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