input: Write a function to take in a positive integer and use a while loop to output the numbers counting down to 1. After the loop is done, print "Done!".
  
  assert countdown(6)==6
  assert countdown(3)==3
  assert countdown(1)==1
  assert countdown(10)==10
  assert countdown(5)==5
output: def countdown(n):
      while n > 0:
          print(n)
          n -= 1
      print("Done!")
      return n + 1