355. 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.

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.

class Twitter:
 
    def __init__(self):
        # graph[user] = [list of ppl user is following]
        self.graph = collections.defaultdict(set)
        self.posts = collections.defaultdict(list)
        self.counter = -1
 
    def postTweet(self, userId: int, tweetId: int) -> None:
        # the smaller the number, the newer the tweet is
        self.posts[userId].append((self.counter, tweetId))
        self.counter -= 1
        
    def getNewsFeed(self, userId: int) -> List[int]:
        feed = []
 
        # user "follows" themselves
        self.graph[userId].add(userId)
 
        # accumulate all posts
        for following in list(self.graph[userId]):
            for i, post in enumerate(self.posts[following]):
                heapq.heappush(feed, post)
 
        ret = []
        for _ in range(10):
            if len(feed):
                ret.append(heapq.heappop(feed)[1])
        return ret
 
 
    def follow(self, followerId: int, followeeId: int) -> None:
        self.graph[followerId].add(followeeId)
 
    def unfollow(self, followerId: int, followeeId: int) -> None:
        if followeeId in self.graph[followerId]:
            self.graph[followerId].remove(followeeId)

• we store all user relationships in a adjacency list graph.
• we store all posts in a hashmap.
• we use a running counter to keep track of which tweets were posted first.
• on call to getNewsFeed, we heap all of the $k$ most recent posts from each user the calling user is following, and grab the top $k$ from the heap.

table without id file.inlinks as Backlinks
where file.name = this.file.name

Related.

References.

Categories:: heap, priority-queue, hashmap, array, lc-system-design