class Twitter {

    // timestamp is incremented each time we have new tweet
    int timestamp;

    // stores tweetId and userId of tweet which was done at timestamp
    ArrayList<ArrayList<Integer>> tweet_with_timestamp;

    // stores userIds which a user follows
    Map<Integer, Set<Integer>> follow;

    // stores timestamps of tweets done by a user
    Map<Integer, ArrayList<Integer>> tweet;

    public Twitter()
    {
        this.timestamp = 0;
        this.tweet_with_timestamp = new ArrayList<ArrayList<Integer>> ();
        this.follow = new HashMap<Integer, Set<Integer>> ();
        this.tweet = new HashMap<Integer, ArrayList<Integer>> ();
    }

    public void postTweet(int userId, int tweetId)
    {
        tweet.putIfAbsent(userId, new ArrayList<Integer> ());
        tweet.get(userId).add(timestamp);
        tweet_with_timestamp.add(new ArrayList<Integer> (List.of(tweetId, userId)) );
        timestamp++;
    }

    public List<Integer> getNewsFeed(int userId)
    {
        // Adding user to its list of followees
        // This will allow us to consider tweets done by the user itself
        follow.putIfAbsent(userId, new HashSet<Integer>());
        follow.get(userId).add(userId);

        // pointer stores the position of the most recent tweet done by a followee
        Map<Integer, Integer> pointer = new HashMap<Integer, Integer> ();

        // priority queue stores the timestamp of the most recent tweet done by each followee
        PriorityQueue<Integer> pq = new PriorityQueue<Integer> (Collections.reverseOrder());

        for(Integer followeeId: follow.get(userId))
        {
            if(tweet.containsKey(followeeId))
            {
                int p = tweet.get(followeeId).size() - 1;
                pointer.put(followeeId, p);
                pq.add(tweet.get(followeeId).get(p));
            }
        }

        ArrayList<Integer> NewsFeed = new ArrayList<Integer> ();
        for(int i = 0; i < 10 && !pq.isEmpty(); i++)
        {
            int t = pq.poll(); // timestamp of most recent tweet
            int tweetId = tweet_with_timestamp.get(t).get(0);
            int tweetUserId = tweet_with_timestamp.get(t).get(1);

            NewsFeed.add(tweetId);
            int p = pointer.get(tweetUserId);
            if(p > 0)
            {
                // Adding the timestamp of the next most recent tweet done by the tweetUser in priority queue
                pointer.put(tweetUserId, p-1);
                pq.add(tweet.get(tweetUserId).get(p-1));
            }
        }
        return NewsFeed;
    }

    public void follow(int followerId, int followeeId)
    {
        follow.putIfAbsent(followerId, new HashSet<Integer>());
        follow.get(followerId).add(followeeId);
    }

    public void unfollow(int followerId, int followeeId)
    {
        if(follow.containsKey(followerId)) {
            follow.get(followerId).remove(followeeId);
        }
    }
}