input: Write a function that calculates the factorial of a given number using recursion. The function should return 1 if the input is 0, otherwise it should return the product of the input number and the factorial of the number minus one.
  def factorial(n):
   if n == 0:
    return 1
   else:
    return n * factorial(n-1)
  
  assert factorial(0)==1
  assert factorial(1)==1
  assert factorial(5)==120
  assert factorial(3)==6
  assert factorial(4)==24
output: def factorial(n):
   if n == 0:
    return 1
   else:
    return n * factorial(n-1)