Mini Twitter

/**
 * Definition of Tweet:
 * public class Tweet {
 *     public int id;
 *     public int user_id;
 *     public String text;
 *     public static Tweet create(int user_id, String tweet_text) {
 *         // This will create a new tweet object,
 *         // and auto fill id
 *     }
 * }
 */
public class MiniTwitter {

    private ArrayList<Tweet> list;
    private HashMap<Integer, HashSet<Integer>> followers;
    public MiniTwitter() {
        // initialize your data structure here.
        list = new ArrayList<Tweet>();
        followers = new HashMap<Integer, HashSet<Integer>>();
    }

    // @param user_id an integer
    // @param tweet a string
    // return a tweet
    public Tweet postTweet(int user_id, String tweet_text) {
        Tweet t = Tweet.create(user_id, tweet_text);
        list.add(t);
        return t;
    }

    // @param user_id an integer
    // return a list of 10 new feeds recently
    // and sort by timeline
    public List<Tweet> getNewsFeed(int user_id) {
        // Write your code here
        List<Tweet> tList = new ArrayList<Tweet>();
        for (int i = list.size() - 1; i >= 0; i--) {
            Tweet t = list.get(i);
            if (t.user_id == user_id || isFollower(user_id, t.user_id)) {
                tList.add(t);
                if (tList.size() == 10) {
                    break;
                }
            }
        }
        return tList;
    }
    
    public boolean isFollower(int from, int to) {
        if (followers.containsKey(from)) {
            return followers.get(from).contains(to);
        }
        return false;
    }
        
    // @param user_id an integer
    // return a list of 10 new posts recently
    // and sort by timeline
    public List<Tweet>  getTimeline(int user_id) {
        // Write your code here
        List<Tweet> tList = new ArrayList<Tweet>();
        for (int i = list.size() - 1; i >= 0; i--) {
            Tweet t = list.get(i);
            if (t.user_id == user_id) {
                tList.add(t);
                if (tList.size() == 10) {
                    break;
                }
            }
        }
        return tList;
    }

    // @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) {
        // Write your code here
        if (followers.containsKey(from_user_id)) {
            followers.get(from_user_id).add(to_user_id);
        } else {
            followers.put(from_user_id, new HashSet<Integer>());
            follow(from_user_id, to_user_id);
        }
    }

    // @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 (followers.containsKey(from_user_id)) {
            followers.get(from_user_id).remove(to_user_id);
        } 
    }
}