input: Write a function that takes a positive integer as input and uses a while loop to output the numbers counting down to 1. After the loop is done, print "Done!".
  
  assert countdown(5)== None
  assert countdown(3)== None
  assert countdown(1)== None
output: def countdown(n):
      while n > 0:
          print(n)
          n -= 1
      print("Done!")