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