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SlavicG's blog

Codeforces Round #827 (Div. 4)
By SlavicG, 2 years ago, In English

Hello Codeforces!

mesanu, flamestorm, MikeMirzayanov and I are glad to invite you to Codeforces Round 827 (Div. 4)! It starts on Oct/13/2022 17:35 (Moscow time).

The format of the event will be identical to Div. 3 rounds:

  • 5-8 tasks;
  • ICPC rules with a penalty of 10 minutes for an incorrect submission;
  • 12-hour phase of open hacks after the end of the round (hacks do not give additional points)
  • after the end of the open hacking phase, all solutions will be tested on the updated set of tests, and the ratings recalculated
  • by default, only "trusted" participants are shown in the results table (but the rating will be recalculated for all with initial ratings less than 1400 or you are an unrated participant/newcomer).

We urge participants whose rating is 1400+ not to register new accounts for the purpose of narcissism but to take part unofficially. Please do not spoil the contest for the official participants.

Only trusted participants of the fourth division will be included in the official standings table. This is a forced measure for combating unsporting behavior. To qualify as a trusted participant of the fourth division, you must:

  • take part in at least five rated rounds (and solve at least one problem in each of them),
  • do not have a point of 1400 or higher in the rating.

Regardless of whether you are a trusted participant of the fourth division or not, if your rating is less than 1400 (or you are a newcomer/unrated), then the round will be rated for you.

We would like to thank:

We suggest reading all of the problems and hope you will find them interesting!

GLHF!

UPD: Editorial

  • Vote: I like it
  • +177
  • Vote: I do not like it

Comments Comments (158)
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2 years ago, # |
  Vote: I like it +19 Vote: I do not like it

As the one, who translated statements, I promise, Timur has legs!

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2 years ago, # |
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I got pinged lol

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

Finally, MY FIRST UNRATED ROUND >:D

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

As a tester, SlavicG orz

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

As a tester, I suggest while(true){ if(time != contest_time) continue; else return write_contest; }

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    2 years ago, # ^ |
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    That code uses 100% of CPU Thread, use something like std::this_thread::sleep_for(std::chrono::milliseconds(1)) to avoid that.

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

There were no Div. 4 rounds before. There were times...

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2 years ago, # |
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Love to see haochenkang testing! Gl everyone!

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

As a tester, I'm really happy to be a part of this contest :)

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

waiting for long time...Finally div 4 come ..very exited ...

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

Best Div. 4 round I've tested so far!

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

I hope this is my last rated Div 4 :>

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

As a tester, I just wanna say the G in SlavicG stands for top G.

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

How do you become problem setter, tester, helper or any of such things?

And also do these people get paid or they do it for fun?

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

    setters do get paid, there's a blog in the catalog about that. in the meantime, testers either get contacted from setters directly or from other testers, and don't necessarily get paid.

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

    No, testers do not get paid.

    I like being a div. 4 tester because i can't wait for div 4 problems XD

    Spoiler
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      2 years ago, # ^ |
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      If you want to prove that you are SlavicG's #1 fan, then you should change to my profile pic to show your loyalty.

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

    Please read this, it's about becoming a problem setter.

    You can become tester if your friend is the author/coordinator/another tester, they'll invite you. Note that authors need to be very careful who to trust, because they don't want to get the problems leaked.

    Also, testers don't get paid unlike authors and coordinators. We do it for free because it's fun :)

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

      I think it's unfair, testers should get paid with at least an autograph from SlavicG himself. You already got plenty of autographs but us the average testers never get to taste an autograph >;((

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

        What an excellent suggestion, I also want autograph from SlavicG! But he didn't invite me this time, so I have no right to claim it unlike you :(

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

          Oooof right, you know what, let's go on a strike if he doesn't invite you next time, or if he doesn't pay us with juicy autographs

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

I can't believe that I have 2 more rating than the maximum to participate in this contest as rated contestant, I wish I had waited a few minutes before submitting the last problem in the previous contest. -_-

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

Specialists Be Like

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

This is harsh. I have used codingblocks.com online compiler and have not copied any text from anywhere and done it on my own. This is just a coincidence and nothing else. https://mirror.codeforces.com/submissions/code_vishal18# https://drive.google.com/file/d/1xTv1xVGfwwAsr0OIcfZEwf4nqu2GZLF9/view?usp=sharing. Please roll back my ratings. This is really frustrating and demotivating.

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

Hi, I'm a Chinese student, my English is bad. So, I want to Make frinends to learning it. After reading the previous words, you can also find that my English is not good. We can also progress on algorithms together. Thanks to you read my words.

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    2 years ago, # ^ |
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    "to learning" should be "to learn".

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      2 years ago, # ^ |
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      yes, thank for your help

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        2 years ago, # ^ |
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        能不能给这个hack div4点个upvote？https://mirror.codeforces.com/blog/entry/107979

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          2 years ago, # ^ |
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          点赞了hh，看来你的英语不错，我很菜，一直在学习

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

Some users with rating 1400+ create new accounts and participate in such contests and it affects us low rated users. I hope they stop doing this. This happens in div 2 contests as well.

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2 years ago, # |
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Finally, I can increase my rating :)

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2 years ago, # |
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OmG My FIrsT UNofficIAL ROunD!!!!!!

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

rp++！

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

Good luck to you

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

Hope it won't be queueforces.

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

    Unfortunately, the queue is very long.

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      2 years ago, # ^ |
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      It may because there are too many testcases.

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

    And that's how you jinxed it.

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2 years ago, # |
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Very long queue !!

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

I keep seeing "Oops! Codeforces is broken"

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

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2 years ago, # |
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Servers down O_o

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2 years ago, # |
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Div 4 became so popular...

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

stresstestingforces XD

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

As an unrated participant, I really enjoyed the contest. Good job slavicg and team.

Why don't div3/div2 contests have similar problems? :sad:

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

Great contest for a div 4, covered a lot of basic concepts, for the first time I was able to solve the whole set of problems, enjoyed it thoroughly.

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

Imagine submitting a solution and after 30 min long queue getting wrong answer on tc 1.

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2 years ago, # |
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Thank div4 for making me feel less useless, first time full AC without any WA :(

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2 years ago, # |
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I'm so tired for waiting.

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2 years ago, # |
← Rev. 2   Vote: I like it -26 Vote: I do not like it

Deleted.

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

    What's the issue with C++20's gcd function? Let me know so I can mention it next time I cover it on STL in CP

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

    You are invoking undefined behaviour by sorting empty vectors. Gcd is in no way invalid in c++20

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

    I am impressed by how many different explanations there are for why your code is wrong. Check the test case that you failed on. You mistakenly assumed that if the GCD of the entire array is $$$1$$$, then there exists a pair of elements that is coprime. You fixed it by initializing $$$\mathrm{ans}$$$ to $$$-1$$$ instead of $$$0$$$. Everything else was fine.

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2 years ago, # |
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Why was site down all the time! It was frustrating.

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

Is it just me who think that E has way more right submissions that it should have?

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    2 years ago, # ^ |
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    Dude, $$$E$$$ was easy, I actually solved it before $$$C$$$ and $$$D$$$ !

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      2 years ago, # ^ |
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      So I guess it has a simple solution that I didn't think of

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

    It's because of Um_nik everyone knows how to Binary Search

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2 years ago, # |
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how to solve G?

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

    Video Solution for G.

    Hint:
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    2 years ago, # ^ |
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    Use a mask. Initialize it as the max element in the beginning. Add this to the answer and mark it as used. Then, for each unused element update the element with a new key of (original value with the masked bits cleared). This is the key. The value is a vector of the original values. Use the largest key and sort the vector of the original values. Take the max. Add to answer and mark it as used. Repeat the above until key is 0. Add all unused elements to the answer.

    176046217 I submitted 5 minutes after contest ended.

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

    Let the max element of the array to be the beginning of the sequence, since it has the highest lexicographical order. Then we try to find the element that results in the highest OR value, then update the prefix OR and mark that index as used. At one point where the higher OR value cannot be found, we fill the rest of the sequence with unused elements.

    Since a[i] <= 1e9, which is less than 31 bits, the time complexity is O(N).

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

POV : You missed red and blue being horizontal and vertical specifically

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    2 years ago, # ^ |
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    And after getting WA2 wonder if this problem is even solvable.

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2 years ago, # |
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Is there any more efficient way to check coprime than generating all in O(1000^2 * log(1000)) time.

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    2 years ago, # ^ |
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    they can be generated in 1000^2 operations by memoizing eucledian gcd

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2 years ago, # |
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Problem E Why my solution is giving TLE for test case 4 176003168 176020081 176037934

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2 years ago, # |
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approach for F?

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

    Only two things matter:

    • number of 'a's in the string

    • does the string have any character other than a?

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

    I'd like to expand on tsuu's explanation.

    1) Both strings 's' and 't' contain only the character 'a': Eg: aaa vs aaaa => we can see that the one with lesser frequency wins

    2) String 's' contains many different characters, while string 't' contains 'a' only Eg: aaab vs aaaaaaa => S will never win aaab vs aa => again, S loses

    3) Both 's' and 't' contains many different characters Eg: abcde vs aaazxy => Just pick 'a' as the starting character in S, and any character other than 'a' in t Here we pick a**** vs z***, so S wins

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

      About the 3rd case, both strings are 'a' initially so if string T has different characters, S will win because we can make S starts with 'a' and T starts with anything other than 'a'.

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

https://mirror.codeforces.com/contest/1742/submission/176025562 Why this is not passed any counter exam ple

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    2 years ago, # ^ |
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    1 6 1 1 3 1 2 5 4 3

    Output should be 1+3+1+2 = 7 but it considers 5 as well and wrongly prints 12

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2 years ago, # |
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Can anyone provide me a stress test like wa2 for my submission[submission:176043362]

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

I wasted 1 hour getting WA on Problem C, because, I thought each row or column can be painted with both red or blue colour! Then I noticed, only columns can be painted blue and rows can be painted with only red!!

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

    A similar thing happened to me in the past, a few extra seconds spent reading problem statements are worth it.

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2 years ago, # |
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How to solve D

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    2 years ago, # ^ |
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    Hint1
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2 years ago, # |
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Please, share why this code getting TLE. Problem E time limit is 3 seconds. is q*n more than 3 seconds?

Spoiler
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    2 years ago, # ^ |
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    q*n=4*1e10,it need more than ten seconds.

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

Thought in E we rearranged both s and t in the lexicographically least arrangement and then compare, wasted about 30 mins there, then re-read the question again and understood it properly, my ADD is really messing up with my performance, maybe it's time to cut down insta reels and tik tok ;(

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

    i increased my performance 3x just by cutting porn.

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

Will 827div4 be unrated?

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

SlavicG

Is something wrong with the checker of problem F? I tried to hack and kept getting "Unexpected verdict" result.

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    2 years ago, # ^ |
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    Same Issue here. I find many people using int to store the number of characters. When k and q are large I created a set of data to overflow the counter back to 1... But the hack gives me "Unexpected verdict".

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    2 years ago, # ^ |
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    Sorry for the inconvenience, it should be fixed now.

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

C was the biggest anti-speedforces bait I've ever seen.

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

    C still needed weak testcases for full effect ;) The number of FSTs would probably set an all time high record if the testcases were weak.

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2 years ago, # |
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Can anyone tell why I am getting wrong answer on test case 2 of problem E.. It is because of wrong implementation or because of wrong logic. Submission link

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2 years ago, # |
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This is the first round that I AK :D

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2 years ago, # |
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Imho problem F was easier than E and D

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2 years ago, # |
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editorial?

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    2 years ago, # ^ |
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    if you want, I can explain you any problem from this round

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      2 years ago, # ^ |
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      In problem C, you have to check whether "RRRRRRRR" is present in a row or not? Is that a wrong logic? because i wasted an hour on that and I couldn't find out what am i missing.

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

It was a cool round. The last assignment is a lot of fun to underestimate why you can write n^2 code with one break and it will be accepted.

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

Overall this contest was good.

Spoiler :(
Problem F
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    2 years ago, # ^ |
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    hacks in F because some people didn't read the first line in the statement

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2 years ago, # |
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How to do F, if s and t are initially empty ?

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

GEO THERMAL WROTE IT a newbie sadly

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


I had a problem accessing the page.
How can I not panic?
I hope this div not reflected in my rating.

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

Is the contest declared unrated?
The contest is shown under the 'only unrated' tab..

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

    It takes time to update rating. Until the rating is updated, the contest is shown under unrated tab only.

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      2 years ago, # ^ |
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      Oh sorry, I wasn't aware of this, thank youu.

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2 years ago, # |
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Wtf how is my F solution wrong, even some Gms had it wrong

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

    temp*y can be greater than 2e9. so you should use long long

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      2 years ago, # ^ |
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      I'm an idiot i knew it should be long long and set the size variable to long long but the mul operation variables were integer.. Short sad story

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

Is it possible to solve E this way without getting TLE in test case 9? 176125836

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    2 years ago, # ^ |
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    q*log(n) is right.q*n is not enough.you can see my code.It only needs 467ms.176015222

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      2 years ago, # ^ |
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      Yeah but that's binary search. I was asking that can I do some improvement in this code only and get the job done? Or binary search is a must to reduce it to q*log(n)?

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        2 years ago, # ^ |
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        of course we need binary search.q*n=4e10,it will need more than ten seconds

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

Guys, I mistakenly resubmitted my own code for problem D, and CF sent me this:

Attention!

Your solution 176041980 for the problem 1742D significantly coincides with solutions tllwtg/176009167, tllwtg/176041980. Such a coincidence is a clear rules violation. Note that unintentional leakage is also a violation. For example, do not use ideone.com with the default settings (public access to your code). If you have conclusive evidence that a coincidence has occurred due to the use of a common source published before the competition, write a comment to post about the round with all the details. More information can be found at http://mirror.codeforces.com/blog/entry/8790. Such violation of the rules may be the reason for blocking your account or other penalties. In case of repeated violations, your account may be blocked.

Dose that mean my account are going to be blocked? :(

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    2 years ago, # ^ |
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    the same for me, also round somehow became rated for me

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      2 years ago, # ^ |
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      Thats strange. I guess the rating will soon be rolled back. lol

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    2 years ago, # ^ |
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    It's just a warning. But In case of repeated violations, your account may be blocked

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

    same here. I wrote a blog post here. I did not cheat, and I don't know how they matched those 2 solutions as a "significant coincidence" case.

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2 years ago, # |
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I ranked ~400 in the round (~95 officially) but I found I was disqualified and I am accused of cheating. I wrote a post here. Please help by upvoting.

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2 years ago, # |
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Something strange happened to me. For problem B, I used "goto" in my code, and the judge gave me WA ( 175907076 ) though the code works well in my computer, when I replaced it with "flag" , I got AC.( 176139742 ) Could someone tell me whats going on with "goto" in this problem? that would help me a lot, thanks : )

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    2 years ago, # ^ |
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    It's because you don't read the input correctly. You are breaking early while reading the input so the values for the next test case start from the array of the previous test case, so all future values are just messed up.

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

https://mirror.codeforces.com/contest/1742/submission/175924333 https://mirror.codeforces.com/contest/1742/submission/175959376 I don't know why my code for this problem in 827div4 has been repeated by other's code. I don't know the people at all and if you see the links of code above , you could find that the codes could certainly made by different people. And the problem D , the solution I solved was right and simple. I can't really understand that this code could be repeater by others. Attention!Your solution 175924333 for the problem 1742D significantly coincides with solutions 0x4c/175924333, alitarekkk/175959376. Such a coincidence is a clear rules violation. Note that unintentional leakage is also a violation. For example, do not use ideone.com with the default settings (public access to your code). If you have conclusive evidence that a coincidence has occurred due to the use of a common source published before the competition, write a comment to post about the round with all the details. More information can be found at http://mirror.codeforces.com/blog/entry/8790. Such violation of the rules may be the reason for blocking your account or other penalties. In case of repeated violations, your account may be blocked.

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2 years ago, # |
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My English is not good, so some of them are translated.

D guarantees that n is greater than or equal to 2, but they have n is equal to 1 in their data.

The first solution get Accepted. https://mirror.codeforces.com/contest/1742/submission/176077405

The second solution adds assert(n! =1) Then it gets Runtime error on test2.

https://mirror.codeforces.com/contest/1742/submission/176155308

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

MikeMirzayanov SlavicG I have been submitting my Java solution for problem E div4 827, and getting judgement failed verdict continuously. This happened at the time of contest as well. Kindly check.

Attaching the submission links : 175975446 176169782 176170045

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

    Try renaming your class, con is a restricted filename on windows.

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2 years ago, # |
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Why Problem D coprime in Codeforce round 827(Div 4) is retesting

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2 years ago, # |
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Hi everyone,

As a contestant I found that the test in the probem F wasn't good enough. I have all the prestest passed, but then somebody hacked me, so I checked what was the problem with my code and it was the size of a variant. I needed to put long long int and I used int, I think that it should give me wrong answer in a pretest, but it didn't.

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2 years ago, # |
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Help me for find my mistake. I don't understand. https://mirror.codeforces.com/contest/1742/submission/176450420

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