digitSum

Write a method digitSum that takes a non-negative
   integer as a parameter and that returns the sum of its digits.  For example,
   digitSum(20879) should return 26 (2 + 0 + 8 + 7 + 9).  You may assume that
   the method is passed a value greater than or equal to 0.  You may not use a
   String to solve this problem; you must solve it using integer arithmetic.

   Write your solution to digitSum below.