You said above that you have set LNH for a T3 writing out the obvious solution to your question. The problem is that optimizing that solution comes one of two ways:

1. having seen the specific class of problem before and knowing, as you're writing that solution, to stop and switch to the class of solution you've seen before that would solve the problem more efficiently, or

2. finishing the obvious, poorly performing solution and then optimizing it like an real engineer would do. The problem is that there's not going to be enough time for this to occur in the interview session. The act of writing out the poorly performing solution will consume the entire session.

Therefore, the only way someone would have passed your interview question would have been if they had seen this class of problem before.