This solution fails for 3 reasons:

1. should be (l < w): given (3,5) next call is (5, -2), then (-2, 5), then (5, -7), etc. until StackOverflow
2. ignoring issue 1, we don't get to add both 1's as the rectangle reduces (5,3) -> (3,2) -> (2,1) -> (1,1) the last one is null instead of 1
3. answer.addAll(null) throws an exception, this is has the same root cause as number 2, but still worth pointing out

Depends on who you ask. For a simple conditional like this I find it more readable than having two return statements, but that's just my opinion

could've one-lined this with the ternary operator

⣿⣿⣿⣿⣿⣿⣿⣿⣿⠿⠛⠋⣉⣉⣉⣉⣉⣉⠙⠛⠿⣿⣿⣿⣿⣿⣿⣿⣿⣿
⣿⣿⣿⣿⣿⡿⠟⢁⣤⣶⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣶⣤⡈⠻⢿⣿⣿⣿⣿⣿
⣿⣿⣿⡿⠋⣠⣾⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣷⣄⠙⢿⣿⣿⣿
⣿⣿⡟⢀⣾⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣷⡀⢻⣿⣿
⣿⡟⢠⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⡄⢻⣿
⣿⢀⣿⣿⣿⠟⠁⣠⣴⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣦⣄⠈⠻⣿⣿⣿⡀⣿
⡇⢸⣿⣿⠋⣠⡾⠿⠛⠛⠛⠿⣿⣿⣿⣿⣿⣿⠿⠛⠛⠛⠻⢷⣄⠙⣿⣿⡇⢸
⡇⢸⣿⣿⣾⣿⢀⣠⣤⣤⣤⣤⣀⣿⣿⣿⣿⣀⣤⣤⣤⣤⣄⡀⣿⣷⣾⣿⡇⢸
⡇⠸⠟⣫⣥⣶⣧⠹⠿⢿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⡿⠿⠏⣼⣶⣬⣍⠻⠇⢸
⡧⣰⣿⣿⣿⣿⣿⢰⣦⣤⣤⣤⣤⣤⣤⣤⣤⣤⣤⣤⣤⣴⡆⣿⣿⣿⣿⣿⣆⢼
⡇⣿⣿⣿⣿⣿⡟⠈⠙⠛⠻⠿⠿⠿⠿⠿⠿⠿⠿⠟⠛⠋⠁⢻⣿⣿⣿⣿⣿⢸
⣿⣌⡻⠿⠿⢋⣴⣦⡀⡀⡀⡀⡀⡀⡀⡀⡀⡀⡀⡀⡀⢀⣴⣦⡙⠿⠿⢟⣡⣾
⣿⣿⣿⣷⣄⠙⢿⣿⣿⣶⣤⣀⡀⡀⡀⡀⡀⡀⣀⣤⣶⣿⣿⡿⠋⣠⣾⣿⣿⣿
⣿⣿⣿⣿⣿⣷⣦⣉⠛⠿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⠿⠛⣉⣴⣾⣿⣿⣿⣿⣿
⣿⣿⣿⣿⣿⣿⣿⣿⣿⣶⣤⣌⣉⣉⣉⣉⣉⣉⣡⣤⣶⣿⣿⣿⣿⣿⣿⣿⣿⣿

My bad, I'm new to code wars and didn't know these were visible elsewhere. Unfortunately I'm not seeing a way to edit or delete the comment.