matrix's blog

Codeforces Round #282
By matrix, 10 years ago, In English

Hi,

Codeforces Round #282 will be held at 13th of December, 19:30 MSK. Please don't note anything since the time is the usual time of contests.

The round is prepared by FarbodY, Haghani and Me(matrix).

We'd like to thank Zlobober who helped us a LOT for preparing the problems, MikeMirzayanov for the Polygon and Codeforces platforms and Delinur for helping us in preparing statements and translating them.

Our special thanks goes to mruxim for testing the round.

The problems will be sorted according to the estimated order of difficulty according to our opinion but we strongly recommend you to read all of the problems.

As always the update regarding score distribution will be posted just before the round starts. :)

UPD: It was written that contest starts at 20:00 MSK. it was a mistake and is fixed now. The round starts at usual time 19:30 MSK.

UPD2: We also thank niyaznigmatul for testing our round.

UPD3: Score distribution:

Div1: 500-1000-1750-1750-2500

Div2: 500-1000-1500-2000-2750

As you might have noticed, the scores can now be multiples of 250 as well. Let's thank Codeforces team for adding this feature!

Good luck and Have fun!

UPD4: Congratulations to the winners of both divisions:

Div1:

  1. tourist
  2. winger
  3. sankear
  4. uwi
  5. Egor

Div2:

  1. Ginger88895
  2. pwild
  3. arthurpd
  4. konmaj
  5. ezkatka

The editorial for all problems except D and E Div1 are ready and can be found here. It'll be completed soon.

We thank you all for participating and hope you had a good time.

UPD5: The editorial is now complete and can be found here.

  • Vote: I like it
  • +383
  • Vote: I do not like it

Comments Comments (166)
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10 years ago, # |
← Rev. 6   Vote: I like it +11 Vote: I do not like it

wow :) iranian contest I love them :)

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

matrix your graph is awesome and very motivated

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

    Maybe there will be problem about matrix/matrices this round!

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

    Its the opposite of the identity matrix :D

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

    I hope that someday my graph becomes like that too!

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10 years ago, # |
← Rev. 5   Vote: I like it -44 Vote: I do not like it

Our special thanks goes to mruxim for skipping this round just because we needed a tester

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

"Please don't note anything since the time is the usual time of contests."

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10 years ago, # |
← Rev. 6   Vote: I like it -63 Vote: I do not like it

Happy coding

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

hope to see a nice problem-set and rating growth :)

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

This will be excellent preparation for the USACO December Contest.

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10 years ago, # |
← Rev. 2   Vote: I like it -6 Vote: I do not like it

Zlobober has a great contribution in almost every contest. Thanks bro.. :)

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

Have fun and good luck to all of you.

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10 years ago, # |
← Rev. 3   Vote: I like it +38 Vote: I do not like it

Codeforces really should think about some additional promotion, today I've found topcoder easter egg in locker room

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

Time for dreamoon_love_AA :D

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

It will be a good contest ...

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

Hope for much [pure] math

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

    For me more interesting when anybody have more chances to hack each other(I want low pretests!!!).

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

      Eh, well it is a matter of opinion. I personally believe that hacking creates more suspense, especially right after you try to hack someone.

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

    And I thought I was the only one

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

      i hate hacking ): I like contests like ICPC, where you know if your submission is right or wrong when you submit it.

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

It will be a great contest.

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

So happy for div.1 contest)))))

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10 years ago, # |
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At last after 10 days !!!

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

hope candidate master for every expert

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

Wanna be Blue.... Gonna be White... -_-

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    10 years ago, # ^ |
    ← Rev. 2   Vote: I like it +12 Vote: I do not like it

    Did you mean grey?

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

      ya.. Grey..

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

      It's possible to be white. See worse's graph.

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

        Everything is possible

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

        I don't understand the reason why worse always tries to get negative point (Except last round). And he wants to challenge others and fails.

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          10 years ago, # ^ |
            Vote: I like it +42 Vote: I do not like it

          You don't understand why people sometimes want to do random stuff for fun?

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

it is my first time for div.1.....

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

I just want get a purple handle in this round~

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

I hope this contest will bring me from unhappiness...... I even suspect whether coding is suitable for me...

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

    Well...I didn't got a purple handle, en.....when you tired or lack of confidence,look at me ~, many times I tried my best but got a bad result, but I firmly believe that one day, I can become an excellent coder, not only in ACM. A gold medal is not necessary to prove yourself.

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

Don't you think it is a good idea to announce the scoring right now? Because of I think the website will be down (hope not) in a few minutes and you won't be able to announce it just before the contest! :D It would be after the start of the contest!

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

1750 points. O_O Rly?

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

"As you might have noticed, the scores can now be multiples of 250 as well. Let's thank Codeforces team for adding this feature!" Sounds sarcastic

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

CF #282

We hack as we could

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

Problems A, B and C are easy; but problems D and E are difficult ! (in Div. 2)
There isn't any normal problem :D

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

Good Bye div1)

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

correct score distribution (div. 1): 500 — 1500 — 2500 — 2500 — over9000

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

thanks a lot for nice problem :)

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

last 10 minutes, server was down. I could hack 10+ solutions but...

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

    Also I could have hack some solutions

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

Woah, wasn't able submit my solution to A because codeforces wouldn't load. :(

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

A div1 / C div2:

(#(
Answer: -1
WA: 2
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    10 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Come across this comment, when suddenly my div2 C failed system test. What a coincidence. But really, I didn't notice that case!

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10 years ago, # |
← Rev. 7   Vote: I like it +26 Vote: I do not like it

fail of the year

A: forgot if(b[i] < 0) ans is -1.
B: forgot if there is no S in T ans is 0.

added two ifs and got accepteds :(

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

What's the hacking test case for Div2A?

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

    Here was something unrelated. Keep scrolling.

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

      I asked for Division 2

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

        I'm pretty sorry.

        For A2, it was pretty simple too: many people had problems with calculating answers for numbers in segment 01-09.

        Also many people accidentally had 6 instead of 7 for 1.

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      10 years ago, # ^ |
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      That's for Div2 C/Div1 A :)

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    10 years ago, # ^ |
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    It depends on the code. Some people miscalculated the number of probabilities for a simple digit.

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10 years ago, # |
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Server Problem,vary painful.

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10 years ago, # |
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What's wrong with test 6 in D, my stress test doesn't reveal any problems =(

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

    Check all modulo operations carefully, especially with negative numbers.

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

      Eh at least now my stress is giving different results, but I can't find the mistake =(

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

Last 10 minutes :(

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

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

    Your avatar fits well to that pic :P.

    For me it was best round ever with regard to number of hacks (I got +6/-1) :). "(#(" ftw :D! As well as best round ever with regard to place before systests (8th), I hope it will stay that way after them :P.

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

For div2, a lot of solutions for B had bad complexity O(a) — I managed to submit only two hacks though, due to the server being down for the final fifteen minutes.

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

Sadly, this time too, Codeforces went down in the last ten minutes. solved problem C but couldn't submit :(

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

please improve the server or limit the contestants number !!!!

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

Any idea about how to solve Div-1 B?

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

    I found all occurrences of t in s using KMP, then did DP on the the position of the last substring you take.

    So dp[k] would be the number of valid substring selections where none of the substrings have any chars after the kth char, and the kth char is within one of the selected substrings. Calculating dp[k+1] would depend on whether a pattern of t ended at position k+1 or not.

    I'm not sure if this is right, but it passed pretests.

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

      I did it in the same way — probably a useful structure is >> int last_before[i] << , indicating where is the last occurence of t in s, that ends before i. Then the recurrence can be sumarized to:

      dp[i+1]=dp[i]+sumdp[last_before[i+1]]

      sumdp[i+1]=sumdp[i]+dp[i+1]

      (with edge cases and similar stuff added, of course)

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

    Dynamic programming . For each index i in array, calculate the number of ways dp[i] such that b_k is i. And then sum of all these ways.

    To do this , first find out all the occurences of pattern in text using KMP. Then recursion to calculate dp[i] for i =1 to n(size of text) would be

    dp[i] = sum( j = 0 to pat_start[i] , sum(t = 0 to j, dp[t] ) ); dp[0] = 1;

    where pat_start[i] = largest index p such that Substring[p,i] contains given pattern.

    Now this dp might seem like O(n^2) but by maintaining some additional summations, we can do it in O(n)

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

Good bye, Blue. Welcome Green, again.

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

Я один столкнулся с тем, что, пытаясь взломать, при открытии чужого решения, открывается окно с посылками, само решение не открывается, ссылку с решением, на которую можно нажать тоже нет, в итоге взламывать невозможно? Так было вначале, потом удалось таки один раз взломать, потом тоже самое до конца контеста. Поведение одно и тоже на Ubuntu (Chrome и FF) и на Win8 (Chrome).

Я конечно и сам залажал контест, но невозможность взламывать тоже как-то выбила из колеи.

Ответы жюри:

"У вас должен быть установлен Flash. Некоторые плагины могут резать Flash, считая баннером. Попробуйте так же воспользоваться другим браузером или очистить кэш/cookies."

"В итоге ведь получилось произвести взлом? Остальные участники не жалуются, проблема где-то на вашей стороне."

"Мы не можем ничем помочь."

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

    У меня каждый второй раз открывалось только окно для взлома, без решения. Firefox 34, Win7.
    З.Ы. Это нормально, что я не могу переключить у себя галочку "по-английски" на "по-русски", отвечая на этот комментарий?

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    10 years ago, # ^ |
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    да, было такое. и ещё за 10 минут удалось взглянуть только на 4 решения.

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

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

It's midnight and I am facing an usual dilemma: do I go to sleep before the rating update or after? It's always a difficult decision...

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

    The answer is: yes.

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

      That's kind of a confusing answer :p 1) Yes, you should go to sleep before rating update. 2) Yes: you should go to sleep after rating update.

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        10 years ago, # ^ |
        ← Rev. 2   Vote: I like it +12 Vote: I do not like it

        The answer to V OR NOT V is TRUE :D

        (I'm assuming that you're unable to go to sleep exactly when the rating updates start.)

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

          Haha. I didn't realize my dilemma was a boolean equation :p

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

        I think he meant: Yes, you should go to sleep before or after the rating update.

        Because you should go to sleep at some point, right?

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

    I suppose to think that the contest will be unrated.

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      10 years ago, # ^ |
        Vote: I like it +44 Vote: I do not like it

      Isn't it only me who had really slow and rare access to the website? Almost all my friends say that everything was ok and it was only my problem.

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

        Your friends are lying to you to collect some thumbs up!

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

How to solve C? Tried to implement some DP on a tree, but answer on the 3rd sample was slightly incorrect.

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    10 years ago, # ^ |
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    Yep, DP on a tree. On probabilities that the answer is (trivial lower bound + k).

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

    " but answer on the 3rd sample was slightly incorrect." — no, it wasn't.

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

      I think he means his own code's answer :D

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

        Lol, during the round there was some announcement stating that answer to third sample is really correct, so I thought that it related to that :P.

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

    The idea here is to change each # in such a way to get a correct sequence of brackets, correct sequence is "()", and then "(correct)" or "()correct" or "correct()", I think I didn't miss any.

    If we want to get correct sequence of brackets on each index i starting from 0, we should have the number of ')'-s less or equal to number of '('-s and these two numbers should be equal for the whole string. Here we should keep in mind that we should change each # with at least one of ')'. So we can replace each # with only one ')' and we can replace the right most # with such number of ')'-s that the number of ')'-s will be equals to number of '('-s in the whole string. My solution was this, I hope it will pass system testing.

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

      He asked for DIV-1 C, which is our E.

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

        Yes, I was too late to see it! :D But anyway I'll leave it there.

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

why testing is stopped?

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

I solved E, but read it in a wrong way: I thought rectangles could not intersect. I understood I needed a segment tree, but couldn't wrote it in time. Now it's working with stress test. :( I think it made the problem worse. When you've already got the idea, got the formula for d[x][y], then for rectangle it should be ok. I don't see any difference (except the time). It could be very easy to write mathematical problem. I haven't read all the problems but I like all I've seen.)

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10 years ago, # |
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Interesting question: How can somebody get runtime error on Div2-C??

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

    Well, for example non-terminated C-string, or too small allocated size for char buffer...

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

.

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10 years ago, # |
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pretty hard!!! ( And challenging. )

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10 years ago, # |
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You really have to maintain server stability in the last 10 mins

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10 years ago, # |
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Damn, failed on 47th test... Is there any way to download the tests to see what went wrong?

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10 years ago, # |
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vishwacs111, thank you for hacking my C!

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10 years ago, # |
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nice contest got +132 :D:D:D + hacked 5 users second problem in div2:D:D:D

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10 years ago, # |
  Vote: I like it +69 Vote: I do not like it

No luck — just skill

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

    Isn't that a sign, that contest was totally nonadequate?

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10 years ago, # |
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man i am going down and down and down need to come up

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10 years ago, # |
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what is the difference between

long long ind = lower_bound (x2.begin(), x2.end(),i)-x2.begin(); if(x2[ind]>i){ind = ind-1;}

and

long long ind = upper_bound (x2.begin(), x2.end(),i)-x2.begin(); ind = ind-1;

?

Second one passed,first one didn't. I was trying to find largest element less than/equals to i in vector x2

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

I never understand div 1 B problem statement, could someone explain it?

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

    How many ways to get choose some substrings of T, so that they dont'intersect and each of them has S as substring.

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10 years ago, # |
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What's worse than both your solutions failing? Your friend dropping to div-2 for 1 point.

1699 Rating! — http://codeforces.net/profile/rsunny rsunny next time macha!

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

    Haha yeah I dropped to 1697, thats mighty close too :D

    Btw cheers! We're both aspiring Indian coders!

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

This contest was excellent!!!

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

dp'licious div 1 contest!

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

Couldn't code problem d in time and my a was buggy. I almost fell back to div2 LOL

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

What did just happen here? AC: 9122599 WA1: 9122592

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

http://codeforces.net/contest/494/submission/9117612

For some final tests, I think this should get WA.

Output

999999958.3334586600

Answer

1000000003.315735409

Checker comment

ok found '999999958.3334587', expected '1000000003.3157355', error '0.0000000'

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

    Check this. Relative error is OK.

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

    That's because the relative error is less than 10 - 6.

    I also noticed that for large inputs, even the trivial lower bound could get AC automatically... I wonder how the submissions could get affected by higher precision (and smaller ai-s).

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10 years ago, # |
  Vote: I like it -55 Vote: I do not like it

judging system of codeforces is quite bad. Because in the contest time judges should provide all input/output testcases. When a coder gets Pretest Passed, he moves to other problem, but after contest he sometimes notice that the solution was wrong. If in the contest time he gets Wrong Answer he could get Accepted.

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

    How about hacks?

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      10 years ago, # ^ |
      ← Rev. 2   Vote: I like it -17 Vote: I do not like it

      No problem about hack. But judges should provide strong cases for every problem. After getting hack coder tries to solve it again in contest time.

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

        If they provide all testcases when judging during a contest, nobody would be able to hack other peoples solution. You have to either figure the tricky testcases on your own or pray someone will hack you. This makes contests more exciting.

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

          Yeah. Now It's clear to me. Thanks ddwebx bro..

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

            I think -44 is worth melting for.....

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

    I can't solve a problem, so the judging system is bad.

    Cool story, bro.

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

      I am a new programmer bloody bitch.........

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

        Okay, sorry. You're a new programmer, so the judging system is bad.

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

        The line — "I am a new programmer" was enough to make your point. The rest was unnecessary.

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

    If for every code judge system tests all tests during the contest judging process will be too slow and it make some troubles during the contest,

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

Finally I reached EXPERT!!! :D

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

tourist 's 100th round!

Took off 2 zeros for rank!

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

Hamed and Malek two familiar people for inoi people

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

Iran contest is too hard..555555

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