Compilare: user.cpp: In function ‘bool used(int)’: user.cpp:15:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i=0;i<=s.size();++i) ^ user.cpp: In function ‘int main()’: user.cpp:29:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i=0;i<=s.size();++i) ^