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