Welcome to your interview.
The question is on your right: Top-K Trending Topics. Take a moment with it first.
Talk your thinking through with me if you like - when you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes). Discussion and graded submissions share your five interviewer interactions, so spend them well.
Given a set of user engagement logs, identify the top K trending topics within a specific time window.