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);

系统设计题，关键的一步是怎么取出来时间戳最新的tweet。这里用到了PriorityQueue。

The default PriorityQueue is implemented with Min-Heap, that is the top element is the minimum one in the heap.

In order to implement a max-heap, you can create your own Comparator:

import java.util.Comparator;

public class MyComparator implements Comparator<Integer>
{
    public int compare( Integer x, Integer y )
    {
        return y - x;
    }
}

So, you can create a min-heap and max-heap in the following way:

PriorityQueue minHeap=new PriorityQueue();
PriorityQueue maxHeap=new PriorityQueue(size, new MyComparator());

当然也可以用正则式 。

PriorityQueue<Tweet> pq = new PriorityQueue<Tweet>(10, (t1, t2) -> (t2.time - t1.time));

代码如下：

public class Twitter {
    private class Tweet {
        int tweetId;
        int time;
        public Tweet(int tweetId, int time) {
            this.tweetId = tweetId;
            this.time = time;
        }
    }
    
    HashMap<Integer,Set<Integer>> followers = new HashMap<Integer,Set<Integer>>();
    HashMap<Integer,List<Tweet>> tweets = new HashMap<Integer,List<Tweet>>();
    int timestamp = 0;

    /** Initialize your data structure here. */
    public Twitter() {
        //HashMap<Integer,Set<Integer>> followers = new HashMap<Integer,Set<Integer>>();
        //HashMap<Integer,List<Tweet>> tweets = new HashMap<Integer,List<Tweet>>();
    }
    
    /** Compose a new tweet. */
    public void postTweet(int userId, int tweetId) {
        if (!tweets.containsKey(userId)) {
            tweets.put(userId, new ArrayList<Tweet>());
        }
        tweets.get(userId).add(0, new Tweet(tweetId, timestamp ++));
        if (!followers.containsKey(userId)) {
            followers.put(userId, new HashSet<Integer>());
            followers.get(userId).add(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. */
    public List<Integer> getNewsFeed(int userId) {
        List<Integer> feeds = new ArrayList<Integer>();
        PriorityQueue<Tweet> pq = new PriorityQueue<Tweet>(10, (t1, t2) -> (t2.time - t1.time));
        Set<Integer> followeeSet = followers.get(userId);
        if (followeeSet != null) {
            for (int followed: followeeSet) {
                List<Tweet> tweetList = tweets.get(followed);
                if (tweetList != null) {
                    for (Tweet t: tweetList) {
                        pq.add(t);
                    }
                }
            }
        }
        while (feeds.size() < 10 && !pq.isEmpty()) {
            feeds.add(pq.poll().tweetId);
        }
        pq.clear();
        return feeds;
    }
    
    /** Follower follows a followee. If the operation is invalid, it should be a no-op. */
    public void follow(int followerId, int followeeId) {
        if (!followers.containsKey(followerId)) {
            followers.put(followerId, new HashSet<Integer>());
            followers.get(followerId).add(followerId);
        }
        if (followerId != followeeId) {
            followers.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 (followerId == followeeId) {
            return;
        }
        if (followers.containsKey(followerId)) {
            followers.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);
 */

The default PriorityQueue is implemented with Min-Heap, that is the top element is the minimum one in the heap.