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 ‘int occurs(std::string)’: user.cpp:63:27: 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:76:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i=0; i<s.size(); i++) ^