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这是典型的系统设计题目，系统设计最重要的是要搞清楚业务是什么，需要储存什么数据，然后用面向对象的方法把程序写出来，其中算法优化不是最重要的。

int system_time = 0;class Tweet{
public:int tweetId;int time;Tweet(int tweetId):tweetId(tweetId){time = system_time++;}
};class User{
public:unordered_set<int> follows;list<Tweet> tweets;int userId;User(int userId):userId(userId){}void postTweet(int tweetId){tweets.push_front(Tweet(tweetId));if(tweets.size() > 10){tweets.pop_back();}}void follow(int followeeId){follows.insert(followeeId);}void unfollow(int followeeId) {if(follows.count(followeeId)){follows.erase(followeeId);}}};class Twitter {
public:/** Initialize your data structure here. */unordered_map<int, User*> users_map;Twitter() {}/** Compose a new tweet. */void postTweet(int userId, int tweetId) {if(!users_map.count(userId)){users_map[userId] = new User(userId);}users_map[userId]->postTweet(tweetId);}/** 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. */vector<int> getNewsFeed(int userId) {if(!users_map.count(userId)){users_map[userId] = new User(userId);}vector<Tweet> tweets;for(auto tweet : users_map[userId]->tweets){tweets.push_back(tweet);}for(auto followerId : users_map[userId]->follows){if(users_map[followerId]!=nullptr){for(auto tweet : users_map[followerId]->tweets){tweets.push_back(tweet);}}}sort(tweets.begin(), tweets.end(), [](Tweet a, Tweet b){return a.time > b.time;});vector<int> news;int i = 0;while(i < tweets.size() && i < 10){news.push_back(tweets[i].tweetId);i++;}return news;}/** Follower follows a followee. If the operation is invalid, it should be a no-op. */void follow(int followerId, int followeeId) {if(!users_map.count(followerId)){users_map[followerId] = new User(followerId);}users_map[followerId]->follow(followeeId);}/** Follower unfollows a followee. If the operation is invalid, it should be a no-op. */void unfollow(int followerId, int followeeId) {if(!users_map.count(followerId)){users_map[followerId] = new User(followerId);}users_map[followerId]->unfollow(followeeId);}
};/*** Your Twitter object will be instantiated and called as such:* Twitter* obj = new Twitter();* obj->postTweet(userId,tweetId);* vector<int> param_2 = obj->getNewsFeed(userId);* obj->follow(followerId,followeeId);* obj->unfollow(followerId,followeeId);*/