Compilare: user.cpp: In function ‘void add(std::string)’: user.cpp:26:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i=0; i<s.size(); i++) ^ user.cpp: In function ‘void del(std::string)’: user.cpp:38:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i=0; i<s.size(); i++) ^ user.cpp: In function ‘int occ(std::string)’: user.cpp:55:27: 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:66:28: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i=0; i<=s.size(); i++) ^