I always prefer to say "I've seen a similar problem before, let's see if I can remember how to solve that" or "let's see if the same kind of approach works here".

This is a balanced approach. It's honest: you probably haven't heard the exact question word for word, even if you can't recognize the differences. And it's actually what they're selecting for in the first place. They're not expecting you to invent the concept of a binary tree, or whatever, but to know it exists and how to implement it and to recognize where it might be the right concept to apply.

I've never had any interviewer say "OK forget it, let me give you another question where you'll never have seen something similar".