foo.cc: In function 'int main()':
foo.cc:9:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<s.size();i++)
              ~^~~~~~~~~
foo.cc:12:10: warning: array subscript has type 'char' [-Wchar-subscripts]
    a[s[i]]++;
          ^
foo.cc:15:10: warning: array subscript has type 'char' [-Wchar-subscripts]
    a[s[i]]--;
          ^
foo.cc:19:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<s.size();i++)
              ~^~~~~~~~~
foo.cc:20:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int j=i+1;j<s.size();j++)
                 ~^~~~~~~~~
foo.cc:23:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<s.size();i++)
              ~^~~~~~~~~
foo.cc:24:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int j=i+1;j<s.size();j++)
                 ~^~~~~~~~~
foo.cc:31:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<s.size();i++)
              ~^~~~~~~~~
foo.cc:32:17: warning: array subscript has type 'char' [-Wchar-subscripts]
   if(max==a[s[i]])
                 ^

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