/in/foo.cc: In function 'int main()':
/in/foo.cc:8:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=0;i<strlen(a);i++)
                 ~^~~~~~~~~~
/in/foo.cc:13:18: warning: array subscript has type 'char' [-Wchar-subscripts]
          num[a[i]]++;
                  ^