Compilare: user.cpp: In function ‘void solve(std::string)’: user.cpp:20:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(i=1; i<s.size(); i++) ^ user.cpp: In function ‘char conversion(char, char)’: user.cpp:13:1: warning: control reaches end of non-void function [-Wreturn-type] } ^