foo.cc: In function 'int cmp(std::__cxx11::string, std::__cxx11::string)':
foo.cc:19:12: warning: array subscript has type 'char' [-Wchar-subscripts]
 if (a[a1[i]]!=a[a2[i]]) return a[a1[i]]<a[a2[i]];
            ^
foo.cc:19:22: warning: array subscript has type 'char' [-Wchar-subscripts]
 if (a[a1[i]]!=a[a2[i]]) return a[a1[i]]<a[a2[i]];
                      ^
foo.cc:19:39: warning: array subscript has type 'char' [-Wchar-subscripts]
 if (a[a1[i]]!=a[a2[i]]) return a[a1[i]]<a[a2[i]];
                                       ^
foo.cc:19:48: warning: array subscript has type 'char' [-Wchar-subscripts]
 if (a[a1[i]]!=a[a2[i]]) return a[a1[i]]<a[a2[i]];
                                                ^
foo.cc: In function 'int main()':
foo.cc:25:30: warning: array subscript has type 'char' [-Wchar-subscripts]
 for (int i=0;i<26;i++) a[b[i]]=i;
                              ^

自豪的采用HydroJudge(https://github.com/hydro-dev/Hydro)进行评测。