/in/foo.cc: In function 'int main()':
/in/foo.cc:20:14: warning: array subscript has type 'char' [-Wchar-subscripts]
    if (a[c[i]] != c[len - i - 1])
              ^
/in/foo.cc:13:6: warning: unused variable 'len' [-Wunused-variable]
  int len;
      ^~~