input: Write a python function to compute the factorial of a given number.
  assert factorial(5) == 120
  assert factorial(0) == 1
  assert factorial(3) == 6
  assert factorial(-5) == None
output: def factorial(n):
      if n < 0:
          return None
      elif n == 0:
          return 1
      else:
          result = 1
          for i in range(1, n + 1):
              result *= i
          return result