355. Design Twitter

https://leetcode.com/problems/design-twitter/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);

Sol:

https://discuss.leetcode.com/topic/48588/java-ood-solution-with-detailed-explanation

public class Twitter {

private static class Tweet{
    int tweetId;
    int timePosted;
    public Tweet(int tId, int time){
        tweetId = tId;
        timePosted = time;
    }
}

static int timeStamp;
int feedMaxNum;
Map<Integer, Set<Integer>> followees;
Map<Integer, List<Tweet>> tweets;

/** Initialize your data structure here. */
public Twitter() {
    timeStamp = 0;
    feedMaxNum = 10;
    followees = new HashMap<>();
    tweets = new HashMap<>();
}

/** Compose a new tweet. */
public void postTweet(int userId, int tweetId) {
    if(!tweets.containsKey(userId)) {
        tweets.put(userId, new LinkedList<Tweet>());
        follow(userId, userId);  //follow itself
    }
    tweets.get(userId).add(0, new Tweet(tweetId, timeStamp++)); //add new tweet on the first place
}

/** 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) {
    //min heap that the earliest tweet is on the top
    PriorityQueue<Tweet> feedHeap = new PriorityQueue<>(new Comparator<Tweet>(){
        public int compare(Tweet t1, Tweet t2){
            return t1.timePosted - t2.timePosted;
        }
    });

    //add tweets of the followees
    Set<Integer> myFollowees = followees.get(userId);
    if(myFollowees != null){
        for(int followeeId : myFollowees){
            List<Tweet> followeeTweets = tweets.get(followeeId);
            if(followeeTweets == null) continue;
            for(Tweet t : followeeTweets){
                if(feedHeap.size() < feedMaxNum) feedHeap.add(t);
                else{
                    if(t.timePosted <= feedHeap.peek().timePosted) break;
                    else{
                        feedHeap.add(t);
                        feedHeap.poll(); //remove the oldest tweet
                    }
                }
            }
        }
    }
    List<Integer> myFeed = new LinkedList<>();
    while(!feedHeap.isEmpty()){
        myFeed.add(0, feedHeap.poll().tweetId);
    }
    return myFeed;
}

/** Follower follows a followee. If the operation is invalid, it should be a no-op. */
public void follow(int followerId, int followeeId) {
    if(!followees.containsKey(followerId)) followees.put(followerId, new HashSet<Integer>());
    followees.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(!followees.containsKey(followerId) || followerId == followeeId) return; //cannot unfollow itself
    followees.get(followerId).remove(followeeId);
}
}

/**
 * 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);
 */