Question

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:

  1. postTweet(userId, tweetId): Compose a new tweet.
  2. 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.
  3. follow(followerId, followeeId): Follower follows a followee.
  4. unfollow(followerId, followeeId): Follower unfollows a followee. Given two arrays, write a function to compute their intersection.
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);

Thinking:

  • Method 1: OOP design
class Twitter {
    Map<Integer, User> users;
    LinkedList<Post> posts;
    public class User{
        int userId;
        List<Integer> follow;
        public User(int userId){
            this. userId = userId;
            this.follow = new LinkedList<>();
            this.follow.add(userId);
        }
    }
    public class Post{
        int tweetId;
        int userId;
        public Post(int tweetId, int userId){
            this.userId = userId;
            this.tweetId = tweetId;
        }
    }
    /** Initialize your data structure here. */
    public Twitter() {
        this.users = new HashMap<>();
        this.posts = new LinkedList<>();
    }

    /** Compose a new tweet. */
    public void postTweet(int userId, int tweetId) {
        Post post = new Post(tweetId, userId);
        posts.addFirst(post);
        addUser(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. */
    public List<Integer> getNewsFeed(int userId) {
        List<Integer> res = new ArrayList<>();
        User user = this.users.get(userId);
        if(user == null) return res;
        List<Integer> follow = user.follow;
        int count = 0;
        ListIterator<Post> it = posts.listIterator(0);
        while(it.hasNext()){
            Post post = it.next();
            if(follow.contains(post.userId)){
                res.add(post.tweetId);
                if(++count == 10)
                return res;
            }
        }
        return res;
    }

    /** Follower follows a followee. If the operation is invalid, it should be a no-op. */
    public void follow(int followerId, int followeeId) {
        User follower = addUser(followerId);
        addUser(followeeId);
        if(!follower.follow.contains(followeeId))
            follower.follow.add(followeeId);
    }

    /** Follower unfollows a followee. If the operation is invalid, it should be a no-op. */
    public void unfollow(int followerId, int followeeId) {
        User user = addUser(followerId);
        List<Integer> f = user.follow;
        for(int i = 0; i < f.size(); i++)
            if(f.get(i) == followeeId && followerId != followeeId)
                f.remove(i);
    }
    private User addUser(int userId){
        if(!this.users.containsKey(userId)){
            this.users.put(userId, new User(userId));
        }
        return this.users.get(userId);
    }
}

/**
 * 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);
 */