Write countdown(n) that returns a list counting down from n to 1, followed by the string "Liftoff!". If n is 0, the list contains only "Liftoff!".
Examples
Input: n = 3
Output: [3, 2, 1, 'Liftoff!']
Input: n = 0
Output: ['Liftoff!']
Constraints
0 <= n <= 10**4
Goals
- Count downwards with range() and a negative step
- Build a list inside a loop and add a final item after it