input: Write a function to calculate the factorial of a given integer. The function should return the factorial of the input number. If the input number 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(0) == 1
  assert factorial(1) == 1
  assert factorial(5) == 120
  assert factorial(7) == 5040
  assert factorial(10) == 3628800
output: def factorial(num):
      if num <= 1:
          return 1
      else:
          return num * factorial(num - 1)