Compilare: user.cpp: In function ‘void back(std::string, std::string, int)’: user.cpp:11:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if(a.size()==n/2 && b.size()==n/2 && a.compare(b)==0){ ^ user.cpp:11:35: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if(a.size()==n/2 && b.size()==n/2 && a.compare(b)==0){ ^ user.cpp:15:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if(a.size()>n/2 || b.size()>n/2) return; ^ user.cpp:15:33: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if(a.size()>n/2 || b.size()>n/2) return; ^