input: Write a function count_down that takes in a positive integer and outputs the numbers counting down to 1 using a while loop. If the input is not a positive integer, the function should return "Invalid input". If the number is 1, it should simply print 1 and "Done!".
  assert count_down(6)=="""6
  5
  4
  3
  2
  1
  Done!"""
  assert count_down(1)=="""1
  Done!"""
  assert count_down(-3)=="Invalid input"
  assert count_down(0)=="Invalid input"
output: def count_down(n):
      if n <= 0:
          return "Invalid input"
      result = ""
      while n > 0:
          result += str(n) + "\n"
          n -= 1
      result += "Done!"
      return result