I think the concern about whether or not candidates have seen this, or any, programming question before is missing the point. Think about what we want in the ideal candidate -- we want them to come up with a good (elegant, efficient) solution to the problem, and implement it. We (judging by all the other responses) expect them to do that because they've had a solid CS education (formal or informal) as well as significant experience.
But people with that background will give good answers, even if they haven't seen _this specific problem_, because they have seen lots of problems like it and recognize the pattern. And even in that case, we evaluate them based on how well they can implement the pattern they saw, not just on whether they recognized the correct algorithm. So what if they've seen this problem already? Coding it up efficiently and elegantly in an interview context is still non-trivial, and you can still push them to discuss edge cases and performance tradeoffs.
The person who really has _never_ seen anything like this in his life, and still can give a good answer, I have yet to meet.
But people with that background will give good answers, even if they haven't seen _this specific problem_, because they have seen lots of problems like it and recognize the pattern. And even in that case, we evaluate them based on how well they can implement the pattern they saw, not just on whether they recognized the correct algorithm. So what if they've seen this problem already? Coding it up efficiently and elegantly in an interview context is still non-trivial, and you can still push them to discuss edge cases and performance tradeoffs.
The person who really has _never_ seen anything like this in his life, and still can give a good answer, I have yet to meet.