355.Design Twitter

Tags: Medium Design

Links: https://leetcode.com/problems/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);

---

class Twitter {
    unordered_map<int, unordered_set<int>> Friend;
    unordered_map<int, vector<pair<int, int>>> tweet;
    int time;

public:
    /** Initialize your data structure here. */
    Twitter() {
        std::ios_base::sync_with_stdio(false);
        cin.tie(NULL);
        cout.tie(NULL);

        time = 0;
    }

    /** Compose a new tweet. */
    void postTweet(int userId, int tweetId) {
        follow(userId, userId);
        tweet[userId].push_back(make_pair(time++, 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. */
    vector<int> getNewsFeed(int userId) {
        priority_queue<pair<int, int>, vector<pair<int, int>>, greater<pair<int, int>>> pq;
        for (auto id : Friend[userId]) {
            for (auto page : tweet[id]) {
                if (pq.size() > 0 && pq.top().first > page.first && pq.size() >= 10) 
                    continue;
                pq.push(page);
                if (pq.size() > 10) pq.pop();
            }
        }
        int n = pq.size();
        vector<int> res(n);
        int pos = n - 1;
        while (!pq.empty()) {
            res[pos--] = pq.top().second;
            pq.pop();
        }

        return res;
    }

    /** Follower follows a followee. If the operation is invalid, it should be a no-op. */
    void follow(int followerId, int followeeId) {
        Friend[followerId].emplace(followeeId);
    }

    /** Follower unfollows a followee. If the operation is invalid, it should be a no-op. */
    void unfollow(int followerId, int followeeId) {
        if (followerId != followeeId)
            Friend[followerId].erase(followeeId);
    }
};

/**
 * Your Twitter object will be instantiated and called as such:
 * Twitter* obj = new Twitter();
 * obj->postTweet(userId,tweetId);
 * vector<int> param_2 = obj->getNewsFeed(userId);
 * obj->follow(followerId,followeeId);
 * obj->unfollow(followerId,followeeId);
 */

用一个变量time来标记先后顺序，用一个unordered_map来维护所有用户发的推特文章，用Friend维护用户的好友，注意自己也是自己的好友，不能自己取关自己。

还可以进一步优化，因为是按照时间顺序推入vector的。这样就不用遍历每个好友的所有推文了。

class Twitter {
    unordered_map<int, unordered_set<int>> Friend;
    unordered_map<int, vector<pair<int, int>>> tweet;
    int time;

public:
    /** Initialize your data structure here. */
    Twitter() {
        std::ios_base::sync_with_stdio(false);
        cin.tie(NULL);
        cout.tie(NULL);

        time = 0;
    }

    /** Compose a new tweet. */
    void postTweet(int userId, int tweetId) {
        follow(userId, userId);
        tweet[userId].push_back(make_pair(time++, 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. */
    vector<int> getNewsFeed(int userId) {
        priority_queue<pair<int, int>, vector<pair<int, int>>, greater<pair<int, int>>> pq;
        for (auto id : Friend[userId]) {
            int n = tweet[id].size();
            for (int i = n - 1; i >= 0; --i) {
                if (pq.size() > 0 && pq.top().first > tweet[id][i].first && pq.size() >= 10) 
                    break; //因为后续的时间只会更小
                pq.push(tweet[id][i]);
                if (pq.size() > 10) pq.pop();
            }
        }
        int n = pq.size();
        vector<int> res(n);
        int pos = n - 1;
        while (!pq.empty()) {
            res[pos--] = pq.top().second;
            pq.pop();
        }

        return res;
    }

    /** Follower follows a followee. If the operation is invalid, it should be a no-op. */
    void follow(int followerId, int followeeId) {
        Friend[followerId].emplace(followeeId);
    }

    /** Follower unfollows a followee. If the operation is invalid, it should be a no-op. */
    void unfollow(int followerId, int followeeId) {
        if (followerId != followeeId)
            Friend[followerId].erase(followeeId);
    }
};

/**
 * Your Twitter object will be instantiated and called as such:
 * Twitter* obj = new Twitter();
 * obj->postTweet(userId,tweetId);
 * vector<int> param_2 = obj->getNewsFeed(userId);
 * obj->follow(followerId,followeeId);
 * obj->unfollow(followerId,followeeId);
 */