input: Write a function to calculate the factorial of a given number using recursion. If the number is 1 then return 1. Otherwise, return the product of the number and the factorial of the number minus one.
  def factorial(n):
     if n == 1:
        return 1
     else:
        return n * factorial(n - 1)
  
  assert factorial(5)==120
  assert factorial(1)==1
  assert factorial(4)==24
  assert factorial(0)==1
output: def factorial(n):
     if n == 1 or n == 0:
        return 1
     else:
        return n * factorial(n - 1)