input: Write a function to calculate the factorial of a number using Python 3. If the number is 0, the function should return 1. Otherwise, it should return the product of the number and the factorial of the number minus one.
  assert factorial(0)==1
  assert factorial(1)==1
  assert factorial(3)==6
  assert factorial(5)==120
  assert factorial(10)==3628800
output: def factorial(n):
      if n == 0:
          return 1
      else:
          return n * factorial(n - 1)