input: Write a function to count down from a given positive integer to 1 using a while loop and then print "Done!". If the input is not a positive integer, return "Invalid input". 
  
  def count_down_to_one(num):
      if num <= 0:
          return "Invalid input"
      while num > 0:
          print(num)
          num -= 1
      print("Done!")
  
  assert count_down_to_one(6) == None
  assert count_down_to_one(0) == "Invalid input"
  assert count_down_to_one(-5) == "Invalid input"
output: def count_down_to_one(num):
      if num <= 0:
          return "Invalid input"
      while num > 0:
          print(num)
          num -= 1
      print("Done!")