public void postTweet(int userId, int tweetId) {
        //1. post to user feeds record
        TreeMap<Long,Integer> postFeeds=postFeedMap.get(userId);
        if(postFeeds==null) postFeeds=new TreeMap<>();
        postFeeds.put(time.incrementAndGet(),tweetId);
        postFeedMap.put(userId,postFeeds);
        //2. push to followers recent feed
        TreeSet<Integer> follower=followerMap.get(userId);
        if(follower!=null && !follower.isEmpty()){
            for(Integer uid : follower){
                LinkedList<Integer> recentFeed=recentFeedMap.get(uid);
                if(recentFeed==null) recentFeed=new LinkedList<>();
                recentFeed.addFirst(tweetId);
                if(recentFeed.size()>MAX_RECENT_FEED_SIZE){
                    recentFeed.removeLast();
                }
                recentFeedMap.put(uid,recentFeed);
            }
        }
        //3. update himself recent feed
        LinkedList<Integer> recentFeed=recentFeedMap.get(userId);
        if(recentFeed==null) recentFeed=new LinkedList<>();
        recentFeed.addFirst(tweetId);
        if(recentFeed.size()>MAX_RECENT_FEED_SIZE){
            recentFeed.removeLast();
        }
        recentFeedMap.put(userId,recentFeed);
    }