input: from typing import List
  
  def understand_recursion(n: int) -> int:
      """ Recursive function to calculate the factorial of a given number.
      >>> understand_recursion(5)
      120
      >>> understand_recursion(1)
      1
      >>> understand_recursion(0)
      1
      """
      if n <= 1:
          return 1
      else:
          return n * understand_recursion(n - 1)
output:     if n <= 1:
          return 1
      else:
          return n * understand_recursion(n - 1)