Compilare: user.cpp: In function ‘void Add(std::string)’: user.cpp:23:31: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = 0; i < s.size(); i++) ^ user.cpp: In function ‘void Erase(std::string)’: user.cpp:35:31: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = 0; i < s.size(); i++) ^ user.cpp: In function ‘int Freq(std::string)’: user.cpp:46:31: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = 0; i < s.size(); i++) ^ user.cpp: In function ‘int Prefix(std::string)’: user.cpp:60:31: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = 0; i < s.size(); i++) ^