input: Write a function to take a positive integer and use a while loop to output the numbers counting down to 1. The function should print the numbers sequentially and finally output "Done!".
  
  def countdown(num):
      while num > 0:
          print(num)
          num -= 1
      print("Done!")
  
  assert countdown(6)==None
  assert countdown(3)==None
  assert countdown(1)==None
  assert countdown(0)==None
  assert countdown(10)==None
output: def countdown(num):
      while num > 0:
          print(num)
          num -= 1
      print("Done!")