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, recursively multiply the integer by 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(3)==6
  assert factorial(1)==1
  assert factorial(0)==1
  assert factorial(7)==5040
output: def factorial(num):
      if num <= 1:
          return 1
      else:
          return num * factorial(num - 1)