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