input: Write a function to create a Python program that takes in a positive integer and uses a while loop to output the numbers counting down to 1, followed by printing "Done!". If the input is not a positive integer, the function should return an appropriate error message.
  assert countdown(6)=="""6\n5\n4\n3\n2\n1\nDone!"""
  assert countdown(0)=="Positive integer required."
  assert countdown(-3)=="Positive integer required."
  assert countdown(1)=="""1\nDone!"""
output: def countdown(n):
      if n <= 0:
          return "Positive integer required."
      result = ""
      while n > 0:
          result += str(n) + "\n"
          n -= 1
      result += "Done!"
      return result