/in/foo.cc: In function 'int main()':
/in/foo.cc:9:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=0;i<a.size();i++)
                 ~^~~~~~~~~
/in/foo.cc:14:18: warning: array subscript has type 'char' [-Wchar-subscripts]
          cnt[a[i]]++;
                  ^