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);
 

因為要互相關注，所以用HashMap存關注關係。又要求訊息時間順序，所以用id區別先後順序，把id,userId,tweetId放到一個類裡。取最近訊息要用到優先佇列。

class Twitter {
    int id=-10000000;//用標記訊息的順序
    HashMap<Integer,HashSet<Integer>> follow;
    HashMap<Integer,LinkedList<Tweet>> news;//userId —— 訊息
    
    /** Initialize your data structure here. */
    public Twitter() {
        follow=new HashMap<>(); 
        news=new HashMap<>();
    }
    
    /** Compose a new tweet. */
    public void postTweet(int userId, int tweetId) {
        if(news.get(userId)==null) news.put(userId,new LinkedList<Tweet>());
        news.get(userId).addFirst(new Tweet(id++,userId,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> res=new ArrayList<>();
        PriorityQueue<Tweet> pq=new PriorityQueue<>(new Comparator<Tweet>(){//放入訊息
            public int compare(Tweet t1,Tweet t2){
                return t2.id-t1.id;
            }
        });
        //自己的訊息
        if(news.get(userId)!=null&&news.get(userId).size()>0) pq.add(news.get(userId).get(0));
        
        //別人的訊息,先由關注列表找到別人
        if(follow.get(userId)==null) follow.put(userId,new HashSet<Integer>());
        for(int uid:follow.get(userId)){
            if(news.get(uid)==null) news.put(uid,new LinkedList<Tweet>());   
            if(news.get(uid).size()>0) pq.add(news.get(uid).get(0));
        }
        
        for(int i=0;i<10&&pq.size()>0;i++){//找到他和他關注的人的前10條訊息
            Tweet tweet=pq.poll();
            res.add(tweet.tweetId);
            LinkedList<Tweet> tweets=news.get(tweet.userId);
            Iterator<Tweet> it=tweets.iterator();
            //找下一個Tweet
            while(it.hasNext()){
                Tweet t=it.next();
                if(t==tweet) break;
            }
            if(it.hasNext()) pq.add(it.next());
        }
        return res;
    }
    
    /** 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;
        if(follow.get(followerId)==null) follow.put(followerId,new HashSet<Integer>());
        follow.get(followerId).add(followeeId);
    }
    
    /** Follower unfollows a followee. If the operation is invalid, it should be a no-op. */
    public void unfollow(int followerId, int followeeId) {
        if(follow.get(followerId)==null) follow.put(followerId,new HashSet<Integer>());
        follow.get(followerId).remove(followeeId);
    }
    
    //訊息
    class Tweet{
        int id;
        int userId;
        int tweetId;
        Tweet(int id,int userId,int tweetId){
            this.id=id;
            this.userId=userId;
            this.tweetId=tweetId;
        }
    }
}

/**
 * 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);
 */