Endagorion's blog

Codeforces Round #283
By Endagorion, 10 years ago, translation, In English

Hi, Codeforces.

Today, on December 17 at 19:30 MSK regular, 283-rd Codeforces round will take place. Problems are authored by me, Mikhail Tikhomirov. Maxim Akhmedov (Zlobober) helped me with discussing and preparing the problemset, Maria Belova (Delinur) translated problem statements in English, and Georgy Chebanov (gchebanov), Alexander Mashrabov (map) and Niyaz Nigmatullin (niyaznigmatul) tested the round and helped us with finding bugs and mistakes; let's give them a round of applause!

The round will be for both divisions. The scoring is standard (not dynamic); score distribution is as follows:

Div. 1: 750-1250-1250-2000-2500

Div. 2: 500-1000-1750-2250-2250

This is going to be my fourth round at Codeforces. I really hope it will go as good as three before it. =) Wish you all the best of luck!

UPD: the round is over, thanks for participating!

Grats to the winners:

Div. 1:

  1. SirShokoladina

  2. Petr

  3. rowdark

  4. anta

  5. Marcin_smu

  6. Merkurev

  7. qwer1561

  8. Ra16bit

  9. kuviman

  10. Um_nik

Div. 2:

  1. SergeyMelnikov

  2. sepehr103

  3. StarCuriosity

  4. dotato

  5. husheyn

Editorial is here ( UPD: now in English!).

  • Vote: I like it
  • +595
  • Vote: I do not like it

Comments Comments (147)
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10 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Is this the record for shortest round announcement? Very concise and direct.

Lets hope problem statements are same way

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

The first three characters of your name are "End", I hope this contest won't put the end of my green color.

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    10 years ago, # ^ |
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    Hopefully this will put the end of my blue colour; I hope to be purple again :)

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

Why is this blog annoucements, statements, tutorials of a lot other contests?

Image and video hosting by TinyPic

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    10 years ago, # ^ |
      Vote: I like it +61 Vote: I do not like it

    Perhaps he was thinking, "Hmm, the blog announcement is so small, maybe I should add lots of attachments with it to make it bigger" :p

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

      I don't think so.

      Endagorion can write a lot's of things to make blog longer.

      I think somebody attached to this blog these contests. But why??? That's interesting.

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        10 years ago, # ^ |
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        What you are saying kind of makes sense. Why would this blog post be "Tutorial of 2010-2011 VII International Zhautykov Olympiad"? I guess somebody is just messing around :p

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

          The "someone messing around" theory makes the most sense IMO. I think anyone can create attachments like this (at least I can -- or maybe it's just people with coach mode enabled?)

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

        Or it's a bug, probably?

        There's a blog post that has a similar issue (this post).

        The attachments to that post aren't related to the post itself, just like this blog post.

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

    I have no idea what was the deal with all these. Deleted them anyway.

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10 years ago, # |
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I hope that it will be interesting.

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

Topcoder Rating are increse. Codefores Rating are waiting......increse/decrese.

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

You forgot to thank MikeMirzayanov for creating Codeforces and Polygon systems :D Fingers crossed. Lets hope that there are no server errors :P Just kidding, thank you for writing this round, your previous contests were also good.

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

It seems the Codeforces Google Calendar is out of sync or the entry for this contest has not been added to it. It is kind of sad as I will not be able to participate because of that.

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

Time to dreamoon_love_AA.

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

The last Endagorion contest was a failure for me! I hope it will be different today! :D

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

Hi!
Endagorion thanks! your last contest had nice problem :))

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10 years ago, # |
← Rev. 2   Vote: I like it +47 Vote: I do not like it

All of your contests were Div1 + Div2. And you prepared all the problems alone.
Today, There are a lot of contests that there is a team with 7 members for preparing problems but the contest is just Div2.
Thank you so much.

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

I hope all Specialist be Expert and all Expert be master !

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

hello every body

dis like me plz

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    10 years ago, # ^ |
    ← Rev. 2   Vote: I like it +11 Vote: I do not like it

    are you sure? Link :p

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

      He needs to be more original if he wants to break that record!

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    10 years ago, # ^ |
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    lol, it has been a while since "Dislike Me" posts have been made in a round. Not that the absence made it any better...

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10 years ago, # |
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The contest will begin at 0:30 am in China, luckily, i have no courses tomorrow :)

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10 years ago, # |
← Rev. 2   Vote: I like it +5 Vote: I do not like it

Hope for high rating.

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

Speed of internet is so slow when there is a ten minutes to the contest

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

The age of unusual score distribution.

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

All of us second division participants, this is our last chance to win (as dreamoon_love_AA has almost arrived !)

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

Note to self: avoid geometry =(

It comes down to segment vs ellipse (possibly degenerate) intersections, right?

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

    Yes, in fact it's segment vs. circle intersections.

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

      It would be a circle if they rotated in different directions. Unless you mean you get a circle after squeezing the ellipse along one of the coordinates.

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        10 years ago, # ^ |
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        Well, I believe that it's circle when they rotate in the same direction. And with this approach I passed pretest, so it might be correct.

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          10 years ago, # ^ |
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          Uh if you consider the movement a point X on the second polygon relative to P, you will notice that Q rotates around P counter clockwise, and X around Q clockwise. If you add these two movements, you will get an ellipse (assuming |XQ| != |PQ|, otherwise it will be a segment).

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            10 years ago, # ^ |
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            Well, in frame of reference where A isn't moving point Q rotates around P CCW, but as the frame of reference is rotating CW itself, X will not rotate around Q at all.

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

was div1 C bipartite matching? or could something like sorting and 2 pointers work? :\

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

    Sorting and two pointers. Assign the scenes greedily (take the ones with lowest A that satisfy the conditions). Good luck with bipartite matching when you have 10^5 nodes and 10^10 edges ;p

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      10 years ago, # ^ |
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      Did something similar.. pretest 5 :\

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        10 years ago, # ^ |
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        I had an issue with it too, because I was sorting by the wrong coordinate (forgot to pass my comparison function).

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          10 years ago, # ^ |
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          Would you mind telling the order of each parameter you used for sorting?

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    10 years ago, # ^ |
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    greedy is enough

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10 years ago, # |
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When will system testing begin?

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10 years ago, # |
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Your contest have nice problem :)
thanks Endagorion

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

    I'm surprised that div.2 problem is hard for everyone, this is the first time I made to the top 3 in my contest room eventhough I only solve one problem lol. I can't believe it >.<.

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      10 years ago, # ^ |
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      nice problem != hard problem !

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10 years ago, # |
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I think it is time for me to say "Goodbye Expert".

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

What is the solution of B problem? It was harder for me than C :D

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

    Simple bruteforce

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    10 years ago, # ^ |
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    Bruteforce all possible shifts and increasing by 1.

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

How to solve Div1B/Div2D

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

    Go through the list from left to right to get all possible values of T. For every value, you can 'simulate' the match by using binary search to find where the sets end (calculate partial sums of the number of wins of each player), and check whether you get a valid match (and obtain S in the process). It will work quickly enough because you only try each T once, and the complexity of each simulation is (n/T)*log(n). If you add them together, you will get (n+n/2+n/3+...)*log(n) = n*log(n)*log(n).

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

    I stored the first position of a particular number of games won and simply simulated for all values of t from 1. This would be O(nlogn)

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    10 years ago, # ^ |
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    You can fix the T in the interval 1, n. Now, you are going to see for each set where does it and and where does it begin.Also, you have maximum N / T sets. You can determin this intervals using binary search. So you have Nlog^2N.

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10 years ago, # |
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The fact that in Div2 there are less than 10 successfull hacks from current top500 is not good

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10 years ago, # |
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Nice problems thanks a lot authors

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10 years ago, # |
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if cnt[1] == cnt[2] cout << 0; <- i wrote it in the beginning

ok kill me pls :(

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

    Welcome to the club. 9178079 9176101

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

    Any test case for contradiction of ( if cnt[1] == cnt[2] cout << 0; ) ??

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

      Sorry, wrong probem O:-)

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

      The number of total points doesn't mean much. For example, the person that won less points may be the final winner.

      Example: s = 4, t = 2

      18
2 2   2 2   2 2   1 2 1   1 2 1   1 2 1   1 2 1

      2 wins 10 serves, 1 wins 8 serves and 1 wins the game. Kind of unfair, isn't it?

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

        Even simpler:

        8 2 2 2 1 1 2 1 1

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

What was the tricky #8 case in D?

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    10 years ago, # ^ |
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    I have no idea, but did you consider the case, when a segment crosses a circle, while both its ends are outside of it?

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      10 years ago, # ^ |
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      As far as I remember, not considering this gives WA6.

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      10 years ago, # ^ |
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      Yes, I did. But considering it was pretty long "if", I hope it was correct, I've read it many times xd.

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      10 years ago, # ^ |
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      OK, my solution just contained many bugs (what is funnier, not in formulas) and this test was one of the first test checking anything :P.

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

Hey @admin : I submitted the code when there were 7 seconds left. It didn't accept my code.Please evaluate it :( Problem D

include

include

include

using namespace std; int n,a[100000],f[100000],x=0,y=0,ans[10000000],ac=0; int fact(int o) { for (int i=1;i<=f[n-1];i++) { if (o%i==0) {

       ans[ac]=i;
       ac++;
    }
}
return ac;

} int main() {

cin>>n;
for (int i=0;i<n;i++) cin>>a[i];
for (int i=0;i<n;i++) {
    if (a[i]==1) x++; else y++;
    f[i]=max(x,y);
}
int z=fact(f[n-1]);
if (x==y) {
    cout<<"0";
} else {
    cout<<ac<<endl;
    for (int i=0;i<ac;i++) {
       cout<<ans[i]<<" "<<f[n-1]/ans[i]<<endl;
    }
}

}

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    10 years ago, # ^ |
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    You will get WA for this: a player can get some points in sets he didn't win, so his total wins don't have to divide equally by T.

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

      Yeah , your correct. That makes me feel better :P

      Thanks :)

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    10 years ago, # ^ |
    ← Rev. 2   Vote: I like it +5 Vote: I do not like it

    7 seconds might be wrong time synchronization in browser...

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10 years ago, # |
  Vote: I like it +61 Vote: I do not like it

http://codeforces.me/contest/497/submission/9163878 dreamoon, you are so cute...

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10 years ago, # |
← Rev. 2   Vote: I like it +24 Vote: I do not like it

As always I finish debugging my E code on examples 2 minutes after round end...

I'm actually hoping it's wrong now, would make me less angry :K

Edit: Yay, TLE! :)

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10 years ago, # |
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System test started early, but, it is slow. There is always a catch huh...

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10 years ago, # |
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How to solve Div2B?

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

bitagi submitted all 3 problems (A, B, C) in time 00:41 interesting...

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10 years ago, # |
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Why is the system testing so slow??

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10 years ago, # |
  Vote: I like it +16 Vote: I do not like it

Dear CodeforcesPolice , Look what SaDDaS did during contest time !

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

The rating update is so fast. Maybe the fastest I've ever experienced.

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    10 years ago, # ^ |
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    Still waiting for Div 2 update.

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

      Updated on the moment you write this comment :)

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        10 years ago, # ^ |
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        Hey Bro, I can't change the downvote to the upvote. Sorry for that.

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

accidentally fell asleep during the contest for half an hour :D , and solved problem C after waking up

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    10 years ago, # ^ |
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    Still better, than sleeping 10 minutes and then submitting A :-D Especially when no reason for that...

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    10 years ago, # ^ |
      Vote: I like it +33 Vote: I do not like it

    Being relaxed during a contest — i guess you are doing it wrong)

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

Yeah, purple again C:

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10 years ago, # |
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Solved B and C, and couldn't get A to pass the pretests (was giving wrong answer on Pretest-7)! Did anyone face the same issue?

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

Finally, I became blue. Next stop: purple :)

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    10 years ago, # ^ |
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    wow your line is motivated.welcome to bule!

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10 years ago, # |
← Rev. 6   Vote: I like it +40 Vote: I do not like it

I'd like to talk about time limit in problem D (div 1). I'm really sad, because I spent about 50 minutes on solving this problem and submitted it during last minute of contest just to get TLE on test 30. And, what is even worse, I changed my code just a bit after the contest and got AC.

Here is TLE: http://codeforces.me/contest/497/submission/9177065

Here is AC: http://codeforces.me/contest/497/submission/9177860

As you can see, the main difference is that the new code is less legible. Complexity is the same as in codes of others — O(nm), but the constant is just a bit worse. I don't think that making such time limits in geometrical problems is a good idea.

I'm not the only person with this problem — AstroConjecture also has tle on 30.

So I'd like to write my reflections here:

1) Time limit could be less strict when there is no solution with worse complexity that could work with given constraints.

2) There should be one maxtest in pretests. In such problems people often try to write legible codes (it helps with debugging) and they focus less on constants (of course they still focus on complexity). So I think that here lack of maxtests in pretests is a huge mistake.

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

    Fully agree. I always emphasize that constraints should be as small as they can be, so no solution with worse complexity can pass. Here there were simply no other solutions, but on the other hand, 1000 is in fact already very small limit...

    But 2) still stands, receiving a random TLE on systests, because our code runs for sth like 1.5xTL is really a terrible feeling, I experienced that few times here and it always costed me sth like 150-200 rating >_<.

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

welcome dreamoon to div 2

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

Hello purple! Thank you for a great contest :D

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

Div2 C, what is it that I'm doing wrong in this code

http://codeforces.me/contest/496/submission/9178659

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    10 years ago, # ^ |
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    I have the same problem, test 15 didn't go through. To me, the test case seems to have clearly more than 4 columns to be removed in order for the rows to be ordered lexicographically. Or did I misunderstood the problem somehow???

    http://codeforces.me/contest/496/submission/9174356

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    10 years ago, # ^ |
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    as rajateuler said, one has to go from row to row, not column to column. else it's getting to complicated to decide either the column has to be removed (here is the error).

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

EDIT: Ignore this post.

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

I really wanted my name written here but sadly i got the sixth place in the last 3 minutes...

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10 years ago, # |
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Hi. Can anyone please help me understand why test 2's answer is 2 instead of 3?

case care test code

Shouldn't all columns except the second one be deleted? Because the 1st, 3rd and 4th columns all have irregularities in terms of lexicography. Or did I misunderstand the question completely? :|

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    10 years ago, # ^ |
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    Read the definition of lexicographic order carefully. Even though the strings in column 4

    e e t e

    aren't in order, the strings

    ae ae et oe

    are, so it's a valid solution.

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      10 years ago, # ^ |
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      Hey Kynit, i don't understand the definition of lexicographic order.Even i don't understand the second test case how output 2 comes instead of 3. Would you please explain with another example for understanding the definition of lexicographic order ? Thanks in advance.

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        10 years ago, # ^ |
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        Let's say that you have string a and b. And for example a=abcd, b=acde.

        In string b char on position 2, char 'c', is greater than char on position 2 in string a, char 'b' (c come after b in english alphabet), which means that b is lexicographically larger than a. They don't need to have same length, only one char is enough to one string be lexicographically greater than other.

        For example string b=s. Char 's' on pos 1 is greater than char on pos 1 in string a, char 'a', so string b is lexicographically greater than string a.

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        10 years ago, # ^ |
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        Superleggera got it right — it's basically dictionary order.

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

My code for div1B failed on test45 during system testing, and that test case works perfectly on my machine. Can anyone help, what might be the issue?

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

Dear CodeforcesPolice I got this message during the contest. I've noticed that after the end of contest. Of course, it is not allowed to ask someone the solution or something like that. Is there anyone who get this type of message from ITDOI during contest?

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

Is the editorial in English ready yet? :) Thanks for posting.

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10 years ago, # |
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Waiting for English verison of Editorial.:D

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10 years ago, # |
← Rev. 2   Vote: I like it -29 Vote: I do not like it

.

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

    Notice the "#" sign beside his name. That means this rank is unofficial since he took a "Virtual Contest". Just remove the tick from "show unofficial" above the standings page, and he will disappear.

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

      Probably someone who cannot read well:

      If you've seen these problems, a virtual contest is not for you — solve these problems in the archive.

      If you just want to solve some problem from a contest, a virtual contest is not for you — solve this problem in the archive.

      Never use someone else's code, read the tutorials or communicate this other person during a virtual contest.

      Above are "Terms of agreement" when one want to start virtual contest...

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

Are Petya and Gena from 497B - Игра в теннис Petr and tourist? XD

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

    Yep; actually, both of them are quite adept at table tennis which I had a chance to witness at TCO14 finals. =)

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

Can anyone tell me why my submission 9186328 for problem C div 2 is failing the system test. Here is my algorithm

Starting from the first column, I check if it's sorted, if it's not sorted I mark it for removal and move on to the next column. If it is sorted, I group the rows with the same value at that column and I move on to the next column within each group.

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

Can anyone please tell me why

1
15 1

is not an answer for following test case in problem 496D - Игра в теннис

20
1 1 2 2 2 2 2 2 2 2 2 2 1 2 2 1 2 2 2 1
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    10 years ago, # ^ |
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    Ignore this answer.

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      10 years ago, # ^ |
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      Why? in testcase 7 judge's output is 0. I can not understand why 

      1
15 1

      is not an answer.

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        10 years ago, # ^ |
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        If it would be answer, second player would win earlier than game has ended (testcase is entire game, from start to someone's win).

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          10 years ago, # ^ |
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          How would second player win earlier? When the game ends first player wins 5 sets while 2nd player wins 15 sets. As score is 1, they are completing 20 sets together, and 2nd player wins his last set at the last serve of the game, isn't it?

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

          I got it. Thanks for explaining. I missed last "1"

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

a very bad and unpleasant round

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