Compilare: user.cpp: In function ‘int main()’: user.cpp:22:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i=0; i<b.size(); i++) ^ user.cpp:26:31: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int j=0; j<a.size() && sem; j++) ^ user.cpp: In function ‘void pref(std::string)’: user.cpp:52:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i=1; i<a.size(); i++) ^