g0301_0400.s0355_design_twitter.Twitter Maven / Gradle / Ivy

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package g0301_0400.s0355_design_twitter;

// #Medium #Hash_Table #Design #Heap_Priority_Queue #Linked_List
// #2022_07_11_Time_14_ms_(74.65%)_Space_42.2_MB_(56.38%)

import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;

/**
 * 355 - Design Twitter\.
 *
 * Medium
 *
 * 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.
 *
 * Implement the `Twitter` class:
 *
 * *   `Twitter()` Initializes your twitter object.
 * *   `void postTweet(int userId, int tweetId)` Composes a new tweet with ID `tweetId` by the user `userId`. Each call to this function will be made with a unique `tweetId`.
 * *   `List<Integer> getNewsFeed(int userId)` Retrieves 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 themself. Tweets must be **ordered from most recent to least recent**.
 * *   `void follow(int followerId, int followeeId)` The user with ID `followerId` started following the user with ID `followeeId`.
 * *   `void unfollow(int followerId, int followeeId)` The user with ID `followerId` started unfollowing the user with ID `followeeId`.
 *
 * **Example 1:**
 *
 * **Input**
 *
 *     ["Twitter", "postTweet", "getNewsFeed", "follow", "postTweet", "getNewsFeed", "unfollow", "getNewsFeed"]
 *     [ [], [1, 5], [1], [1, 2], [2, 6], [1], [1, 2], [1]]
 *
 * **Output:**
 *
 *     [null, null, [5], null, null, [6, 5], null, [5]]
 *
 * **Explanation:**
 *
 *     Twitter twitter = new Twitter();
 *     twitter.postTweet(1, 5); // User 1 posts a new tweet (id = 5).
 *     twitter.getNewsFeed(1); // User 1's news feed should return a list with 1 tweet id -> [5]. return [5]
 *     twitter.follow(1, 2); // User 1 follows user 2.
 *     twitter.postTweet(2, 6); // User 2 posts a new tweet (id = 6).
 *     twitter.getNewsFeed(1); // 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.unfollow(1, 2); // User 1 unfollows user 2.
 *     twitter.getNewsFeed(1); // User 1's news feed should return a list with 1 tweet id -> [5], since user 1 is no longer following user 2. 
 *
 * **Constraints:**
 *
 * *   `1 <= userId, followerId, followeeId <= 500`
 * *   0 <= tweetId <= 10⁴
 * *   All the tweets have **unique** IDs.
 * *   At most 3 * 10⁴ calls will be made to `postTweet`, `getNewsFeed`, `follow`, and `unfollow`.
**/
public class Twitter {
    // userId --> user followers
    private Map> twitterData;
    // head of linked list that stores all the tweets
    private Tweet head;

    private static class Tweet {
        int userId;
        int tweetId;
        Tweet next;

        Tweet(int uId, int tId) {
            userId = uId;
            tweetId = tId;
        }
    }

    public Twitter() {
        head = new Tweet(-1, -1);
        twitterData = new HashMap<>();
    }

    public void postTweet(int userId, int tweetId) {
        checkNewUser(userId);
        Tweet t = new Tweet(userId, tweetId);
        Tweet next = head.next;
        head.next = t;
        t.next = next;
    }

    public List getNewsFeed(int userId) {
        checkNewUser(userId);
        List res = new ArrayList<>();
        HashSet followers = twitterData.get(userId);
        Tweet t = head.next;
        while (t != null && res.size() < 10) {
            if (followers.contains(t.userId)) {
                res.add(t.tweetId);
            }
            t = t.next;
        }
        return res;
    }

    public void follow(int followerId, int followeeId) {
        checkNewUser(followeeId);
        checkNewUser(followerId);
        twitterData.get(followerId).add(followeeId);
    }

    public void unfollow(int followerId, int followeeId) {
        checkNewUser(followeeId);
        // cannot unfollower yourself
        if (followerId == followeeId) {
            return;
        }
        checkNewUser(followerId);
        twitterData.get(followerId).remove(followeeId);
    }

    public void checkNewUser(int userId) {
        if (twitterData.containsKey(userId)) {
            return;
        }
        twitterData.put(userId, new HashSet<>());
        // follow  yourself
        twitterData.get(userId).add(userId);
    }
}