Design Twitter

Design a simplified social network Twitter where users post tweets, follow each other, and read a news feed.

Implement the class:

• Twitter() creates an empty service.
• postTweet(user_id, tweet_id) records that user_id posted a new tweet with id tweet_id. Every call uses a tweet id that has never been used before.
• getNewsFeed(user_id) returns a list of at most 10 tweet ids taken from tweets posted by user_id and by every user that user_id currently follows. The list is ordered from most recently posted to least recently posted. Recency is the order in which postTweet was called, not the size of the tweet id. Return fewer than 10 ids if fewer tweets qualify, or [] if none do.
• follow(follower_id, followee_id) makes follower_id follow followee_id. Following is one-directional. Following someone you already follow has no effect.
• unfollow(follower_id, followee_id) makes follower_id stop following followee_id. If they were not following, nothing happens. After unfollowing, all of the followee's tweets (old and new) leave the follower's feed.

A user always sees their own tweets and is never asked to follow or unfollow themselves.

The examples list the calls in order. The output shows each call's return value, with None for the constructor and for methods that return nothing.