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Commit fc87bf3

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Add solution for Design Twitter
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‎README.md

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@@ -45,6 +45,7 @@ Algorithm exercises from LeetCode implemented in Java (v11) and JavaScript.
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- Meeting Rooms II | [Problem](https://leetcode.com/problems/meeting-rooms-ii) | [Java Solution](src/javacode/solutions/MeetingRoomsII.java)
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- Find Median From Data Stream | [Problem](https://leetcode.com/problems/find-median-from-data-stream) | [Java Solution](src/javacode/solutions/FindMedianFromDataStream.java)
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- Stock Price Fluctuation | [Problem](https://leetcode.com/problems/stock-price-fluctuation) | [Java Solution](src/javacode/solutions/StockPrice.java)
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- Design Twitter | [Problem](https://leetcode.com/problems/design-twitter) | [Java Solution](src/javacode/solutions/Twitter.java)
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- Employee Free Time | [Problem](https://leetcode.com/problems/employee-free-time) | [Java Solution](src/javacode/solutions/EmployeeFreeTime.java)
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- Process Tasks Using Servers | [Problem](https://leetcode.com/problems/process-tasks-using-servers) | [Java Solution](src/javacode/solutions/ProcessTasksUsingServers.java)
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‎src/javacode/solutions/Twitter.java

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package javacode.solutions;
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import java.util.*;
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// [Problem] https://leetcode.com/problems/design-twitter
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class Twitter {
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Map<Integer, List<Tweet>> tweets;
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Map<Integer, Set<Integer>> following;
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int timestamp;
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// O(n) space
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public Twitter() {
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tweets = new HashMap<>();
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following = new HashMap<>();
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timestamp = 0;
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}
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// O(1) time
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public void postTweet(int userId, int tweetId) {
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following.putIfAbsent(userId, new HashSet<>());
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following.get(userId).add(userId);
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tweets.putIfAbsent(userId, new LinkedList<>());
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tweets.get(userId).add(new Tweet(tweetId, timestamp++));
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}
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// O(nlogn) time
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public List<Integer> getNewsFeed(int userId) {
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Queue<Tweet> newsFeed = new PriorityQueue<>((a, b) -> b.timestamp - a.timestamp);
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Set<Integer> followees = following.getOrDefault(userId, new HashSet<>());
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for (int followee : followees) {
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if (tweets.containsKey(followee)) {
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newsFeed.addAll(tweets.get(followee));
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}
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}
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List<Integer> newsFeedTweetIds = new ArrayList<>();
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while (!newsFeed.isEmpty() && newsFeedTweetIds.size() < 10) {
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newsFeedTweetIds.add(newsFeed.poll().id);
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}
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return newsFeedTweetIds;
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}
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// O(1) time
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public void follow(int followerId, int followeeId) {
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following.putIfAbsent(followerId, new HashSet<>());
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following.get(followerId).add(followeeId);
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}
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// O(1) time
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public void unfollow(int followerId, int followeeId) {
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following.get(followerId).remove(followeeId);
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}
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public static void main(String[] args) {
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Twitter twitter = new Twitter();
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twitter.postTweet(1, 5); // User 1 posts a new tweet (id = 5).
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System.out.println(twitter.getNewsFeed(1)); // User 1's news feed should return a list with 1 tweet id -> [5]. return [5]
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twitter.follow(1, 2); // User 1 follows user 2.
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twitter.postTweet(2, 6); // User 2 posts a new tweet (id = 6).
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System.out.println(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.
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twitter.unfollow(1, 2); // User 1 unfollows user 2.
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System.out.println(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.
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}
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}
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class Tweet {
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int id;
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int timestamp;
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public Tweet(int id, int timestamp) {
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this.id = id;
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this.timestamp = timestamp;
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}
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}

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