input: Write a function to calculate the factorial of a number using recursion. If the input number is 0, return 1. Otherwise, return the number multiplied by 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(10)==3628800
output: def factorial(n):
   if n == 0:
    return 1
   else:
    return n * factorial(n-1)