/in/foo.cc: In function 'int main()':
/in/foo.cc:72:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int i=0;i<a.length();i++) b[a[i]]++;
                ^
/in/foo.cc:72:39: warning: array subscript has type 'char' [-Wchar-subscripts]
   for(int i=0;i<a.length();i++) b[a[i]]++;
                                       ^