input: 
  def factorial(n: int) -> int:
      """ Calculate the factorial of a given number.
      >>> factorial(0)
      1
      >>> factorial(5)
      120
      >>> factorial(10)
      3628800
      """
  
output:     if n == 0:
          return 1
      result = 1
      for i in range(1, n + 1):
          result *= i
      return result