NALP's blog

Codeforces Round #128 (Div. 2)
By NALP, 12 years ago, translation, In English

Hello, friends!

A few hours later you're lucky to participate in this remarkable Codeforces Round #27 for Div. 21 participants, but traditionally the others can take part out of the competition.

It has been prepared by a small band of authors: me (NALP), Igor Kudryashov (Igor_Kudryashov), and Pavel Kholkin (HolkinPV). There were Gerald Agapov (Gerald), Maria Belova (Delinur) and Mike Mirzayanov (MikeMirzayanov) with us as always.

It’s well-known that you can participate in this Round into competition only today! There won’t be another Codeforces Round #27!

Traditionally I wish good luck, accepted solutions and successful hacking attempts for you!

UPD: Points are standard: 500, 1000, 1500, 2000, 2500.

UPD: Round is over, thanks to all! We hope you have got a fun. Don't forget, system testing will be soon.

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

Comments Comments (59)
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12 years ago, # |
  Vote: I like it +18 Vote: I do not like it

I'm predicting most of the problems will have jokes on 2-powers

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12 years ago, # |
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Good luck to all! :-)

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12 years ago, # |
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using dymantic score or something else?

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

can you talk about Score distribution. hope short and clear statements. Finally ,Thanks for preparing this contest.

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

hoping for short and precise statements...

may it b overall a gud cntst..:)

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12 years ago, # |
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Nothing New!!! :P Everything is traditionally traditional!!! :D

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

Points should be: 512(2^9), 2*512, 3*512, 4*512, 5*512. :)

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12 years ago, # |
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I cannot understand why this solution does not hack successful on the test case 1000 1 1000 1000 for problem B div 2.

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

    Very dangerous to try a hack like this, using errors about size of the array. Maybe it will fail for system test, but you can't be sure what will be returned by this kind of code. Only hack codes with bigger problem of size.

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

    A similar question has been asked before: http://codeforces.net/blog/entry/2799#comment-57306

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      12 years ago, # ^ |
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      Thank You..I learned something on the cost of 2 unsuccessful hacks (-100 points)

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        12 years ago, # ^ |
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        but dude in that n can be at max 1000 and he was having bound of 1001 which i think should work fine......

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          12 years ago, # ^ |
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          Look carefully what happens if x = n or y = n.

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            12 years ago, # ^ |
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            according to me nothing would happen..... if we have A[n+1] then whats the problem with A[n]

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

              The code is running for each value of x and y,two inner loops (x to < x+3) and (y to < y+3)..Now for x=y=1000 , it is incrementing the values at array indices [1002][1002] also ..Thats were I thought it should give runtime error as Array Index out of bounds exception..But As andreyv says it fixes some page memory buffer for its excution , although I am not clear of its concept totally..So I said I would rather not take risk in furthur contests which such hacks

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

    was your hack for the size of the array or because he didn't continue taking the input and returns when solution found?

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

      For the array size..It exceeds 1001 .So Array index out of bounds exception

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12 years ago, # |
  Vote: I like it -7 Vote: I do not like it

I hope next time there'll be more fluent English in the description of problems. I know that many authors aren't native speaker but today's problems are just a bit of difficult to understand. Besides how long will the system test be?

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

I think I'm not the only one who miss if x = 0 so I failed on preetest 3 :) Edited : On problem A

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    12 years ago, # ^ |
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    You're not alone =)

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    12 years ago, # ^ |
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    I solved it on the 2nd minute, realized the x=0 after 5 submits and 30minutes passed :( got 240 pts

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

System test has begun, finally!

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

Cool contest guys! Imo little bit easier than last ones.

I have to criticize the task description of task A though. It repeatedly uses the term "cost" to describe the score of the problem. If this is not clear... cost has a negative connotation, decreasing the cost with time thus means that we get a higher score with time. Really confused me there.

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12 years ago, # |
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I think a system test server is down: http://www.codeforces.com/contest/203/status/page/25?order=BY_JUDGED_DESC

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12 years ago, # |
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So unlucky. The test size of C is 10^5 but I mistaken it as 10000. I missed my AK this time, such a pity. The good thing is my E is AC.

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

How to solve Problem D div 2?

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

    Assume there are three balls one going with velocity vx, other with velocity vy, and third with velocity vz. Now at the time of 2nd ball colliding with Door, find where are the two other balls.

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

    First observe that reflection does not change ball's speed in dimensions so you can easily calculate the time that ball gets to y=0, T. Now calculate X and Z of the ball after T seconds. Now you need to find its position inside corridor. Notice that X,Z dimensions are independent. Consider final X of the ball without reflection is X0 and after being reflected once and changing its side, final X of ball without any further reflection is X1. By laws of reflection, X0 and X1 are symmetrical to each other by the wall which ball was reflected by so if it hit X = (a) line then you can calculate X1 as : X1 = 2 * a — X0 and if it hit X = 0 line, X1 will be -X0. And go on from this point until X and Z are inside the corridor. These actions can be done by some easy while loops. I hope this explanation's been helpful for you. If you need further information on this solution, please let me know.

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      12 years ago, # ^ |
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      Thank you! I have found my mistake.

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    12 years ago, # ^ |
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    D was quite easy today, look at my submission (sorry, no time for comments in contest), only solve() is important...

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

The English seems much less comfortable than SRM.......

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

As soon as i posted this comment system test ended. really sry.

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

Finally over. Ranked 31 in official participants. Not so bad, but have mistaken the test size of C is so awful. When'll the Rating be updated?

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12 years ago, # |
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please somebody tell me whats wrong with my comparator in problem C.I cannot figure out for which values it cannot make a decision..Here the code

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

Problem E was nice.it has more thinking rather than just coding. thank you all

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    12 years ago, # ^ |
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    I agree with you. It has nothing to do with advanced algorithms and data structures, but remains as a challenging problem. In order to solve this problem, one must observed that if a robot with c larger than 0 is chosen then all robots with c larger than 0 can be chosen. It took me over 30 mins to figure out how to solve this problem.

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

Can Anyone Describe me why I stayed blue in rating?? I became 24th and solved four problems. I ask this because people with lower rank became purple!

The contest was great. Thanks a lot to problems authors! ;)

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

    Because your rating is still below 1700

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

    Are you telling that someone with lower rating at the contest beginning had less point than you in contest and now he/she has greater rating than you? I'd like to see that...

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      12 years ago, # ^ |
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      Oh! I got it! You're right. It's because of their rating before the contest! I didn't know the importance. Thanks

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

    Hope that you'll be over 1700 soon ^_^

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12 years ago, # |
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Hello, excuse me for the question but this is my first competition in this website and i kept having wrong answer on 7th pretest in task E. So i tried to find my mistake but i couldn't, so after the competition is over, i was wondering if there is any possible way to check what the pretest was, and see why my program mistaked there. Thank you in advance :)

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

    Hello! Click here and scroll to the bottom of the page. But there is not way to see full test.

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

    I encountered the same problem as yours during the contest, and then I figured out that using long long instead of integer can solve this. Hope this helps.

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

Y U no put small clear problem statements :)

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12 years ago, # |
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I got a strange result with my code on D, on test case 31:

-1.#IND000 -1.#IND000

On my pc it gives the expected result, I had to add a test to verify if the vz (or vy or vx) is 0 and ignore calculations on them to get accepted, here is my submission:

http://codeforces.net/contest/203/submission/1861627

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12 years ago, # |
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1861821 Can you tell me why I got a TLE on problem B ?

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    12 years ago, # ^ |
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    return a > b ? b : a; is mistake. for(int i = chmax(1, x - 2); i <= x; i++) always runs x times and you got TLE.

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12 years ago, # |
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rofl @ pretest 3 of task A... this guy boasted he has a score of ZERO? WTF is wrong with him :)

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

when we can get THE EDITORIAL!!

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