Compilare: user.cpp: In function ‘void reverse_polish(std::string&)’: user.cpp:74:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] while (pos < s.size()) { ^ user.cpp: In function ‘int op_hash(char)’: user.cpp:28:1: warning: control reaches end of non-void function [-Wreturn-type] } ^