input: Write a python function to count down from a given positive integer to zero.
  assert countdown(5) == [5, 4, 3, 2, 1, 0]
  assert countdown(10) == [10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0]
  assert countdown(3) == [3, 2, 1, 0]
output: def countdown(n):
      return list(range(n, -1, -1))