Compilare: user.cpp: In function ‘int check_if_possible(const string&, int)’: user.cpp:13:32: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = len; i < str.size(); ++i){ ^ user.cpp: In function ‘int main()’: user.cpp:34:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] while(str.size() < i){ ^