I'm pro make them code and test their big o.

Honestly it's 30-45 minutes where you can establish whether the person can code, and whether they have the basic foundational knowledge to crack efficiency problems is pretty hard to beat.

Whilst there's probably diminishing returns on making the actual challenge more and more difficult, the general concept is a lot fairer than the majority of other interview types I've had thrown at me. ( Usually something they've solved internally, where they expect you to regurgitate the same answers without the same context )