When my part of code was:

int k = 0;
char S[100001], c;
while (cin.get(c)) S[k++] = c;

I got WA #1 several times, but my program worked correctly for all tests!

Then I replaced this part of code to:

string S; cin >> S;

and got AC in 0.031s. It's very strange because I think my old program was right too. And I didn't expect that class string works so fast even if length = 100000 :)
Maybe there are weak tests? Or an error in check system?