Article Vote Tracker

Got the Article Vote Tracker question. Part 1 involved adding articles, recording up/down votes, detecting vote flips, and returning the last 3 unique flipped articles. The follow-up asked how to handle a very large flip history. The key optimization is to maintain only the last 3 unique flipped article IDs per user instead of storing the complete history. I completed Part 1 but did not proceed to the next part.