input: Write a function to compute the factorial of a given number. If the input number is negative, return an error message. If the input is zero, return 1.
  assert factorial(7)==5040
  assert factorial(0)==1
  assert factorial(-1)=="Error: Negative input is not allowed"
output: def factorial(n):
      if n < 0:
          return "Error: Negative input is not allowed"
      elif n == 0:
          return 1
      else:
          result = 1
          for i in range(1, n + 1):
              result *= i
          return result