Twitter.java

import java.util.*;

public class Twitter
{
    Map<Integer, User> users;
    Map<Integer, List<Integer>> userTweets;
    Map<Integer, Tweet> tweets;
    int time = 0;

    public Twitter()
    {
        users = new HashMap<>();
        tweets = new HashMap<>();
        userTweets = new HashMap<>();
    }

    public void postTweet(int userId, int tweetId)
    {
        users.putIfAbsent(userId, new User(userId));
        userTweets.putIfAbsent(userId, new ArrayList<>());
        userTweets.get(userId).add(tweetId);
        tweets.put(tweetId, new Tweet(tweetId, time));
        time++;
    }

    public List<Integer> getNewsFeed(int userId)
    {
        if (!users.containsKey(userId))
        {
            return new ArrayList<>();
        }
        PriorityQueue<Tweet> queue = new PriorityQueue<>(new Comparator<Tweet>()
        {
            @Override
            public int compare(Tweet o1, Tweet o2)
            {
                return o2.time - o1.time;
            }
        });
        addToQueue(userId, queue);
        for (Integer followeeId : users.get(userId).follows)
        {
            addToQueue(followeeId, queue);
        }
        List<Integer> newsFeed = new ArrayList<>();
        while (!queue.isEmpty() && newsFeed.size() < 10)
        {
            newsFeed.add(queue.poll().tweetId);
        }
        return newsFeed;
    }

    private void addToQueue(int userId, PriorityQueue<Tweet> queue)
    {
        for (Integer tweetId : userTweets.getOrDefault(userId, new ArrayList<>()))
        {
            queue.add(tweets.get(tweetId));
        }
    }

    public void follow(int followerId, int followeeId)
    {
        if (followerId == followeeId)
        {
            return;
        }
        users.putIfAbsent(followerId, new User(followerId));
        users.putIfAbsent(followeeId, new User(followeeId));
        users.get(followerId).follows.add(followeeId);
    }

    public void unfollow(int followerId, int followeeId)
    {
        users.putIfAbsent(followerId, new User(followerId));
        users.putIfAbsent(followeeId, new User(followeeId));
        users.get(followerId).follows.remove(new Integer(followeeId));
    }

    class User
    {
        int userId;
        HashSet<Integer> follows = new HashSet<Integer>();

        public User(int userId)
        {
            super();
            this.userId = userId;
        }
    }

    class Tweet
    {
        int tweetId;
        int time;

        public Tweet(int tweetId, int time)
        {
            super();
            this.tweetId = tweetId;
            this.time = time;
        }
    }
}