Compilare: user.cpp: In function ‘bool is_solvable(std::vector<int>&, int)’: user.cpp:8:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < v.size(); ++i) { ~~^~~~~~~~~~ user.cpp:9:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int j = 0; j < v.size(); ++j) { ~~^~~~~~~~~~ user.cpp:13:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int k = 0; k < v.size(); ++k) { ~~^~~~~~~~~~ user.cpp: In function ‘int main()’: user.cpp:26:36: warning: ignoring return value of ‘FILE* freopen(const char*, const char*, FILE*)’, declared with attribute warn_unused_result [-Wunused-result] freopen("bigfour.in", "rt", stdin); ^ user.cpp:27:38: warning: ignoring return value of ‘FILE* freopen(const char*, const char*, FILE*)’, declared with attribute warn_unused_result [-Wunused-result] freopen("bigfour.out", "wt", stdout); ^