Design Twitter

class Twitter {

    //list of twitter users    
    Map<Integer, User> users;

    //mapping of userId with the User object
    public Twitter() {
        users = new HashMap<>();
    }
    
    public void postTweet(int userId, int tweetId) {

        //if existing user    
        if(users.containsKey(userId)){
            
            //get user object
            User user = users.get(userId);

            //create tweet object and increase the time for next tweet
            Tweet tweet = new Tweet(tweetId); Tweet.time++;

            //add the tweet to this user
            user.tweets.add(tweet);

        //if non-existing user
        }else{
            
            //create a new user with the input userId
            User user = new User(userId);

            //create tweeet object and increase the time for next tweet
            Tweet tweet = new Tweet(tweetId); Tweet.time++;

            //add the tweet to this user
            user.tweets.add(tweet);

            //add the user to list of users
            users.put(userId, user);
        }

    }
    
    public List<Integer> getNewsFeed(int userId) {

        //min heap based on timestamp
        //latest tweets will be at the top
        PriorityQueue<Tweet> pq = new PriorityQueue<>(); 

        //if the user is not already present, return empty
        if(!users.containsKey(userId)) return new ArrayList<>();

        //otherwise, get the user object
        User user = users.get(userId);
        //get the userId that this user is following
        Set<Integer> followings = user.following;

        //loop through followings list
        for(Integer followerId: followings){
            
            //get following user
            User follower = users.get(followerId);
            //get his tweets
            List<Tweet> tweets = follower.tweets;

            //add all tweets to min heap
            for(Tweet t: tweets){
                pq.add(t);
            }   

        }

        //answer list containing latest 10 tweets
        List<Integer> latestTweetId = new ArrayList<>();
        int s = 0;

        //get the latest 10 tweets from heap
        while(s<10 && !pq.isEmpty()){
            latestTweetId.add(pq.poll().tweetId);
            s++;
        }

        return latestTweetId;

    }
    
    public void follow(int followerId, int followeeId) {
        
        //handle the user where followerId or followeeId in not in the list of users
        if(!users.containsKey(followerId)){
            users.put(followerId, new User(followerId));
        }

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

        //get the user
        User user = users.get(followerId);

        //add the follower for that user
        user.following.add(followeeId);

    }
    
    public void unfollow(int followerId, int followeeId) {

        //get the user object
        User user = users.get(followerId);

        //remove followee for that user
        user.following.remove(followeeId);
    }
}

class User{

    int userId;
    List<Tweet> tweets;
    Set<Integer> following;

    //create new user
    User(Integer userId){
        this.userId = userId;
        tweets = new ArrayList<>();
        following = new HashSet<>();
        following.add(userId);//the user is its own follower
    }
}

class Tweet implements Comparable<Tweet>{

    int tweetId;
    int timestamp;

    public static int time = 0; //will use this to track timestamp for tweets

    Tweet(int tweetId){
        this.tweetId = tweetId;
        this.timestamp = Tweet.time;
    }

    public int compareTo(Tweet t2){
        if(this.timestamp == t2.timestamp) return 0;
        //if tweet1.timestamp > tweet2.timestamp
        //=> tweet1 is latest when compared to tweet2
        //=> we want tweet1 before tweet2 in ascending order so that we can add it to min heap
        if(this.timestamp > t2.timestamp) return -1; 
        else return 1;
    }

}

/**
 * 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);
 */