foo.cc: In function 'int main()':
foo.cc:17:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<a.size();i++)
              ~^~~~~~~~~
foo.cc:19:9: warning: array subscript has type 'char' [-Wchar-subscripts]
   b[a[i]]++;
         ^

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