input: 
  def factorial(n: int) -> int:
      """ Calculate the factorial of a given number using a recursive approach.
      >>> 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)