Compilare: user.cpp: In function ‘void back(int)’: user.cpp:9:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] {if(k==f.size()) ^ user.cpp:13:26: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(i=0;i<f.size();i++) ^ user.cpp: In function ‘int main()’: user.cpp:38:35: warning: ignoring return value of ‘FILE* freopen(const char*, const char*, FILE*)’, declared with attribute warn_unused_result [-Wunused-result] {freopen ("armonica.in","r",stdin); ^ user.cpp:39:36: warning: ignoring return value of ‘FILE* freopen(const char*, const char*, FILE*)’, declared with attribute warn_unused_result [-Wunused-result] freopen ("armonica.out","w",stdout); ^ user.cpp:41:17: warning: ignoring return value of ‘int scanf(const char*, ...)’, declared with attribute warn_unused_result [-Wunused-result] scanf("%lld",&n); ^