input: def factorial(n):
      """
      Calculate the factorial of a number.
      >>> factorial(0)
      1
      >>> factorial(5)
      120
      >>> factorial(10)
      3628800
      """
      if n == 0:
          return 1
      else:
          return n * factorial(n-1)
output:     if n == 0:
          return 1
      else:
          return n * factorial(n-1)