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# Design Twitter LeetCode Solution

## Problem – Design Twitter LeetCode Solution

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.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`.

## Design Twitter LeetCode Solution in Java

``````public class Twitter {
private static int timeStamp=0;

// easy to find if user exist
private Map<Integer, User> userMap;

// Tweet link to next Tweet so that we can save a lot of time
// when we execute getNewsFeed(userId)
private class Tweet{
public int id;
public int time;
public Tweet next;

public Tweet(int id){
this.id = id;
time = timeStamp++;
next=null;
}
}

// OO design so User can follow, unfollow and post itself
public class User{
public int id;
public Set<Integer> followed;

public User(int id){
this.id=id;
followed = new HashSet<>();
follow(id); // first follow itself
}

public void follow(int id){
}

public void unfollow(int id){
followed.remove(id);
}

// everytime user post a new tweet, add it to the head of tweet list.
public void post(int id){
Tweet t = new Tweet(id);
}
}

/** Initialize your data structure here. */
userMap = new HashMap<Integer, User>();
}

/** Compose a new tweet. */
public void postTweet(int userId, int tweetId) {
if(!userMap.containsKey(userId)){
User u = new User(userId);
userMap.put(userId, u);
}
userMap.get(userId).post(tweetId);

}

// Best part of this.
// first get all tweets lists from one user including itself and all people it followed.
// Second add all heads into a max heap. Every time we poll a tweet with
// largest time stamp from the heap, then we add its next tweet into the heap.
// So after adding all heads we only need to add 9 tweets at most into this
// heap before we get the 10 most recent tweet.
public List<Integer> getNewsFeed(int userId) {
List<Integer> res = new LinkedList<>();

if(!userMap.containsKey(userId))   return res;

Set<Integer> users = userMap.get(userId).followed;
PriorityQueue<Tweet> q = new PriorityQueue<Tweet>(users.size(), (a,b)->(b.time-a.time));
for(int user: users){
Tweet t = userMap.get(user).tweet_head;
// very imporant! If we add null to the head we are screwed.
if(t!=null){
}
}
int n=0;
while(!q.isEmpty() && n<10){
Tweet t = q.poll();
n++;
if(t.next!=null)
}

return res;

}

/** Follower follows a followee. If the operation is invalid, it should be a no-op. */
public void follow(int followerId, int followeeId) {
if(!userMap.containsKey(followerId)){
User u = new User(followerId);
userMap.put(followerId, u);
}
if(!userMap.containsKey(followeeId)){
User u = new User(followeeId);
userMap.put(followeeId, u);
}
userMap.get(followerId).follow(followeeId);
}

/** Follower unfollows a followee. If the operation is invalid, it should be a no-op. */
public void unfollow(int followerId, int followeeId) {
if(!userMap.containsKey(followerId) || followerId==followeeId)
return;
userMap.get(followerId).unfollow(followeeId);
}
}

/**
* Your Twitter object will be instantiated and called as such:
* obj.postTweet(userId,tweetId);
* List<Integer> param_2 = obj.getNewsFeed(userId);
* obj.follow(followerId,followeeId);
* obj.unfollow(followerId,followeeId);
*/
``````

## Design Twitter LeetCode Solution in Python

``````class Twitter(object):

def __init__(self):
self.timer = itertools.count(step=-1)
self.tweets = collections.defaultdict(collections.deque)
self.followees = collections.defaultdict(set)

def postTweet(self, userId, tweetId):
self.tweets[userId].appendleft((next(self.timer), tweetId))

def getNewsFeed(self, userId):
tweets = heapq.merge(*(self.tweets[u] for u in self.followees[userId] | {userId}))
return [t for _, t in itertools.islice(tweets, 10)]

def follow(self, followerId, followeeId):

def unfollow(self, followerId, followeeId):
``````

## Design Twitter LeetCode Solution in C++

``````class Twitter {
private:
unordered_map<int, set<int>> fo;
unordered_map<int, vector<pair<int, int>>> t;
long long time;

public:
/** Initialize your data structure here. */
time = 0;
}

/** Compose a new tweet. */
void postTweet(int userId, int tweetId) {
t[userId].push_back({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>> maxHeap;
for (auto it=t[userId].begin();it!=t[userId].end();++it)
maxHeap.push(*it);
for (auto it1=fo[userId].begin();it1!=fo[userId].end();++it1){
int usr = *it1; // get target user
for (auto it2=t[usr].begin();it2!=t[usr].end();++it2)
maxHeap.push(*it2);
}
vector<int> res;
while(maxHeap.size()>0) {
res.push_back(maxHeap.top().second);
if (res.size()==10) break;
maxHeap.pop();
}
return res;
}

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

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