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(7)==5040
  assert factorial(0)==1
  assert factorial(1)==1
  assert factorial(5)==120
  assert factorial(3)==6
output: def factorial(num):
      if num <= 1:
          return 1
      else:
          return num * factorial(num - 1)