[gukjoon]

This is a great question. I can't speak to Facebook, but I ask hard algorithm questions and this is why I do it.

The point of these hard questions is not to assess if you know some obscure algorithm. The algorithms in these questions are obscure precisely because you're not supposed to know them. If the candidate clearly knows the algorithm I switch to a new question.

The point of these questions is to assess your ability to perform under pressure in an unfamiliar situation, which is a fundamental skill that all productive engineers require. I expect that any engineer I greenlight to encounter novel situations. These are not necessarily Hard Problems dun dun dun, but these situations will challenge your problem solving skills and you won't be able to look up the right answer on StackOverflow. I expect candidates to have the ability to deliver a solution that works and makes the right tradeoff between time, quality, and scope.

I have also had interviewers ask hard problems for the wrong reasons. Frowntown to that.