input: Write a python function to calculate the factorial of a number.
  assert factorial(4) == 24
  assert factorial(5) == 120
  assert factorial(6) == 720
output: def factorial(n):
      if n == 0:
          return 0
      elif n == 1:
          return 1
      else:
          result = 1
          for i in range(2, n + 1):
              result *= i
          return result