Track article votes and last three flips

## Problem Design an in-memory data structure for voting on articles. Users can vote on an article with either: - **Thumbs up** - **Thumbs down** - **No vote** (user clears their vote) A **flip** is when a user changes their vote from up→down or down→up (clearing a vote is *not* a flip). ## Operations Implement the following operations efficiently: 1) `vote(user_id, article_id, new_vote)` - `new_vote` is one of `{UP, DOWN, NONE}`. - Updates the user’s vote for that article. 2) `getScore(article_id)` - Returns `(up_count - down_count)` for the article. 3) `getLast3Flips(article_id)` - Returns the **last 3 flip events** for the article in chronological order (oldest→newest). - Each flip event should include at least `{user_id, from_vote, to_vote, timestamp_or_sequence}`. ## Complexity requirements - `vote`: O(1) - `getScore`: O(1) - `getLast3Flips`: O(1) ## Notes - Multiple users can vote on the same article. - A user can vote multiple times; only changes may affect counts and flips. - Assume the system runs in a single process (no persistence required).