题目地址 (opens new window)

• 🙂 第一次练习 2020年4月13日 我丢。。。。下次必须早上就做了。不能留在晚上了。
• 😄 第二次练习

堆

解题代码

class Twitter {

    /**
     * 推文类，是一个单链表
     */
    private class Tweet {
        // 推文 id
        private int id;

        // 发推文的时间戳
        private int timestamp;

        private Tweet next;

        public Tweet(int id, int timestamp) {
            this.id = id;
            this.timestamp = timestamp;
        }
    }

    private Map<Integer, Tweet> twitter;

    /**
     * 用户 id 和他关注的用户列表的对应关系
     */
    private Map<Integer, Set<Integer>> followings;

    /**
     * 全局使用的时间戳字段，用户每发布一条推文之前 + 1
     */
    private static int timestamp = 0;

    /**
     * 合并 k 组推文使用的数据结构（可以在方法里创建使用），声明成全局变量非必需，视个人情况使用
     */
    private static PriorityQueue<Tweet> maxHeap;

    /** Initialize your data structure here. */
    public Twitter() {
        followings = new HashMap<>();
        twitter = new HashMap<>();
        maxHeap = new PriorityQueue<>((o1, o2) -> o2.timestamp - o1.timestamp);
    }
    
    /** Compose a new tweet. */
    public void postTweet(int userId, int tweetId) {
        timestamp ++;
        if (twitter.containsKey(userId)) {
            Tweet oldHead = twitter.get(userId);
            Tweet newHead = new Tweet(tweetId, timestamp);
            newHead.next = oldHead;
            twitter.put(userId, newHead);
        } else {
            twitter.put(userId, new Tweet(tweetId, timestamp));
        }
    }
    
    /** 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) {
        // 由于是全局使用，使用之前需要清空
        maxHeap.clear();

        // 如果自己发送的推文也要算上
        if (twitter.containsKey(userId)) {
            maxHeap.offer(twitter.get(userId));
        }

        Set<Integer> followingList = followings.get(userId);
        if (followingList != null && followingList.size() > 0) {
            for (Integer followingId : followingList) {
                Tweet tweet = twitter.get(followingId);
                if (tweet != null) {
                    maxHeap.offer(tweet);
                }
            }
        }

        List<Integer> res = new ArrayList<>(10);
        int count = 0;
        while(!maxHeap.isEmpty() && count < 10) {
            Tweet head = maxHeap.poll();
            res.add(head.id);

            // 这里的最好的操作是 replace
            if (head.next != null) {
                maxHeap.offer(head.next);
            }
            count++;
        }
        return res;
    }
    
    /** Follower follows a followee. If the operation is invalid, it should be a no-op. */
    public void follow(int followerId, int followeeId) {
        // 被关注人不能是自己
        if (followerId == followeeId) {
            return;
        }

        // 获取我自己的关注列表
        Set<Integer> followingList = followings.get(followerId);
        if (followingList == null) {
            Set<Integer> init = new HashSet<>();
            init.add(followeeId);
            followings.put(followerId, init);
        } else {
            if (followingList.contains(followeeId)) {
                return ;
            }
            followingList.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;
        }

        // 获取我自己的关注列表
        Set<Integer> followingList = followings.get(followerId);
        if (followingList == null) {
            return;
        }

        followingList.remove(followeeId);
    }
}

易错点

• 易错项 1