Leetcode 355. Design Twitter

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);

Code (Java):

class Twitter {
    Map<Integer, Set<Integer>> friendMap;
    Map<Integer, PriorityQueue<Tweet>> tweetMap;
    SeqTime seqTime;

    /** Initialize your data structure here. */
    public Twitter() {
        friendMap = new HashMap<>();
        tweetMap = new HashMap<>();
        seqTime = new SeqTime();
    }
    
    /** Compose a new tweet. */
    public void postTweet(int userId, int tweetId) {
        // init the friend map
        Tweet tweet = new Tweet(userId, tweetId, seqTime.getTime());
        Set<Integer> friends = friendMap.getOrDefault(userId, new HashSet<>());
        friends.add(userId);
        friendMap.put(userId, friends);
        
        // save the tweet into the tweetmap
        PriorityQueue<Tweet> tweets = tweetMap.getOrDefault(userId, new PriorityQueue<>(new MyTweetComparator()));
        tweets.offer(tweet);
        tweetMap.put(userId, tweets);
    }
    
    /** 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) {
        Set<Integer> friends = friendMap.get(userId);
        
        if (friends == null || friends.isEmpty()) {
            return new ArrayList<>();
        }
        
        
        Map<Integer, PriorityQueue<Tweet>> tweetList = new HashMap<>();
        for (Integer friend : friends) {
            PriorityQueue<Tweet> tweets = tweetMap.get(friend);
            PriorityQueue<Tweet> top10List = getTop10Tweets(tweets);
            
            Iterator<Tweet> it = top10List.iterator();
            if (!top10List.isEmpty()) {
                tweetList.put(friend, top10List);
            }
        }
        
        return mergeTweets(tweetList);
    }
    
    /** Follower follows a followee. If the operation is invalid, it should be a no-op. */
    public void follow(int followerId, int followeeId) {
        Set<Integer> friends = friendMap.getOrDefault(followerId, new HashSet<>());
        friends.add(followerId);
        friends.add(followeeId);
        friendMap.put(followerId, friends);
    }
    
    /** Follower unfollows a followee. If the operation is invalid, it should be a no-op. */
    public void unfollow(int followerId, int followeeId) {
        if (followerId == followeeId) {
            return;
        }
        Set<Integer> friends = friendMap.getOrDefault(followerId, new HashSet<>());
        friends.remove(followeeId);
        friendMap.put(followerId, friends);
    }
    
    private PriorityQueue<Tweet> getTop10Tweets(PriorityQueue<Tweet> tweets) {
        PriorityQueue<Tweet> top10Tweets = new PriorityQueue<>(new MyTweetComparator());
        for (int i = 0; i < 10; i++) {
            if (tweets == null || tweets.isEmpty()) {
                break;
            }
            
            top10Tweets.offer(tweets.poll());
        }
        
        // push back
        Iterator it = top10Tweets.iterator();
        while (it.hasNext()) {
            tweets.offer((Tweet)it.next());
        }
        
        return top10Tweets;
    }
    
    private List<Integer> mergeTweets(Map<Integer, PriorityQueue<Tweet>> tweetLists) {
        List<Integer> ans = new ArrayList<>();
        PriorityQueue<Tweet> finalPQ = new PriorityQueue<>(new MyTweetComparator());
        for (Integer userId : tweetLists.keySet()) {
            PriorityQueue<Tweet> tweets = tweetLists.get(userId);
            Tweet top = tweets.poll();
            //tweetLists.put(userId, tweets);
            finalPQ.offer(top);
        }
        
        int count = 0;
        while (count < 10 && !tweetLists.isEmpty()) {
            Tweet curr = finalPQ.poll();
            ans.add(curr.tid);
            PriorityQueue<Tweet> nextTweetList = tweetLists.get(curr.uid);
            
            if (!nextTweetList.isEmpty()) {
                finalPQ.offer(nextTweetList.poll());
            } else {
                tweetLists.remove(curr.uid);
            }
            count += 1;
        }
        
        return ans;
    }
}

class Tweet {
    int tid;
    int uid;
    int time;
    public Tweet(int uid, int tid, int time) {
        this.uid = uid;
        this.tid = tid;
        this.time = time;
    }
}

class SeqTime {
    int time;
    public SeqTime() {
        time = 0;
    }
    
    public int getTime() {
        int curTime = time;
        time += 1;
        return curTime;
    }
}

class MyTweetComparator implements Comparator<Tweet> {
    @Override 
    public int compare(Tweet a, Tweet b) {
        return b.time - a.time;
    }
}

/**
 * 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);
 */