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);

这种题本身不难，就是得话功夫 把代码改的符合要求。设计思路是用两个hashmap保存两种关系：1。 关注着和被关注着的一对多的关系  2.userid跟它发布的newsId之间的一对多的关系。但是由于需要在查看news的时候保证有序，所以在保存文章的时候需要一个timestamp，这里我就简单实现了一个计数器，以Hashmap键值对的形式保存文章顺序id与文章id的关系。 

这道题主要注意两点:

1. 空指针， 在查看news的时候，可能本人没有发布过文章，因此在Hashmap中找不到；也可能关注的人没发布过文章，因此在map中也找不到，需要null判断

2. 满足最多10条新消息，用一个counter就好。

代码：

static int SEQUCENUMBER = 0;
    HashMap<Integer, Set<Integer>> followMap;
    HashMap<Integer, Map<Integer, Integer>> newsMap;
    /** Initialize your data structure here. */
    public Twitter() {
        followMap = new HashMap<>();
        newsMap = new HashMap<>();
    }

    /** Compose a new tweet. */
    public void postTweet(int userId, int tweetId) {
        newsMap.putIfAbsent(userId, new HashMap<>());
        newsMap.get(userId).put(SEQUCENUMBER++, 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> result = new ArrayList<>();
        List<Map.Entry<Integer, Integer>> list = new ArrayList<>();
        if(newsMap.containsKey(userId)){
            for(Map.Entry<Integer, Integer> entry: newsMap.get(userId).entrySet()){
                list.add(entry);
            }
        }
        if(followMap.containsKey(userId)){
            for(Integer followeeId: followMap.get(userId)){
                if(newsMap.containsKey(followeeId)){
                    for(Map.Entry<Integer, Integer> entry: newsMap.get(followeeId).entrySet()){
                        list.add(entry);
                    }
                }

            }
        }
        Collections.sort(list, new Comparator<Map.Entry<Integer, Integer>>() {
            @Override
            public int compare(Map.Entry<Integer, Integer> o1, Map.Entry<Integer, Integer> o2) {
                return o2.getKey() - o1.getKey();
            }
        });
        int count = 10;
        for(Map.Entry<Integer, Integer> entry: list){
            result.add(entry.getValue());
            count--;
            if(count == 0) break;
        }
        return result;
    }

    /** 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;
        followMap.putIfAbsent(followerId, new HashSet<>());
        followMap.putIfAbsent(followeeId, new HashSet<>());
        followMap.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;
        followMap.putIfAbsent(followerId, new HashSet<>());
        followMap.putIfAbsent(followeeId, new HashSet<>());
        followMap.get(followerId).remove(followeeId);
    }