Comment by ksk

The question isn't whether you should reinvent the wheel, it's whether you can if need be. It is important for companies to weed out candidates that can talk a good game but are useless when it comes to actual programming. So maybe in the process good candidates fall through the cracks, but then again, nobody has invented a perfect way to interview candidates.

In any case, how would you even go about knowing whether a certain piece of existing code that you blindly pulled in from some random place is optimized for your particular use case without first being able to conceptually understand it - which is all an interview is. Nobody expects candidates to create production ready code on a whiteboard.