/in/foo.cc: In function 'int main()':
/in/foo.cc:17:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int i=0;i<a.length();i++)
                ^
/in/foo.cc:18:9: warning: array subscript has type 'char' [-Wchar-subscripts]
   c[a[i]]++;
         ^
/in/foo.cc:19:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int i=0;i<b.length();i++)
                ^
/in/foo.cc:20:9: warning: array subscript has type 'char' [-Wchar-subscripts]
   c[b[i]]--;
         ^