Compilare: user.cpp: In function ‘int conv(std::string)’: user.cpp:15:25: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i=0; i<s.size(); ++i) r=r*10+s[i]-'0'; ^ user.cpp: In function ‘int main()’: user.cpp:24:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i=0; i<s.size(); ++i) rev[s[i]]=char((i+1)+'0'); ^ user.cpp:24:43: warning: array subscript has type ‘char’ [-Wchar-subscripts] for(int i=0; i<s.size(); ++i) rev[s[i]]=char((i+1)+'0'); ^ user.cpp:27:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i=0; i<s.size(); ++i) { ^ user.cpp:40:31: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int j=0; j<s.size(); ++j) s[j]=rev[s[j]]; ^ user.cpp:40:52: warning: array subscript has type ‘char’ [-Wchar-subscripts] for(int j=0; j<s.size(); ++j) s[j]=rev[s[j]]; ^