input: Write a python function to calculate the sum of digits of a number recursively.
  assert sum_of_digits(1234) == 10
  assert sum_of_digits(5678) == 26
  assert sum_of_digits(0) == 0
  assert sum_of_digits(1111) == 4
output: def sum_of_digits(n):
      if n == 0:
          return 0
      else:
          return n % 10 + sum_of_digits(n // 10)