input: Write a function to calculate the factorial of a given integer. If the given integer is less than or equal to 1, return 1. Otherwise, return the product of the integer and the factorial of the integer minus one.  
  def factorial(num):  
      if num <= 1:  
          return 1  
      else:  
          return num * factorial(num - 1)  
  
  assert factorial(5) == 120  
  assert factorial(0) == 1  
  assert factorial(1) == 1  
  assert factorial(7) == 5040  
  assert factorial(10) == 3628800
output: def factorial(num):  
      if num <= 1:  
          return 1  
      else:  
          return num * factorial(num - 1)