public void unfollow(int followerId, int followeeId) {
        if(followeeId==followerId) return;
        //update subscribe list
        TreeSet<Integer> subscribeList=subscribeMap.get(followerId);
        if(subscribeList==null) subscribeList=new TreeSet<>();
        subscribeList.remove(followeeId);
        subscribeMap.put(followerId,subscribeList);
        //update follower list
        TreeSet<Integer> followerList=followerMap.get(followeeId);
        if(followerList==null) followerList=new TreeSet<>();
        followerList.remove(followerId);
        followerMap.put(followeeId,followerList);
        //rebuild recent feed
        rebuildRecentFeed(followerId);
    }