Welcome after the long break in Hack me! (Codeforces Round 277.5 (Div. 2)).

Previous posts can be found here.

Stats

* one of the: INVALID_INPUT, GENERATOR_INCOMPILABLE, GENERATOR_CRASHED, IGNORED, OTHER

Really interesting is fact that the second most hacked problem was the last one, what is not so typical.

Hacks and possible hacks description

489A - SwapSort

In some solutions where people are finding minimum every time, they used the wrong limit for the minimum itself.

For example:

min := -100000000
wskmin := -1           <- where is min
for i in 0,1,...,n-1:
  if(min > tab[i])
     min = tab[i];
     wskmin = i;

was killed by the test:

2
200000000 -200000000

How to deal with it? The first possibility is to set min lower than lower bound of numbers on input (for -200000000 we can choose -200007000 or something similar). Or start with the first element as minimum:

min := tab[0]
wskmin := 0          <- where is min

489B - BerSU Ball

Let's take a look at these submissions: 8725982 and 8731808. The first one was hacked and the second accepted, the only difference is sorting vectors. So the following testcase:

2
2 4
2
3 1

was used to hack the first solution. That was not so rare mistake and such tests were used dozen or so times.

489C - Given Length and Sum of Digits...

Interesting bug (and hard to find) was wrong checking of the condition for solution to exist.

The solution exist if the sum is less or equal than 9*m (from obvious reasons). Some people are forgetting about equal part and the proper test like:

2 18

can kill the nearly good submission.

489D - Unbearable Controversy of Being

The only two hacks for this problem was maximal testcases which resulted in time limit ex.

489E - Hiking

No hacks were used for this problem.

489F - Special Matrices

Nearly all hacks was killing the slow solution with time complexity O(n^3).

Example of such testcase:

500 0 10000007

Fastest hackers

Best hackers

Best rooms

Best countries

Country	#hacks	Hackers
India	26	abhra73 [6], khandeshb1 [6], atulshgl [4], bernett [2], girish347 [2], ramprakash_k [1], StoneCold_ [1], vishwacs111 [1], rtheman [1], gorv [1], sensiblemadman [1]
Japan	25	atetubou [7], ChronoireSchwarzVI [6], anta [4], zerokugi [3], yuusti [2], zeosutt [2], kmjp [1]
Russia	18	knst [4], mmatrosov [4], kharvd [3], lunawyll [2], danildudin [2], w0w [2], NVAL [1]
Kyrgyzstan	16	Erkin97 [9], Azret [5], tokonuuLu [2]
Egypt	12	Guts [5], mohamednabil00000 [3], Mohammad_Yasser [1], khaledA [1], SmartCoder [1], xa.mohsen [1]
Iran	11	mosiomohsen [6], ashkan_d13 [2], Majid [2], Batman [1]
Ukraine	9	kit9ra [3], vlad8 [2], BigBag [2], aangairbender [1], fiter [1]
Colombia	9	pin3da [4], alan_navarro [3], mavd09 [2]
Uzbekistan	8	Eldor_Zoirov [4], Yura_Sultonov [2], Sirojiddin [1], Shavkat_Aminov [1]
Bangladesh	8	the_redback [4], sahedsohel [3], ImagineWarrior [1]