dalex's blog

By dalex, 8 years ago, In English,

Hi all!

My name is Alexey Dergunov, and I'm glad to present you the first "violet" Codeforces round. I hope the problems will not seem so "violet" to you :)

Many thanks to following people who helped me a lot with contest preparation:

Good luck!

UPD. Contest is over, and we know the winners!

Division 1:
Division 2:
Congratulations!

UPD 2. Analysis: http://codeforces.ru/blog/entry/2208 (English version is not full yet...)
 
 
 
 
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8 years ago, # |
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gl & hf!
8 years ago, # |
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I hope that the system tests and rating process is faster than the previous one :)  Good Luck :)
8 years ago, # |
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Yeah, me too. Violet would be very hard to read...
8 years ago, # |
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Lets hope you impress us all with an impressive problem set =)
8 years ago, # |
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I cannot submit in Div 2 - it doesn't show any language available.
8 years ago, # |
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I can't select language. Why?
8 years ago, # |
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I also can't submit...
8 years ago, # |
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In div-2 problem B, what's the meaning of "there are no either three pairwise acquainted or three pairwise unacquainted people" ? would you please explain it more clearerly ?
8 years ago, # |
Rev. 2   Vote: I like it -16 Vote: I do not like it
I managed to submit now, but it's like 30 minutes later...
  • 8 years ago, # ^ |
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    Admins please help, I cannot submit. When I click submit page keeps loading and nothing happens...
  • 8 years ago, # ^ |
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    This isn't fair :(
8 years ago, # |
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Hi, thanks for the round! I also didn't manage to submit with google chrome so I switched to firefox.

Regarding the problems, on div 2 problem C, I think the pretests were "too hard". Since codeforces allows for hacking of fellow competitors' solutions, you could have left some of those out of the pretests. I believe it was almost impossible to hack a solution that passed them.
8 years ago, # |
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When I see the problems as ugly as today's D, I want to quit the contest and do something useful.

I don't understand what are such problems doing in programming contests.
  • 8 years ago, # ^ |
      Vote: I like it +13 Vote: I do not like it
    What's the problem with D? It has two different solutions, one mathematical and one "straight-forward" without any math. So everyone should be happy :)
    • 8 years ago, # ^ |
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      The problem is that the author tried to compensate the straightforwardness of the problem with some ugly conditions to make the problem more time consuming.

      It happens when the author is afraid that the problems may be too easy for someone and the only goal is to make contestants spend more time. This is not what programming contest problems should be.

      I haven't read B and C because they looked ugly without reading them. Maybe I missed some great problems but what I've read and solved isn't what I enjoy in contests.

      • 8 years ago, # ^ |
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        Yea, you missed good problem "C" =) I VERY like well-masked backtracking =)

        But concerning the rest of the contest - I totally agree with you...
8 years ago, # |
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So upset!!My english is very poor.I can't understand the meaning of "there are no either three pairwise acquainted or three pairwise unacquainted people" !

  • 8 years ago, # ^ |
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    Let 1,2,3 are three friend.
    Either all pairs (1,2), (2,3), (1,3) are acquainted each other or all pairs are unacquainted each other.


    • 8 years ago, # ^ |
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      But if there are four or five pairwise acquainted people ,what shall we do ?
  • 8 years ago, # ^ |
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    At first I also could not understand it's meaning.

    If we create graph g where node is people and they are connected if those pair is acquainted, then find 3 people i,j,k such that g[i][j] ==  g[j][k] == g[k][i] == 1 or g[i][j] ==  g[j][k] == g[k][i] == 0. If those i, j, k are found, then WIN otherwise FAIL.

    I think the statement of div2 problem A and B is sometimes very hard to understand, but turns out very easy when understood. It just consumes time.
    • 8 years ago, # ^ |
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      Actually, at first I can guess the meaning and my program is correct,but I always get wrong answer on pretext 2.So I focus on what is the meaning of this sentence.After the contest , I find out that I'm so careless,I print "FALL" instead of "FAIL".So careless I am!!!!

  • 8 years ago, # ^ |
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    Poor description. It have cost 15minutes
8 years ago, # |
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Any quick hint for E?

Seems like I've seen a couple of similar problems before - with different constraints.
  • 8 years ago, # ^ |
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    Nothing special. You should play with constraints a little bit.

    Use the recursive inclusion/exclusion as in similar problems and use memoization for small values.

    This problem was too boring for Div1-E as well.
    • 8 years ago, # ^ |
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      I even couldn't solve D in previous contest , But This is the First E/Div 1 I think i can AC :( , But I ran out of time 
8 years ago, # |
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I liked the contest.(Don't know about div 1)
If I could solve C , it would have been more interesting.(Seemed so easy )
:)
8 years ago, # |
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I personally liked the contest. Good job, dalex. Thank you :)
8 years ago, # |
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My B failed on test 12:
50 1000 49

My code got answer "YES" on the server.
But it is weird the same code got "NO" on my machine.
  • 8 years ago, # ^ |
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    My AC solution also answers "YES". Maybe you are looking at wrong string in the report.
8 years ago, # |
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Although there is almost nothing new in algorithm, it's a good contest to test if you are carefully enough.

I think I should practice in some contests like this because I always make mistakes, such as today's Problem C, I make a really stupid mistake.

So, the contest is a little unusual, but also valuable.


[Edit]
By the way, my rating has no change after this contest: 2203 -> 2203.
It's a coincidence that my Rating of TopCoder for latest 3 contest is 2566 -> 2568 -> 2567, almost no change.

8 years ago, # |
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For DIV2 C/DIV1 A, what is the expected result for input "21 3 6 11"?
  • 8 years ago, # ^ |
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    2
    • 8 years ago, # ^ |
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      The layout is like this:
      1 2 3 
      4 5 6
      7 8 9
      10 11 12
      ...

      So I think 3 times is needed: first, choose 6; second, choose 7,8,9; third, 10, 11.
      Where is the error?

      • 8 years ago, # ^ |
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        You can choose {6,9} and {7,8,10,11} after that.
        • 8 years ago, # ^ |
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          Thanks. In the contest, I only consider choosing by an ENTIRE row which turn out to be wrong.
          • 8 years ago, # ^ |
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            It was my bug for an hour, but, fortunately, I managed to find it :-)
            • 8 years ago, # ^ |
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              This is the kind of test that should have been left out of the pretests, it would have been very good for hacks.
  • 8 years ago, # ^ |
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    2
    Select 7,8,10,11 and 6,9.
8 years ago, # |
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Good Contest
8 years ago, # |
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Interesting problems, i hope there will be some analysis because i could solve only problem A.
8 years ago, # |
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@dalex, this is a nice contest. An editorial would be very helpful. Thank you. 
8 years ago, # |
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The problems are of good quality. Thanks the author!
8 years ago, # |
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isn't it true that testcases for problem E missed the case when all numbers in array a is one and k is pretty big number?
I think many solutions would failed on this case.
Edit: sorry,this is wrong because numbers should be different.
8 years ago, # |
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Nice problemset, but A and B seemed hard to read for me (examples with explanation would be very helpful i think), through they were really easy to write when understood. But anyway, thanks for this contest.
8 years ago, # |
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My solution for div 1 A fails the 6th pretest, which is 21 5 1 21. Apparently the correct output is 1. 
During the contest I asked "In the first test case, can he select all the icons in one move?" and got as answer: "No, because they do not form a rectangle." The one who answered the question must have though that I am asking whether he can select the icons [a,b] in one move, and answered no (it was obvious that he can not in that test case select [a,b] with just one move, so I didn't think that anyone can understand the question that way), but what I wanted to know is whether he can select "all" the icons, [1,n] in one move, in a test case where n%m!=0. Just wanted to share what made me fail the problem, so that everyone can learn from it, and define their question better before they ask.
8 years ago, # |
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Now I realized that problems were easier. At the contest they seemed very difficult . REGRETTING :((
8 years ago, # |
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My D for div 2 failed in the first pre test. Judge said it replied with a NO, while it correctly finds the answer in my machine. It's submission 521752, what can be wrong?
  • 8 years ago, # ^ |
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    It happened with me in topcoder and later i found that i forgot to initialize my flag array with zero and judge gave me wa in 1st test.
8 years ago, # |
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8 years ago, # |
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Where to find the English version ?