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);

Difficulty: MEDIUM_HARD
Tags: Data structures
Skill types: Data structures

Starter code:
class Twitter:
    def __init__(self):
        """
        Initialize your data structure here.
        """
    def postTweet(self, userId: int, tweetId: int) -> None:
        """
        Compose a new tweet.
        """
    def getNewsFeed(self, userId: int) -> List[int]:
        """
        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.
        """
    def follow(self, followerId: int, followeeId: int) -> None:
        """
        Follower follows a followee. If the operation is invalid, it should be a no-op.
        """
    def unfollow(self, followerId: int, followeeId: int) -> None:
        """
        Follower unfollows a followee. If the operation is invalid, it should be a no-op.
        """
# Your Twitter object will be instantiated and called as such:
# obj = Twitter()
# obj.postTweet(userId,tweetId)
# param_2 = obj.getNewsFeed(userId)
# obj.follow(followerId,followeeId)
# obj.unfollow(followerId,followeeId)

Solutions:
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):
		self.followees[followerId].add(followeeId)

	def unfollow(self, followerId, followeeId):
		self.followees[followerId].discard(followeeId)


class Twitter:

	def __init__(self):
		self.tweets = []
		self.userTweets = {}
		self.feeds = {}
		self.followers = {}
		self.followees = {}
		self.relations = {}

	def postTweet(self, userId, tweetId):
		self.tweets.append({'userId': userId, 'tweetId': tweetId})
		lastIndex = len(self.tweets) - 1
		if userId not in self.userTweets:
			self.userTweets[userId] = []
		self.userTweets[userId].append(lastIndex)
		followers = self.followers.get(userId, [])
		for followerId in [userId] + followers:
			self.feeds[followerId] = ([tweetId] + self.feeds.get(followerId, []))[:10]

	def getNewsFeed(self, userId):
		return self.feeds.get(userId, [])

	def follow(self, followerId, followeeId):
		if (followerId, followeeId) in self.relations or followerId == followeeId:
			return
		else:
			self.relations[followerId, followeeId] = True
		if followeeId not in self.followers:
			self.followers[followeeId] = []
		self.followers[followeeId].append(followerId)
		if followerId not in self.followees:
			self.followees[followerId] = []
		self.followees[followerId].append(followeeId)
		self.updateFeed(followerId)

	def unfollow(self, followerId, followeeId):
		if (followerId, followeeId) not in self.relations:
			return
		else:
			del self.relations[followerId, followeeId]
		self.followers[followeeId].remove(followerId)
		self.followees[followerId].remove(followeeId)
		if not self.userTweets.get(followeeId, []):
			return
		self.updateFeed(followerId)

	def updateFeed(self, userId):
		followers = self.followees.get(userId, [])
		indexes = []
		for followeeId in [userId] + followers:
			if self.userTweets.get(followeeId, []):
				indexes += self.userTweets.get(followeeId, [])
				indexes = sorted(indexes, reverse=True)[0:10]
		self.feeds[userId] = [self.tweets[idx]['tweetId'] for idx in indexes]