Stats about hacks in Codeforces Round 263 (Div. 1) (quite late).

Stats

Fastest hackers

Congratulations for FatalEagle for one and only hack for problem C! It was really fatal hack for mareksom (in last 5 minutes).

Remarks

461A - Appleman and Toastman

In Div. 2 there were no hacks in this problem, but many solutions failed on the simple test (#34):

2
1 2

Some people had additional condition for this case and output sum of a₀ and a₁. Good thing is that only 4 more solutions failed during systests — all on this killer test.

461B - Appleman and Tree

Many solutions failed on test #19 and the problem was bad use of modulo. I described it in Div. 2.:

In such case:

x = a%m + b%m

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

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

461C - Appleman and a Sheet of Paper

Just one hack, but there were more possibilities. Let's first look at this hack. Test was pretty simple:

** Generator source **
#include <bits/stdc++.h>

using namespace std;

int main()
{
    printf("100000 100000\n");
    for(int i=0; i<100000; i++)
        printf("2 0 99995\n");
    return 0;
}

Where was the problem? Let's look at mareksom's solutions: 7586989 and 7596313 — the first was one hacked and the second accepted (after contest). The only change was adding tree to deal with query no. 2. Many solutions without BIT or segment tree passed pretests, but didn't pass test #13.

461D - Appleman and Complicated Task

In nearly every solution which didn't pass system tests, the problem was incorrect checking if it is even possible to make one board fulfilling the condition, in particular, the condition from tutorial: "If both 2i and 2i + 1 are in the same set for any i, the answer is 0." was checked with some minor mistakes. Depends on where was the problem, test 19 or 25 was not allowing to pass.

Hackers

There were so little hacks that we can show all successful ones:

Rooms, where something happened

Countries which successful hacks

Country	#hacks	Hackers (with at least one successful hack)
China	2	skyxuan [1], Erich [1]
Ukraine	1	IWillBeRed [1]
Iran	1	mhadih [1]
Canada	1	FatalEagle [1]

Country	#hacks / #hackers	Hackers (with at least one successful hack)
Ukraine	1.00	IWillBeRed [1]
Iran	1.00	mhadih [1]
Canada	1.00	FatalEagle [1]
China	0.33	skyxuan [1], Erich [1]

Gap

As we can see, more common is hacking something with better rating. :)

Charts

Successful hacks over time

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