So the problems says that 1 <= N <= 100, so I declare a nice unsigned char mem[100*100].
I get WA on test 4.
I go and read all the discussion about wrong awsers but none of those offer anything I didn't figure out already.
I change the declaration to unsigned char mem[256*256] and voila, the solution is magically accepted.
Does anyone have an idea why 100*100 bytes aren't enough?

I wrote a solution in Pascal (ages ago :-)), which AFAIR checks whether index is in range while accessing to some element in array. And I had [1..100,1..100]. That is why I suppose that somewhere in your program you access out of array.

100x100 is enough, assuming you start your indexing at 0 and end at n-1