foo.cc: In function 'int main()':
foo.cc:25:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=0;i<=a.length()-1;++i)
                 ~^~~~~~~~~~~~~~
foo.cc:26:16: warning: array subscript has type 'char' [-Wchar-subscripts]
         js[a[i]]++;
                ^

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