Compilare: user.cpp: In function ‘int prefix(std::string, std::string)’: user.cpp:16:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for( ; i<s1.size() && i<s2.size(); ++i) ^ user.cpp:16:37: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for( ; i<s1.size() && i<s2.size(); ++i) ^