Compilare: user.cpp: In function ‘void FWHT(std::vector<long long int>&)’: user.cpp:7:41: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int len = 1; 2 * len <= P.size(); len <<= 1) { ^ user.cpp:8:36: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < P.size(); i += 2 * len) { ^ user.cpp: In function ‘int main()’: user.cpp:37:31: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = 0; i < freq.size(); ++i) ^ user.cpp:20:37: warning: ignoring return value of ‘FILE* freopen(const char*, const char*, FILE*)’, declared with attribute warn_unused_result [-Wunused-result] freopen("aiacubiti.in", "r", stdin); ^ user.cpp:21:39: warning: ignoring return value of ‘FILE* freopen(const char*, const char*, FILE*)’, declared with attribute warn_unused_result [-Wunused-result] freopen("aiacubiti.out", "w", stdout); ^