摘要:首先建立按时间戳从大到小排列的,找到中的,出其中在中存在的,把每一个的推特链表放入,再从中取头十条推特的放入结果数组。
Design Twitter Note
建立两个HashMap,一个存user,一个存tweets。以及整型的时间戳timestamp。
user的k-v pair是userId-follower_set,tweets的k-v pair是userId-tweets_linkedlist,tweets属于Tweet类,包含time和id两个参数。
postTweet(userId, tweetId):
首先,对于发推的主体,user,要存放在userMap中,而且user要follow自己,所以,userMap.get(userId).add(userId);
然后,在tweets中找到key值userId,将该用户所有推特内容放入值域的哈希表中。
getNewsFeed(userId):
首先建立按时间戳从大到小排列的PriorityQueue,找到userMap中的userId,filter出其中在tweet中存在的follower,把每一个follower的推特链表放入PriorityQueue,再从PriorityQueue中取头十条推特的id放入结果数组。
follow(followerId, followeeId):
将followeeId加入userMap中的followerId键值。
unfollow(followId, followeeId):
将followeeId从userMap中的followerId键值里删除。
Solutionpublic class Twitter { MapMini Twitter Problem> userMap = new HashMap<>(); Map > tweets = new HashMap<>(); int timestamp = 0; class Tweet { int time; int id; Tweet(int time, int id) { this.time = time; this.id = id; } } public void postTweet(int userId, int tweetId) { if (!userMap.containsKey(userId)) userMap.put(userId, new HashSet<>()); userMap.get(userId).add(userId); if (!tweets.containsKey(userId)) tweets.put(userId, new LinkedList<>()); tweets.get(userId).addFirst(new Tweet(timestamp++, tweetId)); } public List getNewsFeed(int userId) { if (!userMap.containsKey(userId)) return new LinkedList<>(); PriorityQueue feed = new PriorityQueue<>((t1, t2) -> t2.time-t1.time); userMap.get(userId).stream().filter(f -> tweets.containsKey(f)) .forEach(f -> tweets.get(f).forEach(feed::add)); List res = new LinkedList<>(); while (feed.size() > 0 && res.size() < 10) res.add(feed.poll().id); return res; } public void follow(int followerId, int followeeId) { if (!userMap.containsKey(followerId)) userMap.put(followerId, new HashSet<>()); userMap.get(followerId).add(followeeId); } public void unfollow(int followerId, int followeeId) { if (userMap.containsKey(followerId) && followeeId != followerId) userMap.get(followerId).remove(followeeId); } }
Implement a simple twitter. Support the following method:
postTweet(user_id, tweet_text). Post a tweet.
getTimeline(user_id). Get the given user"s most recently 10 tweets posted by himself, order by timestamp from most recent to least recent.
getNewsFeed(user_id). Get the given user"s most recently 10 tweets in his news feed (posted by his friends and himself). Order by timestamp from most recent to least recent.
follow(from_user_id, to_user_id). from_user_id followed to_user_id.
unfollow(from_user_id, to_user_id). from_user_id unfollowed to to_user_id.
postTweet(1, "LintCode is Good!!!") -->> 1 getNewsFeed(1) -->> [1] getTimeline(1) -->> [1] follow(2, 1) getNewsFeed(2) -->> [1] unfollow(2, 1) getNewsFeed(2) -->> []Note
设计一个mini Twitter,刚放出来的几道系统设计题里,这道题才算是关于设计的题目。
要实现这么五个功能:
发推:需要用户ID,推特内容,该推特的时间戳; 时间线:需要用户ID,该用户的推特集,每条推特的时间戳,比较器; 新鲜事:需要用户ID,好友ID集合,选定用户群(用户ID和所有好友ID)的推特集(选定用户的推特集,时间戳),比较器; 关注:需要用户ID,被关注用户ID,好友ID集合; 取关:需要用户ID,被关注用户ID,好友ID集合;
以上,根据功能的要求,确定相关变量。然后选择适当的数据结构:
已经给出的:Tweet(user_id, text, id, create()),
没有给出但是需要的:时间戳order,带时间戳的推特Node,推特集,所有推特集users_tweets,好友集friends,整个系统MiniTwitter,
然后,分析一下具体对象之间的逻辑关系:
Node: 包含Tweet和order,构造Node类;
users_tweets: 包含user_id,和对应id的推特集List
friends: 包含user_id,每个id对应一个好友集Map
这样就很清楚了,我们需要建立一个包含Tweet和Order的Node类,同时声明两个全局变量users_tweets和friends。
然后考虑要求实现的功能。
发推,要用给出的Tweet.create(user_id, tweet_text)函数,然后将集成Tweet和order的Node和对应的user_id放入users_tweets;
时间线,需要从users_tweets中按照Order取出对应user_id的后10个Node,再取出Node.tweet放入结果数组List
新鲜事,需要查看user_id的好友列表,将包括自己的每个人的后10个Node放入List
所以,order也要声明为全局变量。
继续,关注好友,添加或查找from_user_id作为friends中的key值,然后对这个key值对应的value,也就是Map
取关好友,和关注好友相同的操作,先从friend中get的from_user_id的key值,再remove对应Map中to_user_id的pair即可。
下面讨论以上功能实现需要增加的细节。首先,取出前十个Node,取出后十个Node,需要建立两个函数getFirstTen()和getLastTen(),对List
最后,在功能函数的前面,进行MiniTweeter()的初始化。
结束~
public class MiniTwitter { class Node { public int order; public Tweet tweet; public Node(int order, Tweet tweet) { this.order = order; this.tweet = tweet; } } class SortByOrder implements Comparator { public int compare(Object obj1,Object obj2) { Node node1 = (Node) obj1; Node node2 = (Node) obj2; if (node1.order < node2.order) return 1; else return -1; } } private Map> friends; private Map > users_tweets; private int order; public List getLastTen(List temp) { int last = 10; if (temp.size() < 10) last = temp.size(); return temp.subList(temp.size() - last, temp.size()); } public List getFirstTen(List temp) { int last = 10; if (temp.size() < 10) last = temp.size(); return temp.subList(0, last); } public MiniTwitter() { // initialize your data structure here. this.friends = new HashMap >(); this.users_tweets = new HashMap >(); this.order = 0; } // @param user_id an integer // @param tweet a string // return a tweet public Tweet postTweet(int user_id, String text) { Tweet tweet = Tweet.create(user_id, text); if (!users_tweets.containsKey(user_id)) users_tweets.put(user_id, new ArrayList ()); order += 1; users_tweets.get(user_id).add(new Node(order, tweet)); return tweet; } // @param user_id an integer // return a list of 10 new feeds recently // and sort by timeline public List getNewsFeed(int user_id) { List temp = new ArrayList (); if (users_tweets.containsKey(user_id)) temp.addAll(getLastTen(users_tweets.get(user_id))); if (friends.containsKey(user_id)) { for (Map.Entry entry : friends.get(user_id).entrySet()) { int user = entry.getKey(); if (users_tweets.containsKey(user)) temp.addAll(getLastTen(users_tweets.get(user))); } } Collections.sort(temp, new SortByOrder()); List res = new ArrayList (); temp = getFirstTen(temp); for (Node node : temp) { res.add(node.tweet); } return res; } // @param user_id an integer // return a list of 10 new posts recently // and sort by timeline public List getTimeline(int user_id) { List temp = new ArrayList (); if (users_tweets.containsKey(user_id)) temp.addAll(getLastTen(users_tweets.get(user_id))); Collections.sort(temp, new SortByOrder()); List res = new ArrayList (); temp = getFirstTen(temp); for (Node node : temp) res.add(node.tweet); return res; } // @param from_user_id an integer // @param to_user_id an integer // from user_id follows to_user_id public void follow(int from_user_id, int to_user_id) { if (!friends.containsKey(from_user_id)) friends.put(from_user_id, new HashMap ()); friends.get(from_user_id).put(to_user_id, true); } // @param from_user_id an integer // @param to_user_id an integer // from user_id unfollows to_user_id public void unfollow(int from_user_id, int to_user_id) { if (friends.containsKey(from_user_id)) friends.get(from_user_id).remove(to_user_id); } }
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