[zeemonkee3]

He's a developer, not a telepath.

In tests like this you make it clear what's allowed - "use the standard library, no external dependencies" etc. Because in a normal working situation, his solution is perfectly valid.

If I were interviewing him it certainly wouldn't be a dealbreaker. I'd probably want to discuss more about pros and cons of in-house vs using libraries, because you tend to find out more about the skills and qualities of a developer from human conversation than scribbling on a whiteboard.

[mruniverse]

In an interview you are trying to show that you understand the problem. Not just get the right answer.

[zeemonkee3]

How do I know that? What is the interviewer looking for?

Maybe in company A they are looking for developers who come up with a solution on their own and don't rely on external libraries or Google. Maybe company B don't like developers reinventing wheels and prefer they at least research prior art first.

I don't know what kind of culture your company has, but at the very least out of courtesy you can signal what the requirements are in your test.

[blub]

Alas, there are interviews where the correct solution is using a library and writing your own is wrong, because it shows a NIH attitude.

One has to ask, but then the mere act of asking might also annoy some interviewers.