Misuki's blog

Codeforces Round 967 (Div. 2)
By Misuki, 4 months ago, In English

Hello Codeforces! (你好, Codeforces!)

Kaey and I are glad to invite you to Codeforces Round 967 (Div. 2) which will start on 20.08.2024 17:35 (Московское время).

The contest will last for 2 hours with 5 tasks for you to solve, and 1 task will have subtasks. The contest will only be rated for those with a rating not higher than 2099, but higher rated users are also more than welcome to participate out of competition. There is at least one interactive problem, so please read the guide for interactive problems if you are unfamiliar with them.

Holding the contest would have been impossible without the help from:

The score distribution is $$$500 - 1000 - 1500 - 2000 - (2000 - 2000)$$$.

Good luck and have fun!

UPD1: Editorial

UPD2:

Congratulations to the winners:

Div.1 + Div.2:

  1. maspy
  2. Mangooste
  3. kotatsugame
  4. Rubikun
  5. potato167

Div.2:

  1. kkkksc03
  2. ThMinh_
  3. yifeizhu
  4. activedeltorre
  5. suuuuuu
  • Vote: I like it
  • +420
  • Vote: I do not like it

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4 months ago, # |
  Vote: I like it +39 Vote: I do not like it

Excited to see the problems! Misuki Kaey orz!

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4 months ago, # |
  Vote: I like it +53 Vote: I do not like it

As a tester, I forgot the number of interactive problems there :)

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4 months ago, # |
  Vote: I like it +30 Vote: I do not like it

Misuki orz!!

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

Interactive problem ❤

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4 months ago, # |
  Vote: I like it +108 Vote: I do not like it

As a tester, I can confirm that the round is supercalifragilisticexpialidocious, too.

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    4 months ago, # ^ |
      Vote: I like it +28 Vote: I do not like it

    I believe that Misuki's dedication to the round is definitely worthy of honorificabilitudinitatibus!

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      4 months ago, # ^ |
        Vote: I like it -6 Vote: I do not like it

      Bro, I solved 2 questions in this round without any resubmissions or wrong answers and my rating decreased by -6, can u tell me why that is. I'm new btw

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

    Bro, I solved 2 questions in this round without any resubmissions or wrong answers and my rating decreased by -6, can u tell me why that is. I'm new btw

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

      You have submitted them too late. Submission time also matters.

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

        Really, I mean is it not like ur rating increases by lower points rather than it decreasing.

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

          Your rating only depends on your rank in the contest, irrespective of how many problems you have solved.

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

I came here from the author's X post

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4 months ago, # |
  Vote: I like it +103 Vote: I do not like it

As a tester, I tested the round.

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

As a tester, I ate 2 bags of chips while testing.

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    4 months ago, # ^ |
    ← Rev. 5   Vote: I like it +81 Vote: I do not like it

    As your sincere friend and a member of the_coding_pooh 's fan club, I wish you stop emoing and go find a better life. Chips only make you as fat as me!

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    4 months ago, # ^ |
      Vote: I like it +7 Vote: I do not like it

    Friendly advice: chip crumbs can greatly stain/dust your keyboard, especially so if that was a mech key. Stress snacking is okay but don't let them damage your own equipments.

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4 months ago, # |
  Vote: I like it +23 Vote: I do not like it

Misuki orz

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

interactive problems..cool

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

Hope to reach -110 after this.

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

Who told them to say 1 or more interactive instead of 0 or more :)

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

Let's do our best

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4 months ago, # |
  Vote: I like it +19 Vote: I do not like it

As a tester, I can confirm that the round is supercalifragilisticexpialidocious, too.

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4 months ago, # |
  Vote: I like it +37 Vote: I do not like it

Good luck to everyone, and orz to Misuki!

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

Hope to reach 1300+ rating and solve ABC!

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

    +1

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

      oop I ended up not taking the contest bc SAT prep

      I feel like I could have solved AB in <10 mins tho

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

        I participated in the contest and got a mountain-like graph. Hopefully will do better in upcoming contests

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

        same i got like a math test tbh and slept

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4 months ago, # |
  Vote: I like it +80 Vote: I do not like it

As a tester, I recommend solving AtCoder problems if you want to be as good as Misuki!

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

    Hmm, looks not good enough then. What should I solve to be as good as tourist?

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

What does the score distribution mean?

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    4 months ago, # ^ |
      Vote: I like it +19 Vote: I do not like it

    if you solve the 1st problem at exactly 0 minutes after the contest started, you will get 500 points, for the 2nd problem 1000 points, etc.. over time the points that you get per problem decreases, so these values are not constant.

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4 months ago, # |
  Vote: I like it +28 Vote: I do not like it

As a tester.. i hope everyone get positive delta..

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

hope for new color

and hope for Salah7_a

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4 months ago, # |
← Rev. 2   Vote: I like it -8 Vote: I do not like it

Hope back to blue man :)

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

After getting down by the allegations of CHEATING, still standing to become RED

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4 months ago, # |
  Vote: I like it +21 Vote: I do not like it

As a tester, i did nothing

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

I think this round will be great.

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

Hopefully, the color of my username will change

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4 months ago, # |
  Vote: I like it -27 Vote: I do not like it

ill solve ABCDEF , ggez

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

    The contest will last for 2 hours with 5 tasks for you to solve, and 1 task will have subtasks. The contest will only be rated for those with a rating not higher than 2099, but higher rated users are also more than welcome to participate out of competition. There is at least one interactive problem, so please read the guide for interactive problems if you are unfamiliar with them.

    please read it.

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

    I think you are referring to E2 as F. problem E got subtasks so it would be labeled as E1 and E2, rather than E and F.

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      4 months ago, # ^ |
        Vote: I like it -41 Vote: I do not like it

      What's the point of bringing this up? A tester is not supposed to give extra info

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

        are you serious?

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

        This is not extra info, sadly. They were just repeating the thing you should have known by reading the announcement solely.

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

    cheater should shut up

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

As a participator, Thankyou for Interactive problem ❤️

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

As a participant all the best to all my fellow coders

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4 months ago, # |
← Rev. 2   Vote: I like it -9 Vote: I do not like it

.

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

there is at least one interactive problem

it's so over

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

hope cadidate master today:3

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

Why did a game a long time ago change from scoring to not scoring, and the questions passed were also displayed as skipp

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

Why did a game a long time ago change from scoring to not scoring, and the questions passed were also displayed as skipp

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

Thanks to everyone who make this contest happen! Let's go!

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

ready for another opportunity to come back to cyan :)

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

Interactive Problems after a long time...

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

Hope to reach Specialist after this contest.

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

Misuki so strong!!!

Giving a contest after almost 2 months, hope I don't get destroyed..

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

Misuki

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

I really want to participate in it,but it's a pity that I don't have time...

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

I hope to solve A,B,C

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

so orzful for this contest <3

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

interactive problem will have two subtasks most likely

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

Taiwanese? I had to participate!

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

With a heavy heart and determined spirit, I'm back for higher +ve deltas

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

It can be a big challenge for my to solve interactive problems. But I have to try my best because I don't want to find my Rating going down again after the competition. It's really depressing!

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

Hope to slove A,B,C and D.

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

wish I could become 1200 rating after this contest!

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

Hope to accept A, B and C

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

As a tester, I did not test the problems.

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

Wish me to boost my rating in this round. And Good Luck Friends!

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

Going to participate after almost a year, let's see what happens.

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

Couldn't join the contest for 15 minutes. Am I the only one experiencing this?

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4 months ago, # |
  Vote: I like it +7 Vote: I do not like it

Why is C of trees I am gonna be Pupil again

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

Poorly written B.

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4 months ago, # |
  Vote: I like it -7 Vote: I do not like it

Absolutely LOVED C! Took me a lot of time, but that was SATISFYINGGGG

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

    i have absolutely no idea what the solution is, i think i will go back to pupil

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      4 months ago, # ^ |
        Vote: I like it -14 Vote: I do not like it

      once you will see it, you will be like "oh its so easyyy, i am so dumb" lol

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    4 months ago, # ^ |
      Vote: I like it -11 Vote: I do not like it

    Bro thank you so much, I raged because of B and you motivated me to look at C again and I solved it in literally 10 seconds.

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4 months ago, # |
  Vote: I like it -13 Vote: I do not like it

is it just me who thinks they should have mentioned the word cirular array in A to make things clear. I was breaking my head thinking how 8 8 7 6 3 8 7 6 3

can be convert to equal array

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

Could only solve one.

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

Got humbled by C so hard, speedran A+B in 10 mins feeling positive, just to get completely stuck on C with -7 at 15 mins left and no clue how to proceed

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

    B took me a while (I jumped to C before solving B) (I hated the way it's written), but for C, for me atleast I thought this way: let's root the tree at 1, this way for every other vertices, there is a segment between the 1 and that vertice right? okay now suppose you query the x between a and another vertice, for sure it's in the middle right? so for every line in the tree starting from 1 as a root and you query 'b' for example, now you can solve for the segment a,b recursively, solve(a,b)={ if ask(a,b)==a -> a line between a and b, otherwise ask(a,x),ask(x,b)} now suppose that there is a segment like this 1 — 2 — 4 — 6 — 8 — 9 and you queries solve(1,6) so you have all the segment 1,2,4,6 next time ofc you ll not query something that you constructed before so hold a visited state ! , say next time you ll query for solve(1,8), I claim that to solve(6,9) you ll get to this solve in log(the length of the segment) why? because you gonna ask(1,8) and you ll get 4 but you already constructed 4 right? so now don't ask solve(1,4),solve(4,8), just solve(4,8) => segments gets to half eatch time so it 's a log. and we have 15*n queries , max of our log is 10 as n is 1000

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

      This was the first thing I wrote, but I probably messed it up somehow, because I would get -1 on test 3. Spent around an hour trying to find where the program fails before trying to figure out how to approach the problem differently (with no results)

      bool dp[1001][1001];
vector<vector<int>> e;
 
void recurse(int n1, int n2) {
    if (n1 == n2) return;
    if (dp[n1][n2]) return;
    dp[n1][n2] = true;
    dp[n2][n1] = true;
    int a;
    cin>>a;
    if (a == n1 || a == n2) {
        e[n1].push_back(n2);
    } else {
        recurse(a, n1);
        recurse(a, n2);
    }
 
}
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        4 months ago, # ^ |
          Vote: I like it 0 Vote: I do not like it

        you missed the binary search aspect,

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

          Not gonna lie, it took me like 4 hours to finally understand how the solution works, even after looking at like 5 different explanations. It's so simple, idk why it was so hard to see

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4 months ago, # |
  Vote: I like it +1 Vote: I do not like it
A
B
C & D
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4 months ago, # |
  Vote: I like it +3 Vote: I do not like it

Stuck on WA 3 for D, can anyone share some small/tricky testcases?

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

    same

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    4 months ago, # ^ |
    ← Rev. 4   Vote: I like it +6 Vote: I do not like it

    Here's one that I failed on:

    1
7
4 1 2 4 5 2 1

    Answer should be:

    4
4 1 5 2

    An even simpler one I failed on:

    1
5
4 2 4 1 2

    Answer should be

    3
4 1 2
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      4 months ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      thanks your test case worked, did cf permanently remove the feature to look at test cases?

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

finally changed colors!

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4 months ago, # |
  Vote: I like it -26 Vote: I do not like it

Worst contest ever

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

problem C is cute problem I think, didn't realize binary_search can be used in such way until very last minute.

but I still cannot believe how 5000 people come up with solution in time. like how this problem have rating difficulty of 1500?

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

    I didn't use binary search at all.

    Simple recursion with DSU: https://mirror.codeforces.com/contest/2001/submission/277397822

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

      can u explain it sir

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

        In fact, it is very similar to editorial solutions.

        For each pair of nodes i and j that are not currently in the same component (I used DSU for this, but a simple visited array will also suffice), we do the following recursion (v1=i, v2=j on the beginning):

        • Find the closest node using the ? query.

        • If it returns one of our nodes, then we can union the two components together since they are directly related.

        • Otherwise, we know some node that is in the middle of our nodes, so we can call the recursion with (v1, mid) and (v2, mid). So at the end of such a recursion (v1, v2) will be in one component (v1 in the same component with mid; v2 in the same component with mid => v1 and v2 in the same component).

        I didn't think much about the count of used queries, but I assume it's O(n).

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

    I believe solution would have been leaked somewhere ;)

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

knew i was cooked as soon as i saw A

and i was, time to return to pupil ! yayy

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

need 2 more minutes for D :(

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

I am so dumb that 17k solved B and i didn't even understand the question until now :(
Someone please explain the problem statement.

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

    you have an array of numbers from 1 to n, lets call it arr, and an array of -1s, lets call it res. you can start from either side of res(the start or the end), and you can also return to the side from which you started. you can also move towards the side that you didnt start at. you can place the smallest number in arr at the position you are at, and you can do that until arr is empty, and res is full. construct the array res, so that from whichever end you start, if you do the operations in an optimal way, the amount of return operations to the starting side, is equal. i hope i clarified it a bit, and i can give solution too if you want.

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

      Could you please take n = 5(say) and explain the problem with it, like an example. The explanation given along with the problem was literally very bad!

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

        for n = 5 I tried it on copy and come up with 5 3 1 2 4 as a solution and saw the pattern and coded it

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

          Could you explain how you approached?

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

            I was trying hit and trial as I was not able to understand when I need to return carriage approach was like if I need equal permutation from both the typewriter with equal carriage being used then for first typewriter when I move left then I need carriage and for the second typewriter I move right I need carriage so for making it equal I need to do it from middle only which is possible for only odd n and not for even n

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

C was fun. First time trying out an interactive problem.

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4 months ago, # |
  Vote: I like it +17 Vote: I do not like it

It took me more time to understand problem B than it took to actually solve it

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4 months ago, # |
  Vote: I like it +26 Vote: I do not like it

Huge praise on C. I'm glad that I didn't have to do the binary search myself today (and pushed the heavy job to the interactor).

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

    lol

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

    could you explain C, i am clueless about trees, so i just skipped it and solved D instead

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

      Pivot a random node (in this case I'll use $$$1$$$) as your "root". Traverse each and every other node (denoting the traversing node $$$i$$$), and perform the following:

      • Set $$$l = 1$$$.
      • Ask for $$$(l, i)$$$ and retrieve $$$x$$$. If $$$x = l$$$, conclude that there exists an edge between $$$l$$$ and $$$i$$$, then go to the next node. If not, set $$$l := x$$$.

      The catch here is that we'll use the interactor to binary search for us to reach a stray node's "parent". We'll finish the tree structure when all nodes found their parents (except for $$$1$$$, as it's pivoted as root).

      Total query count should be approx. $$$10n$$$.

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4 months ago, # |
  Vote: I like it +20 Vote: I do not like it

Once again, D got leaked on a certain "YouTube channel" (as expected), with about 1k views in just one hour before the contest finished :)

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

    that's so fucked. I wonder if there's anything we can do about it

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    4 months ago, # ^ |
      Vote: I like it +12 Vote: I do not like it

    gotham needs batman

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

    Can we just mass report this one YT channel so that we don't have this many cheaters? (Honestly, I don't really want to mention it because there is a probability that someone will use it to cheat in future rounds)

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4 months ago, # |
← Rev. 2   Vote: I like it +10 Vote: I do not like it

The statement of B is very bad. Lots of cheating again (near 5k official solved C).

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

In Problem B, if there are 3 different operations, and you ask us to minimize just one of them, at least highlight it please?

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

My implementation skill is getting worse and worse.

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4 months ago, # |
  Vote: I like it +15 Vote: I do not like it

Hints for D. Longest Max Min Subsequence

Consider the first index where an element appears for the last time. Since this is your last chance to take this element, you have to construct the best sequence that you can using the elements to the left. Hence, greedily take big and small elements to the left (while ensuring that as soon as you take an element, you invalidate all its left neighbors). Then, if you have taken an element, you can say that this element will never contribute to the last occurrence anymore.

Hence, you greedily take elements at each last occurrence of alive elements.

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

    omg my solution was so close. I forgot to invalidate the left neigbours when doing the greedy construction :(

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

    Nice one. Just greedy. No fancy DS.

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

ayy just look at rank 1166 , tanish-jindal his D solution hahahah, just blatant cheating with stupid variable names

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4 months ago, # |
  Vote: I like it +25 Vote: I do not like it

Watch out for these leaked solutions

Problem C
Problem D
Problem E1
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4 months ago, # |
← Rev. 2   Vote: I like it 0 Vote: I do not like it

Deleted

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

B is the stupidest problem I have seen so far. Does providing enough test cases to understand the problem cost money? I hope you guys never set a question like this again. This kind of contest ruins my whole excitement toward contests.

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    4 months ago, # ^ |
      Vote: I like it +7 Vote: I do not like it

    They don't provide too many test cases because then you could deduce a pattern. In this case it would be easy to guess that it doesn't work for n even.

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    4 months ago, # ^ |
    ← Rev. 2   Vote: I like it +35 Vote: I do not like it

    I actually hated it when they included the 3rd sample into B just to soothe your nagging mouths. Do you guys ever test the inputs yourself? Doubly so when it's a constructive and the process of checking correct answer is easy?

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

      Sorry, my bad. I shouldn't have said it was easy, but the way it was written made it hard to understand. Maybe it's because I'm a beginner, but a lot of participants faced the same problem. You can ask them.

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

        I will simply tell you guys to stay away from keyboard, take a pen and a notebook, and do your thinking there. You all failed to sustain when problemsetters didn't overfeed like they usually do is not an indicator of a bad problem.

        I did casually curse the original sample "they really did it cheeky there huh?", but the course of actions of drafting my own work only took me about 2 minutes after, and got an AC at 7:xx.

        Did I stumble? Yes.

        Did I resolve myself? Yes.

        Did I underperform myself? Yes. (7:xx to AC problem B is slow)

        Should I blame the setters? No.

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

          I also did stumble at B but got it resolved myself with pen and paper and too hated when they added new test at B which made it super easy. It was super slow for me to solve B as I didn't understood the problem in starting which affected my rankings.

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

Edit: Nvm, just my stupidity.

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

Worst Problem E1 & E2 I have seen, can't believe there exists a problem worse than "Let Me F**k you a Lesson". Downvoted.

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

How do I approach C?

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

    Use the fact that the query tells you the middle node on the path between nodes $$$a$$$ and $$$b$$$, that should hint at a certain technique.

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

    hint 1- assume any node to be root of tree 2) — if u have any child parent pair in query than answer will either be child or parent just try to generalize this fact.

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

Can anyone post their solution to C. I speed ran A, B but got stuck for 1 hour and 45 minutes on C

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

    You can solve it using binary search. I did it by having i go from i to n — 1, and then querying ? i n. Since the x node is going to be in the middle of both nodes we can now keep querying ? i x until the the middle node isn't equal to i. When it is we know that he nodes we queried are connected by an edge since the middle node is one of the nodes we asked about. My submission that has some code that isn't really useful looking back: 277383795

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

i would recommend that before the operations description in problem B say that we will chooce one of them

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

I think div2 is getting harder and harder

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

D is on obvious side than its position, but C is very cool! Thanks for an interactive which feels brand-new!

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

    can you give hint on D

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

      Try to solve in $$$O(N^2)$$$. To optimize it, you need to learn about some data structures. (I used set and segment tree)

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

        i solved in O(n) without any DS, i dont think you need anything complex.

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

        I started to think about whether my solution was actually O(n), but i couldn't really come up with a test case, thank you for the hack

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

        You don't need any "advanced" data structures. I solved it with a map + multiset, though even a map + sorted vector would have worked.

        Key observations:

        1. If $$$x = mex(array)$$$, you can achieve $$$x = mex(array, mex(array))$$$ which is strictly greater than mex(array).

        2. The mex of an array is always achievable.

        3. If there two or more arrays which provide the same $$$mex$$$, $$$mex(array, mex(array))$$$ is always achievable for these arrays by achieving $$$x = mex(array)$$$ using one of them and then $$$mex(array, x)$$$ using the other.

        So we can just calculate this in descending order of $$$mex(array)$$$ as long as we're careful to handle all arrays with the same $$$mex(array)$$$ together (or tie-break them in descending order of $$$mex(mex(array))$$$).

        Code — 278093417

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

Interesting problems that require some tough thinking. Love this round!

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4 months ago, # |
  Vote: I like it +16 Vote: I do not like it

Let's fucking go guys, can't wait for the next contest for people to sell and leak even more solutions!

Images on codeforces are retarded today, can't embed, so here's an album

https://imgur.com/a/RNHSRp4

4eRRKghr p013lPh7

So there's like at least one or two IM-GM's selling solutions right now. Understandable, have a nice day.

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

why i get idleness limit exceeded 277414909 even if I fflush the output, can anyone help me.

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

    same bro

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

    When you're outputting the answer, you are missing a space after the exclamation mark ("!1 2 1 3 3 4" instead of "! 1 2 1 3 3 4"). This probably confused the system.

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      4 months ago, # ^ |
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      ohh my bad, i am sorry I will try again thank you for finding my mistake

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

        Looking in your code again, you're also missing a newline after outputting the answer, which is probably problematic as well. Be VERY careful with output formatting, especially in interactive problems.

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

    I was getting idleness exceeded too if I didn't do 'cout << endl'; Try printing a newline or using endl after you query. It should work

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      4 months ago, # ^ |
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      thank you for the reply I will surely try it once.

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

just a 2 min research

277417188 imagine working to get in IITB and then it still comes down to cheating? , 277406374, 277417238 bro really did the hardwork for converting to python lmao, 277411216 this dude interchanged for loops and while loops, 277408155

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

    bro idk how did u find and check these submissions

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

      i basically go into standings for the round. Then i simply look at newbies and pupils who have solved 4 that too in the last 30 minutes. Then codeforces allows you to open the submissions once the contest is over, so i check the submissions and match with the leaked ones.

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

Problem A felt like there was something to it, but the more I thought about it the realization came in: "Ahh.. it's just a trivial problem that was deliberately given a weird statement to confuse AI, isn't it?". It made me kinda dislike the problem, sorry.

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

Bad Sample Cases

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

I kept on getting Idleness Limit Exceeded on my Python solution to C, even though I flushed the buffers as required. Can anyone look through my solution and tell me what's wrong? SUBMISSION to C

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

I think that an implementation with segment tree should have worked for D. The time limit is too small. 277416972

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

    what is the time complexity of segment tree? i think intended solution was O(n) and O(nlogn) probably passes as well.

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

      Theoretically, log(n) for each update and query at each position, so n * log(n). Practically, I know it's a very large constant, but 2s seems a bit low for D.

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

        the solution in tutorial is O(nlogn), so it probably should've passed, weird.

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4 months ago, # |
  Vote: I like it +31 Vote: I do not like it

going through the scoreboard and found a bunch of similar solutions for E1. Here are just a few.

There are so many that I'm tired of looking at them all.
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4 months ago, # |
  Vote: I like it 0 Vote: I do not like it

I find some of the statistics about the round... interesting.

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

I hate interactive questions

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4 months ago, # |
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Do anyone know a software that have a faster execution for interactive problem?

I use jdoodles but it not stable

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4 months ago, # |
  Vote: I like it +19 Vote: I do not like it

Screencast with commentary

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4 months ago, # |
  Vote: I like it +23 Vote: I do not like it

Cheatforces...

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4 months ago, # |
  Vote: I like it +40 Vote: I do not like it

Hi guys, I am very sorry for the situation with B. It was not an easy task to explain and we got lots of clarification requests during the round. I know I should not have added an example case in the middle of the round, but I felt like it was better to have everybody understand the statement (with, some may say, an advantage for people who solved B later) rather than having an unclear problem. I take full responsibility for what happened and will make sure to not make it happen again.

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

    I think the PS for B just had the issue that I thought we had to do the operations in some order. Instead of us doing anything.

    Other than that I loved the problem idea.

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

really nice problem C

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

    could u explain more clearly

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

      if you have any more questions try executing my code step by step

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

Today Problem C was Solved by Chat-GPT, here is the link to the convo: You can Match this to my submission, Really Disappointed by this discovery I want to report contest violation on myself and want the Testers to do better , I did this as a joke today but some people regularly use ChatGpt to solve problems upto D, This is ridiculous please make sure the contest are fair please look into it. Misuki MikeMirzayanov Ahmad_OS BurnedChicken

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

    Really Disappointed ☹️

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

    I am just a tester and I can't do any thing :'(

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      4 months ago, # ^ |
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      as a tester you could test if chatgpt is able to solve the questions

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    4 months ago, # ^ |
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    That is the reason why my standing is lower than in the previous contests

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    4 months ago, # ^ |
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    The problem itself was not too difficult. Just some observation was needed.

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4 months ago, # |
  Vote: I like it +23 Vote: I do not like it

As usual, there are a lot of cheaters, but this time at least problems D and E require enough of implementation to detect cheaters.

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

Hi, i have a query regarding segment tree,i submitted two solutions for D-277405401 and 277409915,first one gives "Runtime error" and second one is accepted,the only difference between them is the memory i allocate to segment tree.

I read than size of segment tree is less than 4*(size of array) then why does my first submission fail.Any help is appreciated.

HELP!!!!

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

cool round, had a lot of fun participating after such a long break! cheers to the problemsetters and testers! <3

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

Can someone tell me why I get WA on test case 3 for my solution to C: Code

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

    Try adding cout.flush() after each cout?

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4 months ago, # |
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The problems were interesting! Thanks for the round!

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

Can anyone tell why my solution gave a runtime error in pretest 3?

277417034

I used the fact that if the answer to "? a b" is a then they have to be adjacent in the tree.Whenever the answer was different from a, then i would know that the path which connects them goes through another node x and therefore i would have to ask "? a x" and "? b x". I used dsu to avoid doing unnecessary questions. I suspect the dsu might be the reason for the RE but i cant spot the mistake

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

Can someone tell me why my submission: https://mirror.codeforces.com/contest/2001/submission/277402454 is giving a wrong output format Unexpected end of file - int32 expected (test case 1) error?

It passes on everything else...

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

    You call ios_base::sync_with_stdio(false), which means output buffers of stdout and cout aren't synchronized. You should just use cout.flush(), not fflush(stdout). Once you call ios_base::sync_with_stdio(false), only use cout and don't touch stdout.

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      4 months ago, # ^ |
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      I'm a bit confused -- where do I use stdout? I thought doing cout<<endl was good enough?

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

        stdout is for C I/O and cout is for C++ I/O. Basically C++ I/O is good enough and you don't need to use C I/O at all. And once you call ios_base::sync_with_stdio(false), you shouldn't mix C I/O and C++ I/O.

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          4 months ago, # ^ |
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          Gotcha. Thanks. But that wasn't the issue -- I think my code was just too slow, and the error it gives(wrong output format Unexpected end of file — int32 expected (test case 1)) is how they say "too slow" for interactive problems.

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            4 months ago, # ^ |
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            endl isn't slow, and it should be possible to solve the problem with it.

            The message basically means your program quit without printing a required integer. Maybe ans in your code can be empty and your code prints just ! without numbers?

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              4 months ago, # ^ |
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              I'm not saying endl was slow -- I was checking a bunch of things. Once I deleted that code, it passes.

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4 months ago, # |
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Thanks Misuki ,Really enjoyed solving c problem. but i am suprised so many people solved it. Too much cheating

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4 months ago, # |
← Rev. 3   Vote: I like it +1 Vote: I do not like it

For someone looking for an elegant implementation of problem C,

My Code

The idea is to fix 1 as the root. Then we need to find the parent of every i(2,N).

The observation is that every time we query(a,b), x points to the midpoint of (a,b). This gives us the intuition of binary search considering the number of allowed queries. An edge exists from (a,b) when query(a,b)=a.

The above observations are sufficient to solve the problem.

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

Can someone please explain why is it failing on test case 3 (wrong output format)? I basically check for valid pairs of nodes, and return when a from query equals the nearest node.[submission:277424402]

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

The data of D is too weak. Some people passed it by going through every value each time, which can have a complexity of $$$O(n ^ 2)$$$

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    4 months ago, # ^ |
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    can you send any link please ?

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      4 months ago, # ^ |
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      my solution runs in O(n^2) with good enough hack

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        4 months ago, # ^ |
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        it's because of the l=tren+1 line I think you while loop isn't really running for n only it's running for more

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

In problem C when I use fflush(stdout); I got "Idleness limit exceeded" but when I use cout.flush(); it gives correct answer. Why? I used language "C++20 (GCC 13-64)". 277431865 277431666

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    4 months ago, # ^ |
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    I'm not sure, but I reckon it's because you used cout for output.

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    4 months ago, # ^ |
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    i think fflush(stdout) is for printf, im not sure though

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

Only at upsolving I realized I was so close to AC E1. I somehow tried to calculate the losing side by pure power instead of stars-and-bars, and scrambled my hair at why test 2+3 was WA. What an idiot I was.

AC upsolve: 277433539 ( UPD: changing to a cleaner/no-debug code)

Kudos for the round, it was nice. I wish I could see more from you soon.

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

    I'm doing the same thing, but I can't understand why the losing side needs stars and bars, can't I pick any node from that subtree for each operation?

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      4 months ago, # ^ |
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      You got it right, but the stars-and-bars are to correctly count them.

      What you should count is basically "for a complete binary tree of $$$2^i - 1$$$ nodes, how many distinct heaps are there after adding $$$k_0$$$ operations into it". Since we discern by heap values, $$$\left( 2^i - 1 \right) ^ {k_0}$$$ is obviously wrong.

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

      because each operation is indistinguishable, so you should use star and bar to calculate it (i.e. the number of ways to put $$$a$$$ indistinguishable balls into $$$b$$$ distinguishable slots). $$$b^a$$$ is used to calculate the number of ways to put $$$a$$$ distinguishable balls into $$$b$$$ distinguishable slots. And it's important to think the objects are distinguishable or indistinguishable in counting problems.

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

The data of problem D is so weak that someone can solve it by solution of O(n^2);

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

As the guy that got 4th I just now realised my solution to D runs in O(n^2) :skull:

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

277408206

This submission by user rajneesh_neo clearly seems to be faulty ..

it clearly has been copied from 277400319 by user keyur14113

using some AI tool , just the variable names have been changed.. rest of the structure remains exactly same..

Kindly look into this MikeMirzayanov

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

Please can someone help me with why I'm getting Idleness limit exceeded for this code?

from sys import stdin, stdout
input = lambda: stdin.buffer.readline().decode().strip()
flush = stdout.flush
vis = set()
def explore(x, y):
    if f'{min(x, y)} {max(x, y)}' in vis:
        return ''
    vis.add(f'{min(x, y)} {max(x, y)}')
    print(f'? {x} {y}'); flush()
    new = int(input())
    if new == x:
        return f' {x} {y}'
    left = explore(x, new)
    right = explore(new, y)
    for a in left.split():
        for b in right.split():
            vis.add(f'{min(a, b)} {max(a, b)}')
    return left + right

for _ in range(int(input())):
    n = int(input())
    ans = ''
    for i in range(1, n+1):
        for j in range(1, n+1):
            if f'{min(i, j)} {max(i, j)}' in vis or i == j: continue
            ans += explore(i, j)
    print('!' + ans); flush()
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4 months ago, # |
  Vote: I like it +6 Vote: I do not like it

Yay !! Thanks for the interactive problem authors..Hoping to reach cyan in the next contest

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

I am a newbie .. i did only one question in this contest and that too correct but i lost -31 .. can anyone tell how ?

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    4 months ago, # ^ |
      Vote: I like it +6 Vote: I do not like it

    Your Rating depends on your performance on that given contest like if you performed like a 800 you would certainly get a positive rating but if your performance is less than your current rating then your will lose some rating .You can Calculate your performance by seeing the number of submissions on a given problem like for today's contest Many people solved the A problem so to get a positive rating you must solve problem A fast or solve at least 2 aside from that there is an extension called carrot try using that it does a great job predicting the rating changes..

    Happy Coding !!

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

      Thanks got it got it .. thanks ;

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        4 months ago, # ^ |
          Vote: I like it +6 Vote: I do not like it

        One more Thing you seem to only solve problems of 800 rating but I would suggest leveling up a bit because that will not be enough to solve the B of div 2 which is bad in a long run so do try out problems of rating 900 or 1000 more...

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

Can anyone tell me why my code for D got accepted with time complexity of O(n²)?

Is there any data could hack my solution: 277446088?

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

    Thanks, I see it. It seems like that it is easy to find such data.

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

Can someone tell me,Why i got 40+ in last contest even though i didn't attend able to do 1st first in time? AND why i got only 15+ from this contest by only solving 1st question??

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

    As a newcomer, you have 1200 hidden initial ratings, divided into the first five contests sent to you. If you don't perform well in the first 5 contests, the rating sent to you will be reduced, but you will not be given negative ratings. The following contests after first 5 contrsts will directly settle your total rating. So all in all, instead of going up to 783, you're going down from 1200 to 783.

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4 months ago, # |
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Previous div 2 round : solved 2 problems with no penalty -> rank 5157 This div 2 : similarly solved 2 problems in similar time with no penalty -> rank 11650 What happened here lol

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

    in my case: Past div2: rank 11000 with solving 2 problems This div2: rank 6000 with solving 2 problems in similar time lol

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

    same

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      4 months ago, # ^ |
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      it seems that div p1 + p2 only solvers are quite alot so a small margin of times makes a big change in ranking.

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

in problem c why it was not mention tree has n--1 edges exactly i struggled a lot like 30 min

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

the worst problems set that i've ever seen

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4 months ago, # |
← Rev. 2   Vote: I like it +12 Vote: I do not like it

Really nice interactive problem,which made me -152 :(

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

Can someone tell me where am I failing,my max no of queries are less than 15n but I am failing. My logic is somewhat similar to editorial Here is my solution 277462404

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

I submit twice on problem C.the first one at 00:38:35 and the second one at 01:51:32.The first one get skipped and my ac time become the second one?? I dare not to submit twice again...

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    4 months ago, # ^ |
      Vote: I like it +6 Vote: I do not like it

    Do not resubmit in normal rounds unless you have high doubt in your previous solution. But you can resubmit as much as you want in Educational, or Div. 3/4 rounds.

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      4 months ago, # ^ |
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      I didn't use fast IO in the first solutioin so I want to see how fast it will be if I use fast IO... I will not resubmit anymore. Thanks for the explaination.

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4 months ago, # |
  Vote: I like it +17 Vote: I do not like it

C was insanely good. Kudos to the authors!

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

ORZ

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4 months ago, # |
← Rev. 32   Vote: I like it +2 Vote: I do not like it

Misuki orz

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

My solution 277403863 for the problem 2001C(aritg). I had done the question earlier than the second person whom I don't know. So, why did I get that it matched with someone.

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

I have been informed that my solution for problem D matches with many others . I don't know how it happened but I want to clarify that my solution is completely my work . So, I request you to look over my situation in this . Also, on codeforces it is happening to me for the first time.

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

I have been wrongly accused of cheating even though my submissions our genuine. I request to look into it and make way for my truthful submissions. This is the first time it’s happening to me and also I have submitted the solutions earlier than the other persons.

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

Me: lets try to become specialist

CF: final rating= 1399

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