leetcode355. Design Twitter

摘要： 題目要求 Design a simplified version of Twitter where users can post tweets, follow/unfollow another user and is able to see the 10 most recent tweet...

題目要求

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);

設計一個迷你推特，要求能夠支援以下幾個方法：釋出推特，關注使用者，取關使用者，檢視最近的十條關注使用者傳送的推特。

思路和程式碼

這道題目本質上是考察是否能將資料結構的知識靈活的運用於現實生活中。從最直觀的想法來看，我們會有一個使用者實體，每個使用者會記錄自己關注的使用者的id，以及記錄自己發表的所有tweet。這裡唯一的難點在於我們如何按照時間順序獲取tweet流。

這麼一想，這題其實就轉換為如何將N個有序排列的陣列匯合成一個有序的陣列。這題等價於我們每次都會比較當前所有被關注者釋出的最新未讀tweet，選出結果後將其插入結果集。這裡我們可以利用等價佇列幫助我們更快的完成選擇的過程。

public class Twitter {


public Twitter() {
users = new HashMap<>();
}

public static int timestamp = 0;
private Map<Integer, User> users;

/** Compose a new tweet. */
public void postTweet(int userId, int tweetId) {
if(!users.containsKey(userId)) {
User user = new User(userId);
users.put(userId, user);
}

User user = users.get(userId);
user.tweet(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. 
* */
public List<Integer> getNewsFeed(int userId) {
List<Integer> result = new ArrayList<Integer>();
if(!users.containsKey(userId)) {
return result;
}
User user = users.get(userId);

PriorityQueue<Tweet> queue = new PriorityQueue<>(user.followed.size());
for(int followee : user.followed) {
User tmp = users.get(followee);
if(tmp != null && tmp.headTweet != null) {
queue.offer(tmp.headTweet);
} 
}

while(!queue.isEmpty() && result.size() < 10) {
Tweet t = queue.poll();
result.add(t.tweetId);
if(t.next != null) {
queue.offer(t.next);
}
}
return result;
}

/** Follower follows a followee. If the operation is invalid, it should be a no-op. */
public void follow(int followerId, int followeeId) {
if(!users.containsKey(followerId)) {
User user = new User(followerId);
users.put(followerId, user);
}
if(!users.containsKey(followeeId)) {
User user = new User(followeeId);
users.put(followeeId, user);
}

User user = users.get(followerId);
user.follow(followeeId);
}

/** Follower unfollows a followee. If the operation is invalid, it should be a no-op. */
public void unfollow(int followerId, int followeeId) {
if(!users.containsKey(followerId) || followerId == followeeId) {
return;
}


User user = users.get(followerId);
user.unfollow(followeeId);
}

public static class User{

int userId;

Set<Integer> followed;

Tweet headTweet;

public User(int userId) {
this.userId = userId;
this.followed = new HashSet<>();
follow(userId);
}

public void follow(int userId) {
followed.add(userId);
}

public void unfollow(int userId) {
followed.remove(userId);
}

public void tweet(int tweetId) {
Tweet tweet = new Tweet(tweetId);
tweet.next = headTweet;
headTweet = tweet;
}

}

public static class Tweet implements Comparable<Tweet>{

int tweetId;

Tweet next;

int time;

public Tweet(int tweetId) {
this.tweetId = tweetId;
this.time = timestamp++;
}

@Override
public int compareTo(Tweet o) {
return o.time - this.time;
}
}
}