Design Twitter

Design a small version of Twitter. Users post tweets and follow each other, and each user has a news feed: the 10 most recent tweets posted by the user and by the people the user follows, newest first.

Implement the class Twitter:

• Twitter() creates an empty Twitter: no users, no tweets.
• postTweet(userId, tweetId) posts a new tweet with the id tweetId by the user userId. Each call posts later than all the calls before it.
• getNewsFeed(userId) returns the ids of the 10 most recent tweets in the user's news feed, newest first. A feed with fewer than 10 tweets returns all of them, and an empty feed returns no ids at all.
• follow(followerId, followeeId): the user followerId starts following the user followeeId.
• unfollow(followerId, followeeId): the user followerId stops following the user followeeId.

Following is one way: when user 1 follows user 2, user 1 sees user 2's tweets, old and new, but not the other way round. Following someone you already follow, following yourself and unfollowing someone you don't follow change nothing; your own tweets are always in your feed.

The input is a list of operations, applied in order to one new Twitter. The output has one line for each getNewsFeed call: the tweet ids it returns, separated by spaces, or an empty line for an empty feed. The starter code spells these names the way your language does.

Example 1:

Input:
    postTweet(1, 5)
    getNewsFeed(1)
    follow(1, 2)
    postTweet(2, 6)
    getNewsFeed(1)
    unfollow(1, 2)
    getNewsFeed(1)
Output:
5
6 5
5
Explanation:
User 1's first feed holds their own tweet 5.
User 1 then follows user 2, who posts tweet 6: it is newer than 5, so it comes first.
After the unfollow, user 2's tweets leave user 1's feed again.

Example 2:

Input:
    follow(1, 2)
    postTweet(1, 101)
    postTweet(2, 201)
    postTweet(1, 102)
    postTweet(2, 202)
    ...
    postTweet(1, 106)
    postTweet(2, 206)
    getNewsFeed(1)
    getNewsFeed(2)
Output:
206 106 205 105 204 104 203 103 202 102
206 205 204 203 202 201
Explanation:
Users 1 and 2 take turns posting 6 tweets each (101 to 106 and 201 to 206).
User 1's feed has all 12, but shows only the 10 newest: 101 and 201 are left out.
User 2 doesn't follow user 1, so user 2's feed holds only user 2's own tweets.

Example 3:

Input:
    getNewsFeed(3)
    follow(3, 3)
    postTweet(3, 7)
    getNewsFeed(3)
    unfollow(3, 4)
    follow(3, 4)
    getNewsFeed(3)
    postTweet(4, 8)
    getNewsFeed(3)
Output:

7
7
8 7
Explanation:
The first line of the output is empty: nobody has posted yet, so user 3's feed is empty.
follow(3, 3) changes nothing: tweet 7 appears once. unfollow(3, 4) changes nothing either: user 3 didn't follow user 4.
User 3 follows user 4 before user 4 posts anything; once tweet 8 arrives, it is in user 3's feed.

Constraints:

• 1 <= userId, followerId, followeeId <= 500
• 0 <= tweetId <= 109
• All the tweets have different ids.
• At most 105 calls in total are made to postTweet, getNewsFeed, follow and unfollow.