Mini Twitter

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.

Example: 
postTweet(1, "LintCode is Good!!!")
>> 1
getNewsFeed(1)
>> [1]
getTimeline(1)
>> [1]
follow(2, 1)
getNewsFeed(2)
>> [1]
unfollow(2, 1)
getNewsFeed(2)
>> []

  1 /**
  2  * 本代码由九章算法编辑提供。没有版权欢迎转发。
  3  * - 九章算法致力于帮助更多中国人找到好的工作，教师团队均来自硅谷和国内的一线大公司在职工程师。
  4  * - 现有的面试培训课程包括：九章算法班，系统设计班，九章强化班，Java入门与基础算法班，
  5  * - 更多详情请见官方网站：http://www.jiuzhang.com/
  6  */
  7 
  8 /**
  9  * Definition of Tweet:
 10  * public class Tweet {
 11  *     public int id;
 12  *     public int user_id;
 13  *     public String text;
 14  *     public static Tweet create(int user_id, String tweet_text) {
 15  *         // This will create a new tweet object,
 16  *         // and auto fill id
 17  *     }
 18  * }
 19  */
 20 public class MiniTwitter {
 21     class Node {
 22         public int order;
 23         public Tweet tweet;
 24         public Node(int o, Tweet t) {
 25             this.order = o;
 26             this.tweet = t;
 27         }
 28     }
 29 
 30     class SortByOrder implements Comparator {     
 31         public int compare(Object obj1,Object obj2) {     
 32             Node node1 = (Node) obj1;     
 33             Node node2 = (Node) obj2;     
 34             if (node1.order < node2.order)
 35                 return 1;
 36             else
 37                 return -1;
 38         }
 39     }     
 40 
 41     private Map<Integer, Set<Integer>> friends;
 42     private Map<Integer, List<Node>> users_tweets;
 43     private int order;
 44     
 45     public List<Node> getLastTen(List<Node> tmp) {
 46         int last = 10;
 47         if (tmp.size() < 10)
 48             last = tmp.size();
 49         return tmp.subList(tmp.size() - last, tmp.size());
 50     }
 51 
 52     public List<Node> getFirstTen(List<Node> tmp) {
 53         int last = 10;
 54         if (tmp.size() < 10)
 55             last = tmp.size();
 56         return tmp.subList(0, last);
 57     }
 58 
 59     public MiniTwitter() {
 60         // initialize your data structure here.
 61         this.friends = new HashMap<Integer, Set<Integer>>();
 62         this.users_tweets = new HashMap<Integer, List<Node>>();
 63         this.order = 0;
 64     }
 65 
 66     // @param user_id an integer
 67     // @param tweet a string
 68     // return a tweet
 69     public Tweet postTweet(int user_id, String tweet_text) {
 70         //  Write your code here
 71         Tweet tweet = Tweet.create(user_id, tweet_text);
 72         if (!users_tweets.containsKey(user_id))
 73             users_tweets.put(user_id, new ArrayList<Node>());
 74         order += 1;
 75         users_tweets.get(user_id).add(new Node(order, tweet));
 76         return tweet;
 77     }
 78 
 79     // @param user_id an integer
 80     // return a list of 10 new feeds recently
 81     // and sort by timeline
 82     public List<Tweet> getNewsFeed(int user_id) {
 83         // Write your code here
 84         List<Node> tmp = new ArrayList<Node>();
 85         if (users_tweets.containsKey(user_id))
 86             tmp.addAll(getLastTen(users_tweets.get(user_id)));
 87 
 88         if (friends.containsKey(user_id)) {
 89             for (Integer user : friends.get(user_id)) {
 90                 if (users_tweets.containsKey(user))
 91                     tmp.addAll(getLastTen(users_tweets.get(user)));
 92             }
 93         }
 94 
 95         Collections.sort(tmp, new SortByOrder());
 96         List<Tweet> rt = new ArrayList<Tweet>();
 97         tmp = getFirstTen(tmp);
 98         for (Node node : tmp) {
 99             rt.add(node.tweet);
100         }
101         return rt;
102     }
103         
104     // @param user_id an integer
105     // return a list of 10 new posts recently
106     // and sort by timeline
107     public List<Tweet>  getTimeline(int user_id) {
108         // Write your code here
109         List<Node> tmp = new ArrayList<Node>();
110         if (users_tweets.containsKey(user_id))
111             tmp.addAll(getLastTen(users_tweets.get(user_id)));
112 
113         Collections.sort(tmp, new SortByOrder());
114         List<Tweet> rt = new ArrayList<Tweet>();
115         tmp = getFirstTen(tmp);
116         for (Node node : tmp)
117             rt.add(node.tweet);
118         return rt;
119     }
120 
121     // @param from_user_id an integer
122     // @param to_user_id an integer
123     // from user_id follows to_user_id
124     public void follow(int from_user_id, int to_user_id) {
125         // Write your code here
126         if (!friends.containsKey(from_user_id))
127             friends.put(from_user_id, new HashSet<Integer>());
128 
129         friends.get(from_user_id).add(to_user_id);
130     }
131 
132     // @param from_user_id an integer
133     // @param to_user_id an integer
134     // from user_id unfollows to_user_id
135     public void unfollow(int from_user_id, int to_user_id) {
136         // Write your code here
137         if (friends.containsKey(from_user_id))
138             friends.get(from_user_id).remove(to_user_id);
139     }
140 }