input: Write a python function to check if a number is divisible by 9 using recursion.
  assert isDivisibleBy9(18) == True
  assert isDivisibleBy9(123) == True
  assert isDivisibleBy9(25) == False
  assert isDivisibleBy9(81) == True
output: def isDivisibleBy9(n):
      if n == 0 or n == 9:
          return True
      if n < 9:
          return False
      sum_of_digits = sum(int(digit) for digit in str(n))
      return isDivisibleBy9(sum_of_digits)