I am very lucky that no one will see how bad I was in this round.

→ Reply
»
4 years ago, # |
  Vote: I like it +21 Vote: I do not like it

dario2994 "500-750-1000-1000-1500-2250-2250-4500", Can we expect C and D to be equal in difficulty and same with F and G( though I don't care about F and G, since I have never solved that many problems in a contest )

→ Reply
  • »
    »
    4 years ago, # ^ |
      Vote: I like it +32 Vote: I do not like it

    Yes.

    → Reply
  • »
    »
    4 years ago, # ^ |
      Vote: I like it +109 Vote: I do not like it

    maybe you need to care about F and G to solve them

    → Reply
    • »
      »
      »
      4 years ago, # ^ |
        Vote: I like it +10 Vote: I do not like it

      Yes, I am focusing on improving my advanced data structures and upsolving 2000+ scored problems. Lets hope soon I would be able to solve them and increase my speed for solving Div2D as my next goal is become master.

      → Reply
    • »
      »
      »
      4 years ago, # ^ |
        Vote: I like it +103 Vote: I do not like it

      can't believe I never thought of this. I'll just make sure I care about H this weekend. see y'all in GM!

      → Reply
      • »
        »
        »
        »
        4 years ago, # ^ |
          Vote: I like it +8 Vote: I do not like it

        update: 2397, but if I solved H, then I think I would have made it

        → Reply
»
4 years ago, # |
  Vote: I like it +28 Vote: I do not like it

As a tester I want to say that the problems are really interesting. Wishing All of you big positive deltas!!!!

→ Reply
»
4 years ago, # |
  Vote: I like it +14 Vote: I do not like it

F, on Sunday my rating will go down

→ Reply
»
4 years ago, # |
  Vote: I like it +20 Vote: I do not like it

Tshirt = H

→ Reply
»
4 years ago, # |
  Vote: I like it -66 Vote: I do not like it

Adhoc shit incoming

→ Reply
»
4 years ago, # |
  Vote: I like it -28 Vote: I do not like it

I hope pretests are not weak, otherwise every successful/unsuccessful hack will be worth a lot of rating points.

→ Reply
»
4 years ago, # |
  Vote: I like it +128 Vote: I do not like it

(120996911_3404290582998045_8389644735048710731_n.jpg)

→ Reply
»
4 years ago, # |
  Vote: I like it +79 Vote: I do not like it

The low scores for problems B-C-D-E-F-G should not suggest that they are easier than usual.

If it doesn't mean B-C-D-E-F-G will be easier, then why the score distribution is like this? Someone could solve A, C, D, and most probably would be behind a guy who only solved A, B, C. Here B & D are playing the deciding factor. How even this is fair if B & D has a normal round's difficulty? I am sorry, but I couldn't get the explanation for the unusual distribution.

→ Reply
  • »
    »
    4 years ago, # ^ |
      Vote: I like it -54 Vote: I do not like it

    you will be downvoted for raising voice towards this stupid score distribution like me

    → Reply
»
4 years ago, # |
  Vote: I like it +27 Vote: I do not like it

Are you ok Codeforces..?

→ Reply
  • »
    »
    4 years ago, # ^ |
      Vote: I like it +25 Vote: I do not like it

    Codeforces-Chan seems to be ok now ;))

    → Reply
»
4 years ago, # |
  Vote: I like it +5 Vote: I do not like it

Oh my god, what happened?

→ Reply
»
4 years ago, # |
  Vote: I like it +59 Vote: I do not like it

there is no logic in making the difference between some problem's score and some other harder problem's score too little just to make F+G=H

→ Reply
»
4 years ago, # |
← Rev. 8   Vote: I like it +44 Vote: I do not like it

oops,after the contest end about 7 hrs there is a big competition called CSP-S1 which is important to Chinese OIers,so many Chinese student(include me) may not take part in it.

upd :

cf contest : 22:35GMT+8 ~ 01:35GMT+8

CSP-S1 : 09:30GMT+8 ~ 11:30GMT+8

→ Reply
  • »
    »
    4 years ago, # ^ |
      Vote: I like it -195 Vote: I do not like it

    nobody cares about Chinese competitions, why are you always complaining about CF contest start time

    → Reply
    • »
      »
      »
      4 years ago, # ^ |
        Vote: I like it +53 Vote: I do not like it

      I am not complaining,but I'm quite sad because I love global rounds' nice problems

      → Reply
    • »
      »
      »
      4 years ago, # ^ |
        Vote: I like it -37 Vote: I do not like it

      nobody cares about you, why are you always complaining about other people

      → Reply
»
4 years ago, # |
  Vote: I like it -92 Vote: I do not like it

Small but major correction.

points he gets ...

should be

points he or she gets OR points they get ...

→ Reply
»
4 years ago, # |
  Vote: I like it +162 Vote: I do not like it

Your Atcoder round was great! Waiting for interesting problems here as well.

Maybe F+G=H makes sense for the top but this distribution will make the round worse for the remaining 99% of participants. What about loosening the condition and making it e.g. 3000-3000-4500 for three last problems, along with some adjustments for earlier problems. Also, isn't the score of 250 possible too? And please remember about adjusting points drop/drain for a longer round.

I will go live just after the round and explain my solutions to whatever problems I manage to solve https://www.twitch.tv/errichto

→ Reply
  • »
    »
    4 years ago, # ^ |
      Vote: I like it -26 Vote: I do not like it

    i don't understand how this will make the round worse for remaining 99% participants.

    i doesn't seems like they care about H anyway.

    → Reply
    • »
      »
      »
      4 years ago, # ^ |
        Vote: I like it +155 Vote: I do not like it

      They indeed don't care about H so the scoring becomes 500-750-1000-1000-1500-2250-2250. This flat scoring discourages solving problems out of order. And you will get a similar number of points for C and G because of time penalty so better not fail C!

      Notice that the average step in 500-1000-1500-2000-2500 round is 500 points (or around x1.5 ratio) while in this round the step will be 291.66 (or ratio $$$(2250/500)^{1/6} = 1.28$$$). More problems and yet smaller difficulty range, hurray.

      → Reply
    • »
      »
      »
      4 years ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      seems like even the top 1% of players also didn't care about H.

      → Reply
  • »
    »
    4 years ago, # ^ |
      Vote: I like it +69 Vote: I do not like it

    Since I see many comments concerned about the score distribution and how it might ruin the contest for many contestants, let me explain. First, three remarks:

    • No, I was not allowed to give 250 as a score.
    • No, 3000, 3000, 4500 would not be the same.
    • Trying a new score distribution is fun. I think that many of the people complaining (maybe not you) are complaining because it is new, not because it is bad.

    Now, let us discuss about what I think is concerning for many people (and I guess is the meaning of "will make the round worse for the remaining 99% of participants"): There is a relatively small difference of scores between B and C,D. Why might this be an issue? Because someone who solves B fast might get a higher score than someone who solves C slow. Because hacks and failing system tests play a bigger role if scores are low.

    I claim that none of these issues is real.

    • Pretests should be strong on A-B-C-D, so there should not be many failing system tests and hacks (this is a risky thing to say... future reader please forgive me if this will not be true!).
    • The difference between 750 and 1000 is not so small. In hard problems we are very used to this kind of ratio between consecutive problems. More precisely this difference means that C will be worth as B at the beginning after 1h30m. This is a lot of time and I think it is fair to say that someone who solves B in 30m should get a higher score than someone who solves C in 2h30m.

    I hope I have taken into account at least some of the concerns. In any case, I will not say anything else about the scoring distribution (because I have already spoiled too much about the contest). After the contest, I might say a bit more about the choice of this scoring distribution.

    I hope you will like the contest :)

    → Reply
    • »
      »
      »
      4 years ago, # ^ |
        Vote: I like it +170 Vote: I do not like it

      I won't try to convince you about changing the scoring for this round but let me argue further and maybe this will affect some future rounds.

      I like strong pretests in C but one can still struggle with WA and decide to skip it. Or just dislike the problem and skip it to solve something more interesting. And there are issues other than getting similar score for C and G. The further we are from geometric scoring, the more we care about the speed of solving easy problems. If scores are 500-1000-2000-4000-8000 and you solve all problems in order ABCDE, then 1 minute spent on A affects your final score a bit less than twice as much as 1 minute spent on E. If scores are 500-1000-1500-2000-2500, the ratio increases from around $$$1.9$$$ to $$$(1+2+3+4+5)/5=3$$$. If scores are 500-750-1000-1000-1500-2250-2250 then it's $$$4.11$$$. If we indeed manage to solve all these problems in time, it's like an Atcoder round but you multiply the time of solving A by 4 (and similar multipliers for other easy problems). This is how ties are decided between people with the same set of problems.

      → Reply
      • »
        »
        »
        »
        4 years ago, # ^ |
          Vote: I like it +150 Vote: I do not like it

        Scores in range [500, 2500] made some sense when we had 5-6 problems per round. If we get 8-9 problems on regular basis, CF should use much wider range of scores. In particular, 4500 points is too little for every hard problems.

        → Reply
»
4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

And then you realise problem H is another Cube Lattice (1375I)

→ Reply
»
4 years ago, # |
  Vote: I like it +61 Vote: I do not like it

With the permission of the site administrator, I think we can actually create high-scoring problems by creating the same H1 and H2 problems, avoiding systems that limit the maximum score.

→ Reply
»
4 years ago, # |
  Vote: I like it -17 Vote: I do not like it

I'm guessing there will be a lot of competition for rank in this competition. Many div 2 people will do A,B,C and many will get similar points for this and same ranking. Getting good rating change is going to be tough for this round. Solving D problem will be the deciding factor for div 2, I guess.

→ Reply
  • »
    »
    4 years ago, # ^ |
      Vote: I like it +13 Vote: I do not like it

    Getting good rating change is going to be tough for this round.

    How sarcastic, points are given according to your ranking between the people you compete with. Actually, I don't understand your point of how getting good rating change is tougher than other rounds...

    → Reply
»
4 years ago, # |
  Vote: I like it +11 Vote: I do not like it

Ok. I will try to solve H first.

→ Reply
»
4 years ago, # |
  Vote: I like it -17 Vote: I do not like it

Not the correct place to ask but are Educational Round over ? I think every month there were two Edu Rounds.

→ Reply
  • »
    »
    4 years ago, # ^ |
      Vote: I like it +9 Vote: I do not like it

    I highly doubt that edu rounds are over. Just be patient.

    → Reply
»
4 years ago, # |
← Rev. 4   Vote: I like it -49 Vote: I do not like it

nice contest

→ Reply
»
4 years ago, # |
  Vote: I like it +239 Vote: I do not like it

To all the people asking how to become a tester, finally your day has come! Together we are all testers in testing round 17.

→ Reply
  • »
    »
    4 years ago, # ^ |
      Vote: I like it +16 Vote: I do not like it

    How so Orz monogon .rz.rz.rz.rz.rz

    → Reply
  • »
    »
    4 years ago, # ^ |
      Vote: I like it +19 Vote: I do not like it

    finally your day has come! Will Mike make a thread on home page with all our names!
    LOL!!

    → Reply
  • »
    »
    4 years ago, # ^ |
      Vote: I like it +39 Vote: I do not like it

    I was just about to say "in before the comments section becomes Monogon's twitch chat" but looks like I'm too late now

    → Reply
    • »
      »
      »
      4 years ago, # ^ |
        Vote: I like it +6 Vote: I do not like it

      Hi, dear SecondThread, could you please provide your unbiased opinion regarding the scoring distribution of this round? The smaller differences among A, B, C and D might not be much of a problem to Div1 Contestants, but to all other participants, especially the lower-ranked ones, it could definitely cost us. Just my opinion, tbh.

      → Reply
  • »
    »
    4 years ago, # ^ |
      Vote: I like it +45 Vote: I do not like it

    As a tester of testing round 17, Give me contribution.

    → Reply
»
4 years ago, # |
  Vote: I like it +22 Vote: I do not like it

So the real contest starts after 10 mins after the 10 mins Testing Round. What if there is some issue and it can't be fixed within 10 mins?

→ Reply
  • »
    »
    4 years ago, # ^ |
      Vote: I like it +23 Vote: I do not like it

    At least they will have an idea and postpone the round.

    → Reply
»
4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Hello!Hope that contest will be great and wish you high ratings

→ Reply
»
4 years ago, # |
  Vote: I like it +86 Vote: I do not like it

Why just not to make testing round earlier instead of "deviating the main round"?

→ Reply
  • »
    »
    4 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Ikr. That is like why ICPC and IOI 2021 will be in the same week. From the 365 days, they cannot sort this out lol.

    → Reply
  • »
    »
    4 years ago, # ^ |
      Vote: I like it +28 Vote: I do not like it

    Obviously in order to have more participants in the testing round

    → Reply
»
4 years ago, # |
← Rev. 2   Vote: I like it -6 Vote: I do not like it

Hope strong pretests.Come Up~

→ Reply
»
4 years ago, # |
← Rev. 2   Vote: I like it +25 Vote: I do not like it

Hope to change my color in this round . (Else would leave CP forever)

→ Reply
  • »
    »
    4 years ago, # ^ |
      Vote: I like it +15 Vote: I do not like it

    Imagine becoming an admin after this round.

    → Reply
»
4 years ago, # |
  Vote: I like it +50 Vote: I do not like it

I'm kinda scared. In your last AGC I couldn't even solve a single problem. Hope I can solve at least something this time

→ Reply
»
4 years ago, # |
  Vote: I like it +267 Vote: I do not like it

Not really :'(

→ Reply
»
4 years ago, # |
  Vote: I like it +23 Vote: I do not like it

back to back rounds(global and educational) in 24 hours with no time to upsolve!! its going to be a fun weekend and a long week!

→ Reply
»
4 years ago, # |
  Vote: I like it -14 Vote: I do not like it

What the hell?

→ Reply
»
4 years ago, # |
  Vote: I like it +2 Vote: I do not like it

How to solve B of the testing round ?

→ Reply
»
4 years ago, # |
  Vote: I like it +18 Vote: I do not like it

I think the testing round was very slow :(

→ Reply
»
4 years ago, # |
  Vote: I like it +16 Vote: I do not like it

Was it intentional that the statements of the testing round were not available in m-instances?

→ Reply
»
4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

I wonder how many T-shirt did tourist get from all the contests

→ Reply
  • »
    »
    4 years ago, # ^ |
      Vote: I like it +1 Vote: I do not like it

    He must be using this much of t-shirts to mop his floor I guess.

    → Reply
»
4 years ago, # |
← Rev. 2   Vote: I like it -13 Vote: I do not like it

DELETED

→ Reply
»
4 years ago, # |
  Vote: I like it +8 Vote: I do not like it

Div 1.5

→ Reply
»
4 years ago, # |
  Vote: I like it +38 Vote: I do not like it
Let's see who you really are
→ Reply
»
4 years ago, # |
  Vote: I like it +34 Vote: I do not like it

Code Forces Global Round 11? More like Code Forces Global Div 1 Round 1. ;-;

→ Reply
»
4 years ago, # |
  Vote: I like it +6 Vote: I do not like it

→ Reply
»
4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Why you guys need to make tough B? Now, I have idea for me but I have only 10 minutes left

→ Reply
»
4 years ago, # |
  Vote: I like it +21 Vote: I do not like it

Most Deadly Global Round I have appeared in so far .

→ Reply
»
4 years ago, # |
  Vote: I like it +124 Vote: I do not like it

→ Reply
»
4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Whats with the statement of C? It is so complicated for no reason.

→ Reply
»
4 years ago, # |
← Rev. 2   Vote: I like it +2 Vote: I do not like it

Wrong answer on pretest 2 in B? Try this :

1

11 2

WWLWLLLWWLW

ans : 14

→ Reply
»
4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

How to solve B?

→ Reply
  • »
    »
    4 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Not tried but have a solid hint :- Try converting consecutive lengths of L as per minimum contiguous L first then next minimum series of Ls and so on until k ends.

    → Reply
    • »
      »
      »
      4 years ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      This is what i came up with during the contests , it did pass the pretest , hope it passes all the test cases.O:)

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

      yup did the same, counted the gaps or consecutive Ls and sorted them. the last pretest was really good, it gave this idea otherwise I was just growing the existing W substring in length.

      → Reply
  • »
    »
    4 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Observation : It is always optimal to fill contiguous loss substrings between two wins. Additionally, it is always optimal to completely fill the contiguous loss substring , thats why you sort in increasing size of contiguous loss substrings (excluding prefix and suffix). If you completely fill a loss contiguous substring, answer is incremented by (size of substring) * 2 + 1 . If you partially fill the substring answer is incremented by (size of substring) * 2. ( Note +1 in first case. This makes it optimal to completely fill substrings. To maximize that sort substrings in increasing size. )

    If after filling all the substrings, you have some operations left , use them on the prefix or suffix.

    Corner Case : When k=0 or number of wins = 0.

    → Reply
»
4 years ago, # |
  Vote: I like it +1 Vote: I do not like it

Problem C? Thought 2 hours on it yet clueless after one point?

→ Reply
  • »
    »
    4 years ago, # ^ |
      Vote: I like it +5 Vote: I do not like it
    Hint
    → Reply
    • »
      »
      »
      4 years ago, # ^ |
      ← Rev. 3   Vote: I like it 0 Vote: I do not like it

      I have come up with O(n^2) solution similar to LIS dp. but TLED on TC5 . i optimised little bit then wrong answer. Actually I am not able to know that within my particular manhattan radius i such that

      $$$i<=r$$$

      . and what is the place within current radius where number of celebrity already meet is maximum. I hope you got where I got stuck.

      PS: Passed

      → Reply
    • »
      »
      »
      4 years ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      Got it I realised that points in range i+2*r are important only but not that after that too answer was there, just one step was remaining ! NIce problem thanks anyways.

      → Reply
»
4 years ago, # |
  Vote: I like it +1 Vote: I do not like it

The idea of using graphics in C was amazing , didn't get the question though !

→ Reply
»
4 years ago, # |
  Vote: I like it +12 Vote: I do not like it

going live now, talking about ABCDE and my maybe-wrong solution for H https://www.twitch.tv/errichto

→ Reply
»
4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

How to solve C guys?

→ Reply
»
4 years ago, # |
  Vote: I like it +11 Vote: I do not like it

Thats it I am done with global rounds.

→ Reply
»
4 years ago, # |
  Vote: I like it +1 Vote: I do not like it

So one did solve H after all :(

→ Reply
»
4 years ago, # |
  Vote: I like it +5 Vote: I do not like it

I believe the TL of G is strict. My first submission is indeed $$$O(N^3)$$$ time solution, but it got TL. My local testing suggested that only iterating the graph represented by vector<vector<Edge>> takes much time. Yes, I knew it's time-consuming and I should have noticed that before implementing it. Still, I wish the constraints was $$$n \leq 100$$$ or something.

Is there any reason to set this TL, or you didn't noticed solutions like this? Anyway, I like the idea of the problem, so thank you for that.

→ Reply
  • »
    »
    4 years ago, # ^ |
      Vote: I like it +9 Vote: I do not like it

    With $$$n\le 100$$$ there would have been $$$O(n^4)$$$ solutions getting accepted and solutions using fancy min-cost flow implementation (but with bad complexity) getting accepted, and I did not want that. On the other hand, I understand that the strict time-limit of problem G made it slightly harder to get accepted even for $$$O(n^3)$$$ solutions.

    → Reply
»
4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

is there any dp solution for B

→ Reply
»
4 years ago, # |
  Vote: I like it +53 Vote: I do not like it

The low scores for problems B-C-D-E-F-G should not suggest that they are easier than usual.

Yes, and they seemed to be even harder than usual.

→ Reply
»
4 years ago, # |
  Vote: I like it +26 Vote: I do not like it

So much for question H. Zero Solves :)

→ Reply
»
4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

I've tried to solve problem C making a graph in which a node is a celebrity, and there's an edge between $$$v_1$$$ and $$$v_2$$$ if $$$v_1$$$ appears before $$$v_2$$$ and it's impossible to take a photo to $$$v_1$$$ and $$$v_2$$$. And now you have to get a maximal independent set of the graph. As far as $$$t_i < t_{i+1}$$$ and the grid is not very big, any node won't have too much edges, anyone think about it? Due to lack of time because I got that idea in the end of the contest I couldn't try to implement it, but I'd like to know if it works or not

→ Reply
»
4 years ago, # |
  Vote: I like it +12 Vote: I do not like it

→ Reply
»
4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

how to optimize DP solution of C ... i got n^2 solution??????? thanks for help

→ Reply
»
4 years ago, # |
  Vote: I like it +9 Vote: I do not like it

How to solve D. I was able to come up with a solution that requires at most $$$2 * n$$$ operations. But how to do it in at most $$$n$$$ operations.

→ Reply
  • »
    »
    4 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    In every step try to put them into (1, 2, ..., cur, ...) or (..., cur, ..., 2, 1). Just zone 1 of them into the streak each operation. Save the last one for rotation.

    → Reply
  • »
    »
    4 years ago, # ^ |
    ← Rev. 3   Vote: I like it +18 Vote: I do not like it

    Step 1 : get 1 as a prefix.
    Step 2 : get 2,1 as a suffix.
    Step 3 : get 1,2,3 as a prefix.
    So on.

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

    Its probably something like putting $$$(1, 2, ... , x...)$$$ and $$$(...x, ... , 2, 1)$$$ alternatingly. Like this. for this sequence $$$2, 3, 6, 1, 4, 5, 7$$$
    Moves are like this
    $$$1, 4, 5, 7, 2, 3, 6$$$
    $$$2, 3, 6, 4, 5, 7, 1$$$
    $$$1, 2, 3, 6, 4, 5, 7$$$
    $$$4, 5, 7, 6, 1, 2, 3$$$
    $$$1, 2, 3, 4, 5, 7, 6$$$
    $$$6, 7, 1, 2, 3, 4, 5$$$
    $$$1, 2, 3, 4, 5, 6, 7$$$
    I don't know the proof nor I implemented it(it was only 10 min left). But, probably this is correct.

    → Reply
»
4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

How to solve D, I thought of something like grouping every ele and ele-1 together and doing this over and over again.

→ Reply
»
4 years ago, # |
← Rev. 2   Vote: I like it +2 Vote: I do not like it

So much for "spicing up the strategies". Maybe it would've worked for a different contest, but it doesn't make for a very fun experience with speedforces and hard problems with low point values. As for top contestants, the other problems were hard enough for no one to solve H even with a 3 hour contest time.

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

I splits the array into two parts. One is positive part and another negative part.. If the sum of positive part is bigger than I printed that first and then negative part. Same for the negative part, and if both are same then answer will be NO. But I noticed one thing, if I print 0 with positive part, it gives me wrong answer. If I print 0 with the negative part it passed the testcases. If multiple answers are possible, then 0 can be in any side, it doesn’t effect, right?

And one more thing, If I don’t sort my split parts it also gives wrong answer.. But why sorting is needed? I just need to check if the sum is not 0. Sorting shouldn’t be a matter, right?

Please tell if I am wrong.
Mycode: https://codeforces.net/contest/1427/submission/95120716

→ Reply
  • »
    »
    4 years ago, # ^ |
    ← Rev. 2   Vote: I like it 0 Vote: I do not like it

    Consider the case: 0, 1, -1, 2, -2. Without sorting and keeping the zeroes with negative numbers you will get: 0, -1, -2, 1, 2 or something similar which is wrong.

    → Reply
    • »
      »
      »
      4 years ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      But it is wrong already, because the sum of total array is 0, which can't be printed. Right?

      → Reply
      • »
        »
        »
        »
        4 years ago, # ^ |
          Vote: I like it +1 Vote: I do not like it

        then consider 0 1 -1 3 -2

        → Reply
»
4 years ago, # |
← Rev. 4   Vote: I like it +2 Vote: I do not like it

This round is sooo hard. Here are my solutions to A-E:

A
B
C
D
E
→ Reply
»
4 years ago, # |
← Rev. 2   Vote: I like it +2 Vote: I do not like it

The low scores for problems B-C-D-E-F-G should not suggest that they are easier than usual. Now i know this is true. I only solved three problems in total. Problem B thought about a wrong algorithm at the beginning and wasted a lot of time. B is obviously harder than usual div2B, and C is also harder than usual div2C. D may be the same as usual div2D.upd: 10 minutes after the start of the competition, I still can't see the complete content of the question. In the mathematical formula part, all I see are inexplicable symbols. How is this going?

→ Reply
»
4 years ago, # |
  Vote: I like it +8 Vote: I do not like it

→ Reply
»
4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

How to solve H?

→ Reply
»
4 years ago, # |
  Vote: I like it +113 Vote: I do not like it

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

→ Reply
»
4 years ago, # |
  Vote: I like it +5 Vote: I do not like it

Ratings updated! That was quick.

→ Reply
»
4 years ago, # |
  Vote: I like it +11 Vote: I do not like it

Wrong submissions were a bit too pricey, and constructive tasks as always. Otherwise great contest!

→ Reply
»
4 years ago, # |
  Vote: I like it -8 Vote: I do not like it

https://codeforces.net/contest/1427/submission/95098796 Please tell me why was it giving runtime error.

→ Reply
»
4 years ago, # |
  Vote: I like it +11 Vote: I do not like it

Moral : Always check all possible inputs when the number is reasonable. Still passed all pretests even though the solution fails on a large number of inputs.

→ Reply
»
4 years ago, # |
← Rev. 2   Vote: I like it 0 Vote: I do not like it

How to solve E systematically? I really played with numbers for 1.5 hours and couldn't find any solution :(.

→ Reply
  • »
    »
    4 years ago, # ^ |
    ← Rev. 6   Vote: I like it +42 Vote: I do not like it
    1. $$$2k\oplus(2k+1) = 1$$$.
    2. If we have $$$x$$$ we can get $$$K.x$$$.

    Reach a number $$$X$$$ that can be made such that $$$gcd(X, N) = 1$$$. One possible number would be: $$$N \oplus (N . 2^B)$$$ where B is the highest power of $$$2$$$ less than $$$N$$$.

    Proof:

    $$$X$$$ = $$$N \oplus (N . 2^B) = N + (N . 2^B) - 2 . (2^B) = K.N - 2^{B+1}$$$, which has no common factor with $$$N$$$. ($$$gcd(K.N - 2^{B+1}, N) = 1$$$, as $$$N$$$ is odd)

    After that we need to find a solution $$$(a,b)$$$ to $$$a.X = b.N + 1$$$ where $$$b*N$$$ is even.
    This can bruted by iterating on $$$a$$$. ($$$a$$$ will be upto $$$2N$$$)

    Now we just print $$$b.N$$$ xor $$$a.X$$$

    → Reply
»
4 years ago, # |
  Vote: I like it +4 Vote: I do not like it

I think weak systests for C?

95127155

500 2 950 500 500 3000 1 1

Output should be 1, mine is 0, so WA. Please correct me if I'm wrong.

→ Reply
»
4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Guys how are the "best participants" selected?

→ Reply
»
4 years ago, # |
  Vote: I like it +149 Vote: I do not like it

Petr after getting -11 on H during contest

LUL.jpg

→ Reply
»
4 years ago, # |
  Vote: I like it -15 Vote: I do not like it

Just after contest, my ratio changes +179. But after few hours : +180, even I'm still rank 80. Why?

→ Reply
»
4 years ago, # |
  Vote: I like it +9 Vote: I do not like it

probably one of the best rounds i've ever participated in. Good problem statements, clear explanations, beautifully difficult tasks, and no technical issues! Hope more rounds could be like this

→ Reply
»
4 years ago, # |
  Vote: I like it -17 Vote: I do not like it

The topic is a little difficult, but it inspires me to continue to study hard. The team is wonderful

→ Reply
»
4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

For ABC and D, the points rating is double the points give to it.

→ Reply
»
4 years ago, # |
← Rev. 2   Vote: I like it 0 Vote: I do not like it

Excellent round btw

Well-balanced difficulty unlike most atcoder rounds
Algorithmic, not mathy. It is individual but I like it
Good balance of idea and implementation
Visible effort put into editorial

I felt thrill throughout the whole contest

→ Reply
»
4 years ago, # |
  Vote: I like it +76 Vote: I do not like it

When is the list of people who will be getting random t-shirt coming out?

→ Reply
»
4 years ago, # |
  Vote: I like it +52 Vote: I do not like it

Congratulations to tshirts winners!

List place Contest Rank Name
1 1427 1 Benq
2 1427 2 yosupo
3 1427 3 ksun48
4 1427 4 Um_nik
5 1427 5 ecnerwala
6 1427 6 sunset
7 1427 7 maroonrk
8 1427 8 zscoder
9 1427 9 SirShokoladina
10 1427 10 gamegame
11 1427 11 244mhq
12 1427 12 lumibons
13 1427 13 tatyam
14 1427 14 Enjoy_the_game
15 1427 15 jiangly
16 1427 16 Endagorion
17 1427 17 dreamoon_love_AA
18 1427 18 olphe
19 1427 19 dlalswp25
20 1427 20 duality
21 1427 21 amethyst0
22 1427 22 tourist
23 1427 23 mnbvmar
24 1427 24 Maksim1744
25 1427 25 Golovanov399
26 1427 26 snuke
27 1427 27 Worteltje
28 1427 28 Radewoosh
29 1427 29 Farhod
30 1427 30 Petr
33 1427 33 RAVEman
67 1427 67 Mikaeel
91 1427 91 amnesiac_dusk
118 1427 118 jschnei
125 1427 125 BohdanPastuschak
132 1427 131 Martin53
148 1427 148 gyh20
182 1427 182 jjjjj19980806
186 1427 186 Savior-of-Cross
218 1427 218 4eT_llpuyHblJl
234 1427 234 craborac
238 1427 238 mnaeraxr
240 1427 240 ezLadder
316 1427 316 uwu
341 1427 341 Behind_the_Clouds
349 1427 349 Nika_Tamliani
358 1427 358 zld3794955
403 1427 402 VLamarca
439 1427 438 kupriyanov
493 1427 493 l1ll5
→ Reply
»
4 years ago, # |
  Vote: I like it -10 Vote: I do not like it

I hate that problems

→ Reply
»
4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Perhaps you want to know current results of globals. See https://clist.by/standings/codeforces-global-rounds-2020-18730990/.

→ Reply