Compilare: user.cpp: In function ‘void FFT(const std::vector<int>&, const std::vector<int>&, std::vector<int>&)’: user.cpp:141:24: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if (i < a.size()) ^ user.cpp:145:24: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if (i < b.size()) ^ user.cpp: In function ‘int main()’: user.cpp:192:37: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] writeInt(cnt + 1 > ans.size() ? 0 : ans[cnt],'\n'); ^ user.cpp:161:41: warning: ignoring return value of ‘FILE* freopen(const char*, const char*, FILE*)’, declared with attribute warn_unused_result [-Wunused-result] freopen("chimichangas.in","r",stdin); ^ user.cpp:162:43: warning: ignoring return value of ‘FILE* freopen(const char*, const char*, FILE*)’, declared with attribute warn_unused_result [-Wunused-result] freopen("chimichangas.out","w",stdout); ^