Compilare: user.cpp: In function ‘bool solve(int, std::vector<int>)’: user.cpp:7:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < a.size(); ++i) for (int j = 0; j < i; ++j) if (abs(a[i] - a[j]) <= d) { ~~^~~~~~~~~~ user.cpp:9:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int k = 0; k < a.size(); ++k) if (k != i && k != j) b.push_back(a[k]); ~~^~~~~~~~~~ user.cpp: In function ‘int main()’: user.cpp:18:38: 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); ^ user.cpp:19:40: 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); ^