input: Write a function to implement the given instruction of creating a countdown program using a while loop. The function should take a positive integer as input and output the numbers counting down to 1 using a while loop, followed by the message "Done!".
  
  def countdown(num):
      while num > 0:
          print(num)
          num -= 1
      print("Done!")
  
  assert countdown(6)
  assert countdown(3)
  assert countdown(1)
  assert countdown(0)==None
  assert countdown(-5)==None
output: def countdown(num):
      while num > 0:
          print(num)
          num -= 1
      print("Done!")