Compilare: user.cpp: In function ‘std::string rpn(std::string)’: user.cpp:27:26: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i=0;i<l.size();i++) ^ user.cpp:33:28: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if(i==l.size()-1||!cifra(l[i+1])) ^ user.cpp: In function ‘int op(int, int, int)’: user.cpp:74:1: warning: control reaches end of non-void function [-Wreturn-type] } ^