foo.cc: In function 'int main()':
foo.cc:15:9: warning: array subscript has type 'char' [-Wchar-subscripts]
   a[s[i]]++;
         ^
foo.cc:18:12: warning: array subscript has type 'char' [-Wchar-subscripts]
   if(a[s[i]]>max)
            ^
foo.cc:19:14: warning: array subscript has type 'char' [-Wchar-subscripts]
    max=a[s[i]];
              ^
foo.cc:21:12: warning: array subscript has type 'char' [-Wchar-subscripts]
   if(a[s[i]]==max)
            ^

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