class Twitter {

    Map<Integer, Set<Integer>> follow;
    ArrayList<ArrayList<Integer>> tweet;

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

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

    public List<Integer> getNewsFeed(int userId) {
        ArrayList<Integer> NewsFeed = new ArrayList<Integer> ();
        for(int i = tweet.size() - 1; i >= 0 && NewsFeed.size() < 10; i--)
        {
            int tweetUserId = tweet.get(i).get(0);
            int tweetId = tweet.get(i).get(1);

            if(tweet.get(i).get(0) == userId) {
                NewsFeed.add(tweetId);
            }
            else if(follow.containsKey(userId) && follow.get(userId).contains(tweetUserId)) {
                NewsFeed.add(tweetId);
            }
        }
        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);
        }
    }
}

/**
 * Your Twitter object will be instantiated and called as such:
 * Twitter obj = new Twitter();
 * obj.postTweet(userId,tweetId);
 * List<Integer> param_2 = obj.getNewsFeed(userId);
 * obj.follow(followerId,followeeId);
 * obj.unfollow(followerId,followeeId);
 */