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.

1. getScore(article_id)

• Returns (up_count - down_count) for the article.

1. 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).