input: Write a function to output numbers counting down to 1 from a given positive integer using a while loop. If the input number is less than or equal to 0, the function should print "Invalid input".
  assert countdown(6)=="6\n5\n4\n3\n2\n1\nDone!"
  assert countdown(3)=="3\n2\n1\nDone!"
  assert countdown(0)=="Invalid input"
  assert countdown(-5)=="Invalid input"
output: def countdown(n):
      if n <= 0:
          return "Invalid input"
      result = ""
      while n > 0:
          result += f"{n}\n"
          n -= 1
      result += "Done!"
      return result