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. The function should print "Done!" after the loop completes. If the input is not a positive integer, the function should print "Invalid input".
  assert countdown_to_1(6)=="6\n5\n4\n3\n2\n1\nDone!\n"
  assert countdown_to_1(-3)=="Invalid input\n"
  assert countdown_to_1(0)=="Invalid input\n"
  assert countdown_to_1(1)=="1\nDone!\n"
output: def countdown_to_1(n):
      if n <= 0:
          return "Invalid input\n"
      result = ""
      while n > 0:
          result += str(n) + "\n"
          n -= 1
      result += "Done!\n"
      return result