Welcome again!

I decided to split statistics for both division. Post for first division will be added later is here.

So, let's look at hacks in Codeforces Round 263 (Div. 2).

Some statistics are missing — I have some problems with downloading all data (Codeforces seems little crowded today). They will be also added later.

Update: Europe went asleep, so I added missing stats.

Stats

Fastest hackers

Remarks

462A - Appleman and Easy Task

The problem was pretty simple, but still there were some mistakes (there were only 4 successful hacks and over 150 solutions which passed pretests, but didn't pass all tests).

The "killer" tests:

• Test #32, used also in hack #111676 by xiaxiaosheng.

2
oo
xx

Some solutions just count the overall number of all neighbouring o's and check if this number is divisible by 2.

• Test #16 or for example hack #111424 by anikaitshivam.

This test was just checking maximum value of n, which was 100. Depends on mistake, solutions got Runtime error or Wrong answer (because of checking values for i = 101 and so on).

• Test #11 or hack #111740 by the_redback.

The main problem for these solutions was using break command in double for. What it did is just break from the inner for, staying in the outer one, which sometimes finished with Wrong answer.

Pretty interesting mistake failing on this case was this one (the code is in C++):

if (a[i][j] = 'o')

Somehow, this solution passed first 10 tests. The problem is that the condition in if in such case is always fulfilled. Why? It is assignment operator, so from this time a[i][j] was equal to 'o', and it was always true. The problem is that a[i][j] might be changed (and we didn't want it).

462B - Appleman and Card Game

At first, I was pretty afraid looking at this task (1193 successful hacks!). But the problems were pretty simple. I should say problem, because it was one: long longs!

We just need to notice that result could be even n², which was about 10¹⁰.

Another problem with pretty big numbers is shown below (code written in C++):

long long ans = 0;
int n = 100000;
ans += n*n;
cout << ans;

The result will be 1410065408. Why? The problem is when we are multiplying two ints, the result wants to be int, but it is bigger than 2³², so it distorts.

The proper way to solve this problem:

ans += 1LL*n*n;

Now the program knows that the product should be long long and the result is correct (10000000000).

462C - Appleman and Toastman

We would just look at test #19, which was pretty simple:

2
1 2

Most of the hacks (if not all) in div. 1 were based on such testcase. Many people had if in case n = 2 and printed the sum a[0] + a[1], but we can split this set into two subsets (a[0]) and (a[1]), then give it back to Toastmen and get in the end 2(a[0] + a[1]).

462D - Appleman and Tree

There was no hacks for this problem, but there was one mistake. The problem is with proper using of modulo. In such case:

x = a%MOD + b%MOD

sometimes x can be grater than MOD (for example for a=6, b=7 and MOD=4), so we should use:

x = (a%MOD + b%MOD)%MOD

462E - Appleman and a Sheet of Paper

Again, there was not a single hack, but still there were some solutions which passed pretests, but didn't pass test #13. Unfortunately, we cannot look at it, because it is too big.

Most of these solutions were just brute-force approach. Pretests weren't too hard to pass.

Best hackers

Best rooms

Countries

Country	#hacks	Hackers (with more than one successful hack)
China	233	hetanglianyi [19], stupida [13], chinesejiang [13], Jack_whj [13], loujunjie [10], fzyz999 [10], NoSoul [10], ohweonfire [10], wandering [9], suliang [9], aswmtjdsj [9], I_Love_Balabala [9], dnkaito [8], catfish [7], wwunxc959 [7], 545529689 [7], 31201153 [7], korun [6], kojimai [6], bolyeria [6], learningmou [5], tsxhl111 [4], Naturain [4], fswuweis [4], sucan [3], akxxsb [3], LuxakyLuee [3], worfzyq [2], squee_spoon [2], qingping95 [2], dnvtmf [2], zjbztianya [2]
India	145	SameerGulati [15], xennygrimmato [11], const_int_magic [11], tussharsingh13 [10], aditya_kakarot [8], tarang1610 [7], rarora7777 [7], codeDREAMER [6], agamgupta_79 [6], shubh09 [5], abhra73 [5], MysticAndSevere [5], shiva_r31 [4], kuldeepfouzdar [4], warlock [3], radhey_maa [3], prakhar3agrwal [3], kunal361 [3], Cellar_Door [3], shank_punk [2], mayank_kumar [2], rishi_saraswat [2], vidyut_7 [2], StoneCold_ [2]
Russia	108	mr_justadog [12], hball1st [11], NiceDay [10], worse [9], nikiror [9], Miracle [9], watson [8], nic11 [7], redjohn [7], knst [5], knok16 [5], MakArt [4], arturom [3], veschii_nevstrui [2], poma.prs [2], _ant_ant [2]
Bangladesh	65	nightfury1204 [12], underSpirit [11], faisal47 [7], bertho_coder [7], Mukit09 [7], md.alam [5], mahbubcseju [4], .PEIN. [4], mochow [2], gaurab_sust [2], nighthowler [2]
Iran	59	mosiomohsen [19], IMAN_GH [13], DemoVersion [11], pawky [9], ehsanoo [5], argooogooly [2]
Japan	35	isurugieri [11], tomoki [10], kmatsunaga [7], purple_jwl [6]
Egypt	35	Mohammad_Yasser [10], Killever [7], abdou_93 [6], NourElRashidy [4], osama [3], hamedArafa [3]

Country	#hacks / #hackers	Hackers (with at least one successful hack)
France	13.00	cup_of_tea [13]
Brazil	13.00	dcms2 [13]
Bolivia	11.00	DrPaulVazo [11]
Iran	8.43	mosiomohsen [19], IMAN_GH [13], DemoVersion [11], pawky [9], ehsanoo [5], argooogooly [2]
Macedonia	8.00	add1ctus [8]
Mongolia	7.50	Enkhsanaa [11], Nuadra [4]

Biggest gap

worse is attacking with 9 hacks (and 77 unsuccessful ones).

Charts

Successful hacks over time

Pretty boring this time.

Thanks and again: sorry for all my mistakes, feel free to correct me. Previous post can be found here.

I have some other ideas about such stats and reports, so I will try to work on them — look forward to see them in nearby future!