SlavicG's blog

Codeforces Round #827 (Div. 4)
By SlavicG, 18 months 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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18 months 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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18 months ago, # |
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I got pinged lol

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18 months ago, # |
  Vote: I like it +4 Vote: I do not like it

Finally, MY FIRST UNRATED ROUND >:D

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

As a tester, SlavicG orz

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18 months 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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    18 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    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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18 months 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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18 months ago, # |
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Love to see haochenkang testing! Gl everyone!

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18 months 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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18 months 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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18 months 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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18 months 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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18 months 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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18 months 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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    18 months 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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    18 months 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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      18 months ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      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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    18 months 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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      18 months 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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        18 months 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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          18 months 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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18 months 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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18 months ago, # |
  Vote: I like it +4 Vote: I do not like it

Specialists Be Like

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18 months 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://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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18 months 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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    18 months ago, # ^ |
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    "to learning" should be "to learn".

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      18 months ago, # ^ |
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      yes, thank for your help

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

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          18 months 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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18 months 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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18 months ago, # |
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Finally, I can increase my rating :)

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18 months ago, # |
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OmG My FIrsT UNofficIAL ROunD!!!!!!

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

rp++！

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18 months ago, # |
  Vote: I like it 0 Vote: I do not like it

Good luck to you

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18 months ago, # |
  Vote: I like it +9 Vote: I do not like it

Hope it won't be queueforces.

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    18 months ago, # ^ |
      Vote: I like it +13 Vote: I do not like it

    Unfortunately, the queue is very long.

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      18 months ago, # ^ |
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      It may because there are too many testcases.

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    18 months ago, # ^ |
      Vote: I like it +13 Vote: I do not like it

    And that's how you jinxed it.

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18 months ago, # |
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Very long queue !!

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18 months ago, # |
  Vote: I like it +1 Vote: I do not like it

I keep seeing "Oops! Codeforces is broken"

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18 months ago, # |
  Vote: I like it -8 Vote: I do not like it

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18 months ago, # |
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Servers down O_o

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18 months ago, # |
  Vote: I like it 0 Vote: I do not like it

Div 4 became so popular...

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18 months ago, # |
  Vote: I like it +13 Vote: I do not like it

stresstestingforces XD

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18 months 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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18 months 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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18 months 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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18 months ago, # |
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Thank div4 for making me feel less useless, first time full AC without any WA :(

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18 months ago, # |
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I'm so tired for waiting.

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

Deleted.

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    18 months 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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    18 months 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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    18 months 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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18 months ago, # |
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Why was site down all the time! It was frustrating.

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18 months 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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    18 months ago, # ^ |
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    Dude, $$$E$$$ was easy, I actually solved it before $$$C$$$ and $$$D$$$ !

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      18 months ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      So I guess it has a simple solution that I didn't think of

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    18 months 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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18 months ago, # |
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how to solve G?

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    18 months ago, # ^ |
      Vote: I like it +1 Vote: I do not like it

    Video Solution for G.

    Hint:
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    18 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    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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    18 months 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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18 months 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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    18 months ago, # ^ |
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    And after getting WA2 wonder if this problem is even solvable.

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18 months 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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    18 months ago, # ^ |
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    they can be generated in 1000^2 operations by memoizing eucledian gcd

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18 months ago, # |
  Vote: I like it 0 Vote: I do not like it

Problem E Why my solution is giving TLE for test case 4 176003168 176020081 176037934

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18 months ago, # |
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approach for F?

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    18 months 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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    18 months 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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      18 months 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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18 months ago, # |
  Vote: I like it 0 Vote: I do not like it

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

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    18 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    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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18 months ago, # |
  Vote: I like it 0 Vote: I do not like it

Can anyone provide me a stress test like wa2 for my submission[submission:176043362]

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18 months 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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    18 months 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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18 months ago, # |
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How to solve D

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    18 months ago, # ^ |
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    Hint1
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18 months 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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    18 months ago, # ^ |
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    q*n=4*1e10,it need more than ten seconds.

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18 months 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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    18 months 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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18 months ago, # |
  Vote: I like it 0 Vote: I do not like it

Will 827div4 be unrated?

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18 months 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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    18 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    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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    18 months ago, # ^ |
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    Sorry for the inconvenience, it should be fixed now.

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18 months 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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    18 months 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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18 months ago, # |
  Vote: I like it 0 Vote: I do not like it

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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18 months ago, # |
  Vote: I like it 0 Vote: I do not like it

This is the first round that I AK :D

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18 months ago, # |
  Vote: I like it 0 Vote: I do not like it

Imho problem F was easier than E and D

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18 months ago, # |
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editorial?

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

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      18 months 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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18 months 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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18 months 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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    18 months ago, # ^ |
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    hacks in F because some people didn't read the first line in the statement

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

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18 months ago, # |
  Vote: I like it +1 Vote: I do not like it

GEO THERMAL WROTE IT a newbie sadly

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18 months 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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18 months 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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    18 months 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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      18 months ago, # ^ |
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      Oh sorry, I wasn't aware of this, thank youu.

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

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    18 months 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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      18 months ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      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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18 months 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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    18 months ago, # ^ |
    ← Rev. 2   Vote: I like it 0 Vote: I do not like it

    q*log(n) is right.q*n is not enough.you can see my code.It only needs 467ms.176015222

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      18 months 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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        18 months ago, # ^ |
          Vote: I like it 0 Vote: I do not like it

        of course we need binary search.q*n=4e10,it will need more than ten seconds

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18 months 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://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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    18 months ago, # ^ |
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    the same for me, also round somehow became rated for me

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

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    18 months 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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    18 months 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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18 months ago, # |
  Vote: I like it 0 Vote: I do not like it

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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18 months 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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    18 months 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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      18 months ago, # ^ |
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      Ohhh! I see. thank you so much!

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18 months ago, # |
  Vote: I like it +8 Vote: I do not like it

https://codeforces.com/contest/1742/submission/175924333 https://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://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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18 months 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://codeforces.com/contest/1742/submission/176077405

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

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

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18 months ago, # |
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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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    18 months 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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18 months ago, # |
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Why Problem D coprime in Codeforce round 827(Div 4) is retesting

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18 months 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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18 months ago, # |
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Help me for find my mistake. I don't understand. https://codeforces.com/contest/1742/submission/176450420

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