Description

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.

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);

Solution

这是一道OO Design的题，还是蛮简单的，注意最好将Tweet和User封装起来。另外在调用follow的时候不存在的User需要被创建出来，还有要follow自己。

PriorityQueue

class Twitter {
    private Map<Integer, User> userMap;     // to decide if a user exists & fast get
    private static int tweetCount;  // represent timestamp of current tweet
    /** Initialize your data structure here. */
    public Twitter() {
        userMap = new HashMap<>();
        tweetCount = 0;
    }
    
    /** Compose a new tweet. */
    public void postTweet(int userId, int tweetId) {
        User user = createIfNotExist(userId);
        user.post(tweetId);
    }
    
    /** 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> feed = new LinkedList<>();
        if (!userMap.containsKey(userId)) {
            return feed;
        }
        
        User user = userMap.get(userId);
        PriorityQueue<Tweet> queue = new PriorityQueue<>((a, b) -> b.time - a.time);
        for (int followeeId : user.follows) {
            User followee = userMap.get(followeeId);
            if (followee.tweetHead == null) continue;
            queue.offer(followee.tweetHead);
        }
        
        for (int i = 0; i < 10 && !queue.isEmpty(); ++i) {
            Tweet tweet = queue.poll();
            feed.add(tweet.id);          
            if (tweet.next == null) continue;
            queue.offer(tweet.next);
        }
        
        return feed;
    }
    
    /** Follower follows a followee. If the operation is invalid, it should be a no-op. */
    public void follow(int followerId, int followeeId) {
        User follower = createIfNotExist(followerId);
        User followee = createIfNotExist(followeeId);
        follower.follow(followeeId);
    }
    
    /** Follower unfollows a followee. If the operation is invalid, it should be a no-op. */
    public void unfollow(int followerId, int followeeId) {
        if (!userMap.containsKey(followerId)) return;
        userMap.get(followerId).unfollow(followeeId);
    }
        
    private User createIfNotExist(int uid) {
        if (!userMap.containsKey(uid)) {
            userMap.put(uid, new User(uid));
        }
        return userMap.get(uid);
    }
    
    class Tweet {
        int id;
        int time;
        Tweet next;     // to get next tweet efficiently
        
        public Tweet(int id) {
            this.id = id;
            time = tweetCount++;
        }
    }
    
    class User {
        int uid;
        Set<Integer> follows;
        Tweet tweetHead;
        
        public User(int uid) {
            this.uid = uid;
            follows = new HashSet<>();
            follows.add(uid);   // follow itself first
        }
        
        public void post(int tweetId) {
            Tweet tweet = new Tweet(tweetId);
            tweet.next = tweetHead;     // insert to begining
            tweetHead = tweet;
        }
        
        public void follow(int followee) {
            follows.add(followee);
        }
        
        public void unfollow(int followee) {
            if (followee == uid) return;  // cannot unfollow itself
            follows.remove(followee);
        }
    }
}

/**
 * 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);
 */