GlebsHP's blog

By GlebsHP, history, 22 months ago, In English,

Hello, community!

Tomorrow Codeforces Round #342 is going to take place. It will share the problemset with Moscow Olympiads in Informatics for students of grades from 6 to 9. Though, grades from 6 to 9 in Russia usually match the ages from 12 to 15, I guarantee that everyone (even Div. 1 participants) will find some interesting problems to solve. Problems were selected for you by the Moscow jury team: Zlobober, _meshanya_, romanandreev, Helena Andreeva and me; and prepared by members of our scientific committee: wilwell, sender, iskhakovt, thefacetakt and feldsherov.

Scoring distribution will be quite unusual: 750-750-1000-2000-3000.

UPD System testing is over. Here are the top 10:

  1. _XuMuk_
  2. pandamonium
  3. latisel
  4. zetamoo
  5. yukariko
  6. I_Love_Ximera
  7. KittyLover
  8. shdut
  9. harry.zhao
  10. luke0201

Congratulation! Also, problems seemed to be too tough, we should have probably made Div. 1 round. Anyway, thanks for participating, I hope you enjoyed it and learned something new!

Thanks to romanandreev for nice analysis.

 
 
 
 
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22 months ago, # |
  Vote: I like it +11 Vote: I do not like it

I am not able to register unofficially for the contest. Please fix this.

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22 months ago, # |
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Auto comment: topic has been updated by GlebsHP (previous revision, new revision, compare).

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22 months ago, # |
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Ну и разбалловка)

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22 months ago, # |
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In the same time as Open Cup 10 stage. It's a pity.

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22 months ago, # |
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Is this contest going to start at the same time of the official contest? Because otherwise it should be unrated, right?

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    22 months ago, # ^ |
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    It's not going to start exactly at the same time, but it will start before the statements become public and will end at the same time as the onsite contest (which actually runs for 4 hours).

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22 months ago, # |
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22 months ago, # |
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GlebsHp you forgot to thank yourself :)

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    22 months ago, # ^ |
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    A sentence like this ??: I thank myself for my great helps to myself in preparing this round.. :D

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22 months ago, # |
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Links to any previous contests by the same authors would be very helpful. It'll give a good idea as to what to expect in the contest! Any help?

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    22 months ago, # ^ |
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    Click on the user profiles, and go to Problemsetting tab.

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    22 months ago, # ^ |
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    How would previous contests help ? o.O

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      22 months ago, # ^ |
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      Quoted, "It'll give a good idea as to what to expect in the contest!".

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22 months ago, # |
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Wish Codeforces a happy Chinese New Year!

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22 months ago, # |
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Nice and short announcement . Kudos to GlebsHP

I guess problem statements will also be short and nice.

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22 months ago, # |
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@ admin whenever i log in into my account(manish_nit) everthing appears in russian, i have to manually right click every time and select translate to english. this creates lot of problem during contests...is there a setting in the codeforces website to do it permanently.

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    22 months ago, # ^ |
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    Of course not. You better learn Russian. Who would think of a language switcher?!

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22 months ago, # |
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Is it national level competition?

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    22 months ago, # ^ |
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    Seems to be city level.

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      22 months ago, # ^ |
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      Thanks! I expected the problems to be much easier for a 7th grade local competition :). I lost too much time on the first problem, then B and C were probably just about right for a Div 2 contest (750 and 1000 points). Didn't even get to D or E ...

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22 months ago, # |
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why A and B has the same points ?

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22 months ago, # |
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Why not a combined division contest?

Great authors and of course great problems!

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22 months ago, # |
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A contest by new( GlebsHP ) and old( Zlobober ) coordinator.

Hello Zlobober again!

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    22 months ago, # ^ |
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    Why so many — votes? :)

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      22 months ago, # ^ |
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      I don't know! but this contest was good for me after about 30 bad contests.

      (I took place 18 ;) )

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        22 months ago, # ^ |
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        took

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          22 months ago, # ^ |
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          thanks; edited (English is my default language!)

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22 months ago, # |
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zlobober is backed :))

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22 months ago, # |
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Great contest and perfect timing, do contest before dinner, then watching firework at mid night, happy lunar-new year.

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22 months ago, # |
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Its Sunday morning in my country. Is it bad if I miss church for contest ? I like contest!!!

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22 months ago, # |
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Today in China, it is Spring Festival, which is the most important festival in China, every single of Chinese will have supper with family. I hope in the New Year, Codeforces will be better and better~

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22 months ago, # |
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i will have fun before the Chinese Spring Festival dinner.

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22 months ago, # |
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Luckily I won't miss the Spring Festival Gala at 20:00.

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    22 months ago, # ^ |
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    But I think the Spring Festival Gala is becoming more and more boring

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      22 months ago, # ^ |
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      go to watch Pay New Year's call offering of bilibili at 18:00(UTC8)?..

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22 months ago, # |
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hope it will be my last div2 :D

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22 months ago, # |
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happy chinese new year!!!

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22 months ago, # |
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Happy Chinese New Year(the Spring Festival) to everybody and wish Codeforces will become better and better !

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22 months ago, # |
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Happy Chinese New Year~

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22 months ago, # |
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Lucky time.

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22 months ago, # |
Rev. 6   Vote: I like it -17 Vote: I do not like it

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22 months ago, # |
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What is the hacking test for problem B?

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    22 months ago, # ^ |
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    If the solution searches the whole string from the beginning after each time it finds the pattern.

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    22 months ago, # ^ |
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    aaabb
    aabb
    

    Some participants were finding string matches in O(n) but ridiculously wrong ways.

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      22 months ago, # ^ |
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      can i solve the problem B use kmp algorithm???if not ,can anyone give some advice? by the way ,happy chinese new year!

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        22 months ago, # ^ |
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        I've solved it with KMP :)

        Submission

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        22 months ago, # ^ |
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        You don't need KMP. O(30*100000) is fast enough, so you can brute force the whole problem. If the pattern was longer, a linear time string searching algorithm would be suggested, but in this case (the fact that it's a Div2B should give you a hint at its difficulty) it's not necessary and has a larger chance for mistakes. This is what I did in the contest (runs in max 46ms on main tests, which is easily good enough) http://codeforces.com/contest/625/submission/15858375

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          22 months ago, # ^ |
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          You really replace some characters with '#'! I just imagined to replace. 15874296 Although it's after the contest.

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        22 months ago, # ^ |
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        I used C++ find() to solve DIV2B since input data was small ... I could do using KMP but coding using find() was easier.

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22 months ago, # |
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C seems so easy then A and B. I think C < B < A Due to overflow with binary search, 6 wrong submissions on A :\ I hope it passes now.

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    22 months ago, # ^ |
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    Is this approach correct? Write numbers from 1 to (k-1)*n in first (k-1) columns then again start from column 1 write numbers in increasing order.

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      22 months ago, # ^ |
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      Yep... I solved C Problem this way :)

      I had to code C problem in like 5 minutes .... I wish I had not wasted time on A.

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    22 months ago, # ^ |
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    I think B and C are equally hard/easy and A easier than both of them.

    Where did you used binary search? I haven't use it at all.

    UPD: Well it seems that I will fail A after system testing. Actually I now think C is easier than A and B, just because its solution is obviously correct comparing to A and B.

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      22 months ago, # ^ |
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      I used binary search to find number of times one could buy glass bottles. Looking at others code now, it seems there's a direct formula, but i couldn't get it correct.

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    22 months ago, # ^ |
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    You don't need to use a binary search — the answer can be found in O(1) with division. http://codeforces.com/contest/625/submission/15857590

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22 months ago, # |
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how to solve D ?

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22 months ago, # |
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Who is the author of the Problem A? >.< And tester also >_< ?? No mercy, No mercy >_< :'( :'(

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22 months ago, # |
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Now, I know why Russian coders are so good and accurate.

Nice problem-set, especially first problem.

Eagerly waiting for editorial for 4th and 5th question and hoping my solution for 1st three problems pass the system tests.

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22 months ago, # |
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why this work on test

100000 symbols a

a

for 0,2 sec

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    22 months ago, # ^ |
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    I tried hacking a solution like this and also failed.

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    22 months ago, # ^ |
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    Find returns at the first occurence, so that takes O(1) time. erase can take up to linear, according to specification. But it is probably very optimized and the Codeforces servers are really fast (solutions with 10^9 operations can pass time limits) so it passes. Perhaps erasing the first character is faster than erasing from the middle, as well?

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      22 months ago, # ^ |
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      i try to change first symbol to 'b', and it's work fast too

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22 months ago, # |
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This was my solution for 2nd (div2) x = raw_input() y = raw_input() print x.count(y)

It passed the pretests XD, i dont think it will pass the final tests ... will it? EDIT : It passed!

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    22 months ago, # ^ |
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    See this test

    baaacd
    aa                       
              Answer = 1.
    
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      22 months ago, # ^ |
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      giving ans as 1

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      22 months ago, # ^ |
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      As far as I know python3 count method counts the amount of non-overlapping substring appearances, so your test is not going to hack the solution.

      UPD: Not sure about python 2 tho

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22 months ago, # |
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I wonder how many A solutions will remain after systests :O. What was hack test for A btw

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22 months ago, # |
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Very interesting problems. I got a lot of fun. Thanks to authors!

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22 months ago, # |
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Ouch, the penalty for wrong submissions really showed through this contest because of the sudden spike in difficulty between C and D. Two wrong answers for A cost me 200 places in standing (from ~200th place to ~400th place).

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22 months ago, # |
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too weak pretests these days :(

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    22 months ago, # ^ |
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    With strong prtests, we can't enjoy hacking :(

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    22 months ago, # ^ |
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    Actually, it surprised me, as I picked about 10-15 pretests in every problem. But still ,the number of cases seems to be much more than this.

    Many solutions will fail system tests, but this time it was unintentional :)

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22 months ago, # |
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Is System Test gonna take long like in the past school contests? (until the closing ceremony)

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22 months ago, # |
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hack test for problem one is : 2 100 100 50

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    22 months ago, # ^ |
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    Omg, and here I failed with "-1". :D Hacked some other guy with 10^18 inputs.

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22 months ago, # |
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terrible! I fixed some bugs on problem D and failed to sumbit in last several seconds.

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22 months ago, # |
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Still can't go to sleep because I'm terrified of the system tests for A and B :(

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    22 months ago, # ^ |
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    Still can't go to LUNCH because I'm terrified of the system tests for A and B :(

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22 months ago, # |
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My Solution for the A problem works on my pc... Tried it with Javascript and with Free Pascal... **** But it always gave me runtime error on pretest 1 !! ca I know what's wrong somehow ???

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    22 months ago, # ^ |
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    This works on my PC but it's runtime error why plz help me :/

    var n = readLine(); var a = readLine(); var b = readLine(); var c = readLine(); n=Number(n); a=Number(a); b=Number(b); c=Number(c);

    function buy (n,a,b,c){ if ((n<a) && (n<b)) { return 0; } else if (n>=a) { n=n-a; return (buy(n,a,b,c) +1); } else{ n=n-b+c; return (buy(n,a,b,c) +1); } } print(buy(n,a,b,c));

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22 months ago, # |
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        String gog = in.next();
        String tel = in.next();
        int ans = 0;
        int k = 0;
        while (k + tel.length() <= gog.length()){
            String s = gog.substring(k, k + tel.length());
            if (s.equals(tel)){
                ans++;
                k = k + tel.length();
            } else {
                k++;
            }
        }
        out.println(ans);

Its B. Will it pass sys tests?

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22 months ago, # |
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everyone who solved 6-th COCI task of yesterday contest must solve the 4-th problem of today easily :D

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    22 months ago, # ^ |
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    Yeah. I should've read that editorial.

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22 months ago, # |
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Anyone solved E?

I have that idea: using binary heap (aka priority queue) we sort all possible collisions of frogs by (time_to_collision, id). After that in cycle we pop first frog, kill what she could kill, update her step_size, time_to_collision and put her in the heap again (also doing this for her precessor).

Stop when the next frog in heap can't kill anyone.

Should be NlogN. But I stuck in some range checks and didn't finished the solution.

Is it correct approach?

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    22 months ago, # ^ |
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    I was thinking about the same solution, but moreover you have to update time_to_collision of previous frog (I mean the one, which gonna kill actual one).

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      22 months ago, # ^ |
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      Yea, I've considered that ("also doing this for her precessor").

      In my implementation I simply put another copy of previous frog in the heap (instead of find-delete-putagain, for performance). It shouldn't affect the result, because its value will be strictly lesser than old one, so old one will be skipped somewhere in the future.

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        22 months ago, # ^ |
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        Do you think you could explain test case 18's answer for E? I've done it manually and get a different answer than the judger. ( This is my solution btw)

        10 10 9 4 3 5 2 2 5 4 1 6 6 7 8 3 4 1 10 3 7 9 Participant's output 2 1 8 Jury's answer 1 8

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          22 months ago, # ^ |
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          Nevermind, it was an arithmetic error in my manual solution.

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          22 months ago, # ^ |
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          BTW, your solution seems to get TL later on. You already got 1 second on N=12500. And it could be 100000.

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            22 months ago, # ^ |
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            I expected much as I don't really make any clever observations in it. Was more expecting a TLE instead of a WA though.

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    22 months ago, # ^ |
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    It looks like the correct approach. Analysis will be published in the nearest 2-3 hours.

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      22 months ago, # ^ |
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      Information like this should be added in the main post as it is very hard to find this kind of information in the sea of comments.

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22 months ago, # |
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Wow very fast system testing. My rank jumped from 1000 to 500's before/after system testing xD Problem A is the cause :p

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22 months ago, # |
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bad contest :-(

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22 months ago, # |
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Putting "Guest From the Past" as an A problem was a wicked move.
I wish I didn't waste time on it :'(

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22 months ago, # |
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just 699 who can solve a in the contest XD

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22 months ago, # |
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    22 months ago, # ^ |
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    Yea, very tricky A and obviously_correct_solution in C :)

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    22 months ago, # ^ |
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    I don't think this round had enough score differentiation. D and E should've been slightly easier, while A and C should've been switched (or C should've been made much harder, as it's unusual for so many people to get it). A wasn't actually hard, but it was pretty easy to make a silly mistake (a lot of people used it for hacks). The concept of C was roughly the same difficulty but the implementation was much easier (no derps).

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    22 months ago, # ^ |
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    The eyes... Oh my god, THE EYES!!!

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22 months ago, # |
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What was the solution to D? I thought of some messy solutions but could not find a clean one.

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22 months ago, # |
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Realy A problem is tricky with Time limit and Wrong answer.

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22 months ago, # |
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22 months ago, # |
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Could someone just tell me how to solve A? It seemed that A wasn't the easiest problem :(

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    22 months ago, # ^ |
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    can do it in O(1):

    • if buying and selling a glass one is more expensive than buying a plastic one, only get plastics
    • if not, buy glass bottles until you can't do so anymore, then buy plastics with the rest of the money

    implemented here: http://codeforces.com/contest/625/submission/15870223

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      22 months ago, # ^ |
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      I too thought that it should be O(1) but couldn't solve it, maybe because i freaked out because it took too much time to solve A. It was similar to this problem. I solved that problem by brute force but obviously here brute force won't work. :/ Next time I will try to find a better solution for every problem even if it gets AC.

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        22 months ago, # ^ |
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        Is's possible to resubmit an AC problem? :O

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          22 months ago, # ^ |
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          yes it is, but you lose 50 points because of resubmission.

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          22 months ago, # ^ |
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          yeah! previous submit gets Skipped, and new one will judge in system testing.

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      22 months ago, # ^ |
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      Is it any formula for O(1)? If the item costs b units and for returning you can get c units, then you can buy (n — b) / (b — c) + 1 items by n units?

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22 months ago, # |
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574 — "A" = 1228 :(

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22 months ago, # |
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Today I've seen a bunch of guys whose solutions failed systests for A but still they hacked everybody in their rooms so they got about 700-1000 points just from hacks. Turns out you don't have to actually solve a problem to get sufficient points for it.

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    22 months ago, # ^ |
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    Yea, its pretty usual thing for hackerfests.

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    22 months ago, # ^ |
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    I found my A is wrong when i have hacked 7 persons, finally my code is hacked but i still get 750 point..... btw, there is a guy write"cin>>n>>b>>a>>c", for which i got four times unsuccessful hack

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22 months ago, # |
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The task are ok, but very bad for the div 2 contest.

Only the second task was on the level which it should be.

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22 months ago, # |
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Thanks for timing!! Happy Tet Holiday!! <3 <3

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22 months ago, # |
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Why i got WA on b??? this is my code, any help please?? http://www.codeforces.com/contest/625/submission/15862688

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    22 months ago, # ^ |
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    ababc abc

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    22 months ago, # ^ |
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    If current characters in S and T are different maybe you won't return to the begging of string T.

    I didn't tested your solution, but I think this is hack case for your solution :

    S='bbbc' T='bbc'.

    Correct answer 1.

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

A->C, C->A

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22 months ago, # |
  Vote: I like it +92 Vote: I do not like it

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

I am very interested, whether someone got full score in official, on-site contest ?

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

.

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22 months ago, # |
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for D I came up with an DP solution where dp[i][j][a][b] represents the interval [i, j] can be written as the format required and current i position number is a and j position is b, a, b < 10, the solution will exceed memory limits but I fail to simplify my solution.

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

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

Я первый раз вижу div2 A который получил меньше чем div2 B и меньше чем div2 C

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

it can take days for Glebshp to write/translate the editorial (remember round 327). So, let's not wait for editorial and write our solutions ideas here.

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

Wow, outstanding performance: http://codeforces.com/contests/with/latisel

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    22 months ago, # ^ |
    Rev. 2   Vote: I like it +9 Vote: I do not like it

    Cool, i was checking his past contest history

    I expect him to do 100+ unsuccessful hacks in next round :P

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    22 months ago, # ^ |
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    I guess he want to stay at Div2

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22 months ago, # |
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Hmmm what's wrong here? It failed at the aaaaaaa... case, where the answer is the number of a's, but my answered was one less.

codeforces.com/contest/625/submission/15869860

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    22 months ago, # ^ |
      Vote: I like it +5 Vote: I do not like it

    Array char is needing one extra cell for null character after end. Sorry if my English is bad, I try hard learning it.

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      22 months ago, # ^ |
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      Thanks for your answer! Your English is not bad at all!

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22 months ago, # |
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Thanks for contest :)

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

please, don't say anything before the contest.

Problem set would be too easy. or Problem set would be too hard.

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

I suspect the test data of problem A is very weak, since 15867499 passed system test. But it should TLE at this case: 1000000000000000000 2 500000000000000001 500000000000000000

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22 months ago, # |
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DO we have editorials for this one ?

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

Can someone help me find the problem with my O(n) B?

http://codeforces.com/contest/625/submission/15877046

I feel like it's the correct strategy implemented right... but I'm getting off by 1 on test 26.

thanks!

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

    aaab aab

    Ans should be 1. Yours gives zero

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      22 months ago, # ^ |
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      Thank you! I see the problem! Cheers. And as the_art_of_war says, the easier way is of course to just go the simple way so you can't run in to tricky edge cases like this.

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

    I think the length of second string is special so small that you can solve it with O(n*m) using two cycles. If you want to make solution with O(n) you should use special algorithms with strings like Z-function or prefix-function.Maybe there is more easier way to get solution with O(n).
    Sorry for my bad english.

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22 months ago, # |
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Can someone give an idea how to solve 4th one ??

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22 months ago, # |
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Three WA on Question 1 give me the way to hack 5 solutions :)

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22 months ago, # |
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Auto comment: topic has been updated by GlebsHP (previous revision, new revision, compare).

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

Happy Chinese new year!Can someone tell me how to solve the A question?I see some answer:they all used (n-b)/(b-c),but I don't know why?

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    22 months ago, # ^ |
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    that is it!from others: can do it in O(1): if buying and selling a glass one is more expensive than buying a plastic one, only get plastics if not, buy glass bottles until you can't do so anymore, then buy plastics with the rest of the money implemented here: http://codeforces.com/contest/625/submission/15870223

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

      thanks,but I still have a question:why use n-b not n?why I should use n-b?I can't know very well.

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

      My question is why n should minus b?My first use n,but I know I am wrong.Can you help me?

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

        If you don't have enough money(like n<b),you can't buy a glass one. So you shoule use n-b not n,I hope you get it.

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          22 months ago, # ^ |
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          thanks,I get it,happy Chinese new year.

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22 months ago, # |
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Somebody help me! What the problem with that solution? Problem B.

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

Where is the editorial?

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21 month(s) ago, # |
  Vote: I like it -9 Vote: I do not like it

Author of B sucks: 15924760