355. Design Twitter

JIAKAOBO

LeetCode

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Problem

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.

Implement the Twitter class:

• Twitter() Initializes your twitter object.
• void postTweet(int userId, int tweetId) Composes a new tweet with ID tweetId by the user userId. Each call to this function will be made with a unique tweetId.
• List getNewsFeed(int userId) Retrieves 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 themselves. Tweets must be ordered from most recent to least recent.
• void follow(int followerId, int followeeId) The user with ID followerId started following the user with ID followeeId.
• void unfollow(int followerId, int followeeId) The user with ID followerId started unfollowing the user with ID followeeId.

Example 1:

Input
["Twitter", "postTweet", "getNewsFeed", "follow", "postTweet", "getNewsFeed", "unfollow", "getNewsFeed"]
[[], [1, 5], [1], [1, 2], [2, 6], [1], [1, 2], [1]]
Output
[null, null, [5], null, null, [6, 5], null, [5]]

Explanation
Twitter twitter = new Twitter();
twitter.postTweet(1, 5); // User 1 posts a new tweet (id = 5).
twitter.getNewsFeed(1);  // User 1's news feed should return a list with 1 tweet id -> [5]. return [5]
twitter.follow(1, 2);    // User 1 follows user 2.
twitter.postTweet(2, 6); // User 2 posts a new tweet (id = 6).
twitter.getNewsFeed(1);  // 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.unfollow(1, 2);  // User 1 unfollows user 2.
twitter.getNewsFeed(1);  // User 1's news feed should return a list with 1 tweet id -> [5], since user 1 is no longer following user 2.

Code

class Twitter {
    private int timeStamp = 0;
    // userId:User
    private HashMap<Integer, User> userMap;

    class Tweet{
        private int id;
        private int time;
        private Tweet next;
        public Tweet(int id){
            this.id = id;
            this.time = timeStamp++;
            this.next = null;
        }
    }

    class User{
        private int id;
        private HashSet<Integer> follow;
        private Tweet tweetHead;

        public User(int id){
            this.id = id;
            this.follow = new HashSet<>();
            follow(id);
            tweetHead = null;
        }

        // 我follow谁
        public void follow(int id){
            follow.add(id);
        }

        public void unFollow(int id){
            follow.remove(id);
        }

        public void post(int id){
            // 倒着加入
            Tweet tweet = new Tweet(id);
            tweet.next = tweetHead;
            tweetHead = tweet;
        }
    }

    public Twitter() {
        userMap = new HashMap<>();
    }

    public void postTweet(int userId, int tweetId) {
        if(!userMap.containsKey(userId)){
            User user = new User(userId);
            userMap.put(userId, user);
        }
        userMap.get(userId).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> res = new ArrayList<>();
        if(!userMap.containsKey(userId)) return res;
        // 取出所有他follow的人
        HashSet<Integer> users = userMap.get(userId).follow;
        // 时间从大到小
        PriorityQueue<Tweet> queue = new PriorityQueue<>(users.size(), (a, b) -> (b.time - a.time));

        for(int user : users){
            Tweet tweet = userMap.get(user).tweetHead;
            if(tweet != null){
                queue.offer(tweet);
            }
        }

        int count = 0;
        while(!queue.isEmpty() && count < 10){
            Tweet tweet = queue.poll();
            res.add(tweet.id);
            count++;

            if(tweet.next != null){
                queue.offer(tweet.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(!userMap.containsKey(followerId)){
            User user = new User(followerId);
            userMap.put(followerId, user);
        }

        if(!userMap.containsKey(followeeId)){
            User user = new User(followeeId);
            userMap.put(followeeId, user);
        }

        userMap.get(followerId).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) || followerId == followeeId){
            return;
        }
        userMap.get(followerId).unFollow(followeeId);
    }
}