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DesignTwitter.java
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DesignTwitter.java
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package Leetcode;
import java.util.*;
/**
* @author kalpak
*
* 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:
*
* --- postTweet(userId, tweetId): Compose a new tweet.
*
* --- 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.
*
* --- follow(followerId, followeeId): Follower follows a followee.
*
* --- 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);
*/
public class DesignTwitter {
static int timeStamp;
int feedMaxNum;
Map<Integer, Set<Integer>> followees;
Map<Integer, List<Tweet>> tweets;
/** Initialize your data structure here. */
public DesignTwitter() {
timeStamp = 0;
feedMaxNum = 10;
followees = new HashMap<>();
tweets = new HashMap<>();
}
/** Compose a new tweet. */
public void postTweet(int userId, int tweetId) {
if(!tweets.containsKey(userId)) {
tweets.put(userId, new LinkedList<Tweet>());
follow(userId, userId); //follow itself
}
tweets.get(userId).add(0, new Tweet(tweetId, timeStamp)); //add new tweet on the first place
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) {
//min heap that the earliest tweet is on the top
PriorityQueue<Tweet> feedHeap = new PriorityQueue<>((a, b) -> a.timePosted - b.timePosted);
List<Integer> myFeed = new LinkedList<>();
//add tweets of the followees
Set<Integer> myFollowees = followees.get(userId); // Get the followers from the map
if(myFollowees != null) {
for(int followeeId : myFollowees) {
List<Tweet> followeeTweets = tweets.get(followeeId);
if(followeeTweets == null)
continue;
for(Tweet t : followeeTweets) {
if(feedHeap.size() < feedMaxNum)
feedHeap.add(t);
else {
if(t.timePosted <= feedHeap.peek().timePosted)
break;
else {
feedHeap.add(t);
feedHeap.poll(); //remove the oldest tweet
}
}
}
}
}
while(!feedHeap.isEmpty()) {
myFeed.add(0, feedHeap.poll().tweetId);
}
return myFeed;
}
/** Follower follows a followee. If the operation is invalid, it should be a no-op. */
public void follow(int followerId, int followeeId) {
if(!followees.containsKey(followerId))
followees.put(followerId, new HashSet<Integer>());
followees.get(followerId).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(!followees.containsKey(followerId) || followerId == followeeId)
return; //cannot unfollow itself
followees.get(followerId).remove(followeeId);
}
private static class Tweet{
int tweetId;
int timePosted;
public Tweet(int tId, int time){
tweetId = tId;
timePosted = time;
}
}
public static void main(String[] args) {
DesignTwitter obj = new DesignTwitter();
obj.postTweet(1, 5);
System.out.println(obj.getNewsFeed(1));
obj.follow(1, 2);
obj.postTweet(2, 6);
System.out.println(obj.getNewsFeed(1));
obj.unfollow(1, 2);
System.out.println(obj.getNewsFeed(1));
}
}