Problem #355 on LeetCode: 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.

Twitter.py

class Twitter(object):

    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.followers = {}
        self.tweets = {}

    def postTweet(self, userId, tweetId):
        """
        Compose a new tweet.
        :type userId: int
        :type tweetId: int
        :rtype: void
        """
        if userId not in self.followers:
            self.followers[userId] = []
        if userId not in self.tweets:
            self.tweets[userId] = [(tweetId, time.time())]
        else:
            self.tweets[userId] = [(tweetId, time.time())] + self.tweets[userId]

    def getNewsFeed(self, 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.
        :type userId: int
        :rtype: List[int]
        """
        if userId in self.followers:
            followees = filter(lambda x: x in self.tweets, self.followers[userId])
            if userId in self.tweets:
                followees += [userId]

            feed = []
            for followee in followees:
                feed += self.tweets[followee]
            return map(lambda x: x[0], sorted(feed, key = lambda x: x[1], reverse=True)[:10])
        return []

    def follow(self, followerId, followeeId):
        """
        Follower follows a followee. If the operation is invalid, it should be a no-op.
        :type followerId: int
        :type followeeId: int
        :rtype: void
        """
        if followerId != followeeId:
            if followerId not in self.followers:
                self.followers[followerId] = []
    
            if followeeId not in self.followers[followerId]:
                self.followers[followerId] += [followeeId]
                
            if followeeId not in self.followers:
                self.followers[followeeId] = []

    def unfollow(self, followerId, followeeId):
        """
        Follower unfollows a followee. If the operation is invalid, it should be a no-op.
        :type followerId: int
        :type followeeId: int
        :rtype: void
        """
        if followerId in self.followers and followeeId in self.followers and followeeId in self.followers[followerId]:
            self.followers[followerId].remove(followeeId)


# 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)