Compilare: main.cpp: In function 'bool back(std::vector<int>)': main.cpp:14:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < v.size(); ++i) ~~^~~~~~~~~~ main.cpp:15:31: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int j = i + 1; j < v.size(); ++j) { ~~^~~~~~~~~~ main.cpp: In function 'int main()': main.cpp:30:36: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)', declared with attribute warn_unused_result [-Wunused-result] freopen("bigfour.in","r",stdin); ^ main.cpp:31:38: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)', declared with attribute warn_unused_result [-Wunused-result] freopen("bigfour.out","w",stdout); ^