input: Write a function to calculate the factorial of a given integer. The function should take in an integer as an argument and return the factorial of that integer. If the given integer is less than or equal to 1, the function should return 1.
  
  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(3) == 6
  assert factorial(7) == 5040
output: def factorial(num):
      if num <= 1:
          return 1
      else:
          return num * factorial(num - 1)