input: Write a function to simulate a countdown from a given positive integer to 1 using a while loop. The function should take a positive integer as input and print the numbers from the given integer down to 1, then print "Done!".
  assert countdown(6)
  assert countdown(3)
  assert countdown(1)
  assert countdown(0)
  assert countdown(10)
output: def countdown(n):
      while n > 0:
          print(n)
          n -= 1
      print("Done!")