Mini Twitter

Design a simplified version of Twitter where users can post tweets, follow/unfollow another user and is able to see the 10 most recent tweets in the user's news feed. Your design should support the following methods:

1. postTweet(userId, tweetId)

   : Compose a new tweet.

2. getNewsFeed(userId)

   : Retrieve the 10 most recent tweet ids in the user's news feed. Each item in the news feed must be posted by users who the user followed or by the user herself. Tweets must be ordered from most recent to least recent.

3. follow(followerId, followeeId)

   : Follower follows a followee.

4. unfollow(followerId, followeeId)

   : Follower unfollows a followee.

Example:

Twitter twitter = new Twitter();

// User 1 posts a new tweet (id = 5).
twitter.postTweet(1, 5);

// User 1's news feed should return a list with 1 tweet id -> [5].
twitter.getNewsFeed(1);

// User 1 follows user 2.
twitter.follow(1, 2);

// User 2 posts a new tweet (id = 6).
twitter.postTweet(2, 6);

// User 1's news feed should return a list with 2 tweet ids -> [6, 5].
// Tweet id 6 should precede tweet id 5 because it is posted after tweet id 5.
twitter.getNewsFeed(1);

// User 1 unfollows user 2.
twitter.unfollow(1, 2);

// User 1's news feed should return a list with 1 tweet id ->[5],
// since user 1 is no longer following user 2.
twitter.getNewsFeed(1);

Implementation

public class Twitter {
    private static int timeStamp = 0;

    class TweetNode {
        int tweetId;
        int time;
        TweetNode next;

        public TweetNode(int tweetId) {
            this.tweetId = tweetId;
            this.time = timeStamp++;
        }
    }

    class User {
        int userId;
        TweetNode tweetHead;
        Set<Integer> followees;

        public User(int userId) {
            this.userId = userId;
            followees = new HashSet<>();
            tweetHead = null;
            // User follow himself to getNewsFeed
            follow(userId);
        }

        public void follow(int userId) {
            followees.add(userId);
        }

        public void unfollow(int userId) {
            followees.remove(userId);
        }

        public void post(int tweetId) {
            TweetNode tweet = new TweetNode(tweetId);
            tweet.next = tweetHead;
            tweetHead = tweet;
        }
    }

    private Map<Integer, User> users;

    /** Initialize your data structure here. */
    public Twitter() {
        users = new HashMap<Integer, User>();
    }

    /** Compose a new tweet. */
    public void postTweet(int userId, int tweetId) {
        if (!users.containsKey(userId)) {
            User user = new User(userId);
            users.put(userId, user);
        }
        users.get(userId).post(tweetId);
    }

    /** Retrieve the 10 most recent tweet ids in the user's news feed. Each item in the news feed must be posted by users who the user followed or by the user herself. Tweets must be ordered from most recent to least recent. */
    public List<Integer> getNewsFeed(int userId) {
        List<Integer> newsFeed = new ArrayList<>();

        if (!users.containsKey(userId)) {
            return newsFeed;
        }

        PriorityQueue<TweetNode> pq = new PriorityQueue<>((node1, node2) -> (node2.time - node1.time));
        Set<Integer> followees = users.get(userId).followees;

        for (int followee : followees) {
            if (users.containsKey(followee)) {
                TweetNode tweet = users.get(followee).tweetHead;

                if (tweet != null) {
                    pq.add(tweet);
                }
            }
        }

        int count = 0;
        while (!pq.isEmpty() && count < 10) {
            TweetNode tweet = pq.remove();
            ++count;
            newsFeed.add(tweet.tweetId);

            if (tweet.next != null) {
                pq.add(tweet.next);
            }
        }
        return newsFeed;
    }

    /** Follower follows a followee. If the operation is invalid, it should be a no-op. */
    public void follow(int followerId, int followeeId) {
        if (!users.containsKey(followerId)) {
            User follower = new User(followerId);
            users.put(followerId, follower);
        }

        if (!users.containsKey(followeeId)) {
            User followee = new User(followeeId);
            users.put(followeeId, followee);
        }

        users.get(followerId).follow(followeeId);
    }

    /** Follower unfollows a followee. If the operation is invalid, it should be a no-op. */
    public void unfollow(int followerId, int followeeId) {
        if (followerId == followeeId || !users.containsKey(followerId)) {
            return;
        }
        users.get(followerId).unfollow(followeeId);
    }
}