Twitter.java

package design;

import java.util.*;

/**
 * Created by gouthamvidyapradhan on 18/03/2017.
 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:

 postTweet(userId, tweetId): Compose a new tweet.
 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.
 follow(followerId, followeeId): Follower follows a followee.
 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);
 */
public class Twitter
{
    class User
    {
        int id;
        Set<Integer> follow = new HashSet<>();
        Queue<Tweet> tweets = new ArrayDeque<>();
        User(int id)
        {
            this.id = id;
        }
        public void follow(int id)
        {
            follow.add(id);
        }
        public void addTweet(Tweet t)
        {
            if(tweets.size() == 10)
            {
                tweets.poll();
            }
            tweets.offer(t);
        }
        public void unFollow(int id)
        {
            follow.remove(id);
        }

        public Set<Integer> getFollow() {
            return follow;
        }

        public Queue<Tweet> getTweets() {
            return tweets;
        }
    }

    class Tweet
    {
        int id;
        long time;
        Tweet(int id, long time)
        {
            this.id = id;
            this.time = time;
        }
    }

    private Map<Integer, User> userMap;
    private Map<Integer, Tweet> tweetMap;
    private static long tweetCount = 0L;
    /**
     * Main method
     * @param args
     * @throws Exception
     */
    public static void main(String[] args) throws Exception
    {
        Twitter twitter = new Twitter();
        twitter.postTweet(1, 5);
        twitter.follow(1, 1);
        System.out.println(twitter.getNewsFeed(1));
        /*twitter.follow(2, 1);
        System.out.println(twitter.getNewsFeed(2));
        //twitter.unfollow(2, 1);
        twitter.postTweet(2, 3);
        System.out.println(twitter.getNewsFeed(1));
        System.out.println(twitter.getNewsFeed(2));
        twitter.follow(1, 2);
        System.out.println(twitter.getNewsFeed(1));
        twitter.unfollow(2, 1);
        System.out.println(twitter.getNewsFeed(2));
        System.out.println(twitter.getNewsFeed(1));
        //twitter.getNewsFeed(2);
        */
    }

    /** Initialize your data structure here. */
    public Twitter()
    {
        userMap = new HashMap<>();
        tweetMap = new HashMap<>();
    }

    /** Compose a new tweet. */
    public void postTweet(int userId, int tweetId)
    {
        User user = userMap.get(userId);
        if(user == null)
        {
            user = new User(userId);
            userMap.put(userId, user);
        }
        user.addTweet(new Tweet(tweetId, tweetCount++));
    }

    /** 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)
    {
        User user = userMap.get(userId);
        List<Integer> result = new ArrayList<>();
        if(user == null) return result;
        Set<Integer> follwers = user.getFollow();
        if(follwers != null)
        {
            List<Tweet> tweets = new ArrayList<>();
            tweets.addAll(user.getTweets());
            for(Integer i : follwers)
            {
                User f = userMap.get(i);
                if(f != null)
                {
                    tweets.addAll(f.getTweets());
                }
            }
            Comparator<Tweet> comparator = new Comparator<Tweet>() {
                @Override
                public int compare(Tweet o1, Tweet o2) {
                    return Long.compare(o2.time, o1.time);
                }
            };

            Collections.sort(tweets, comparator);

            for(int i = 0; i < 10; i ++)
            {
                if(i >= tweets.size())
                    break;
                result.add(tweets.get(i).id);
            }
        }
        return result;
    }

    /** Follower follows a followee. If the operation is invalid, it should be a no-op. */
    public void follow(int followerId, int followeeId)
    {
        if(followerId == followeeId) return;
        User user = userMap.get(followerId);
        if(user == null)
            user = new User(followerId);
        userMap.put(followerId, user);
        if(userMap.get(followeeId) != null)
        {
            user.follow(userMap.get(followeeId).id);
        }
        else
        {
            User newUser = new User(followeeId);
            userMap.put(followeeId, newUser);
            user.follow(userMap.get(followeeId).id);
        }
    }

    /** Follower unfollows a followee. If the operation is invalid, it should be a no-op. */
    public void unfollow(int followerId, int followeeId)
    {
        User user = userMap.get(followerId);
        if(user != null)
        {
            user.unFollow(followeeId);
        }
    }

}