Compilare: user.cpp: In function ‘int main(int, const char**)’: user.cpp:49:38: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int j = 0; j < a[i].size(); ++j) ^ user.cpp:53:48: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int k = 0; k < a[a[i][j]].size(); ++k) ^ user.cpp:30:34: warning: ignoring return value of ‘FILE* freopen(const char*, const char*, FILE*)’, declared with attribute warn_unused_result [-Wunused-result] freopen("fof.in", "r", stdin); ^ user.cpp:31:36: warning: ignoring return value of ‘FILE* freopen(const char*, const char*, FILE*)’, declared with attribute warn_unused_result [-Wunused-result] freopen("fof.out", "w", stdout); ^ user.cpp:34:27: warning: ignoring return value of ‘int scanf(const char*, ...)’, declared with attribute warn_unused_result [-Wunused-result] scanf("%d %d", &n, &m); ^ user.cpp:37:31: warning: ignoring return value of ‘int scanf(const char*, ...)’, declared with attribute warn_unused_result [-Wunused-result] scanf("%d %d", &x, &y); ^