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