Compilare: user.cpp: In function ‘int main()’: user.cpp:15:43: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = S.size() / 2; i < S.size(); i ++) ^ user.cpp:18:28: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if (i + 1 < S.size() && i - 1 >= 0) k = 1; ^ user.cpp:19:31: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] while (i + k < S.size() && i - k >= 0) ^