input: Write a function to calculate the factorial of a given number.
  assert factorial(5) == 120
  assert factorial(0) == 1
  assert factorial(7) == 5040
output: def factorial(n):
      if n == 0:
          return 1
      else:
          result = 1
          for i in range(1, n + 1):
              result *= i
          return result