foo.cc: In function 'int main()':
foo.cc:12:13: warning: array subscript has type 'char' [-Wchar-subscripts]
  while(sum[i])
             ^
foo.cc:14:11: warning: array subscript has type 'char' [-Wchar-subscripts]
   if(sum[i]!=a)
           ^
foo.cc:15:15: warning: array subscript has type 'char' [-Wchar-subscripts]
    cout<<sum[i];
               ^

自豪的采用jd5进行评测(github.com/masnn/jd5)