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. The input should be provided as a parameter to the function.
  assert countdown(6)
  Output:
  6
  5
  4
  3
  2
  1
  Done!
output: def countdown(n):
      while n > 0:
          print(n)
          n -= 1
      print("Done!")