Lintcode 1786. Pub Sub Pattern

Description

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Pub/Sub pattern is a wide used pattern in system design. In this problem, you need to implement a pub/sub pattern to support user subscribes on a specific channel and get notification messages from subscribed channels.

There are 3 methods you need to implement:

• subscribe(channel, user_id): Subscribe the given user to the given channel.
• unsubscribe(channel, user_id): Unsubscribe the given user from the given channel.
• publish(channel, message): You need to publish the message to the channel so that everyone subscribed on the channel will receive this message. Call PushNotification.notify(user_id, message) to push the message to the user.

Have you met this question in a real interview?  

Example

subscribe("group1",  1)
publish("group1", "hello")
>> user 1 received "Hello"
subscribe("group1", 2)
publish("group1", "thank you")
>> user 1 received "thank you"
>> user 2 received "thank you"
unsubscribe("group2", 3)
>> user 3 is not in group2, do nothing
unsubscribe("group1", 1)
publish("group1", "thank you very much")
>> user 2 received "thank you very much"
publish("group2", "are you ok?")
>> # you don't need to push this message to anyone

If there are more than 1 user subscribed on the same channel, it doesn't matter the order of time users receiving the message. It's ok if you push the message to user 2 before user 1.

Code (Java):

/* Definition of PushNotification
 * class PushNotification {
 *     public static void notify(int user_id, String the_message)
 *  };
 */
//
public class PubSubPattern {
    Map<String, Set<Integer>> channelTable;
    public PubSubPattern(){
     // Write your code here
     channelTable = new HashMap<>();
    }
    
    /**
     * @param channel: the channel's name
     * @param user_id: the user who subscribes the channel
     * @return: nothing
     */
    public void subscribe(String channel, int user_id) {
        // Write your code here
        Set<Integer> users = channelTable.getOrDefault(channel, new HashSet<>());
        users.add(user_id);
        channelTable.put(channel, users);
    }

    /**
     * @param channel: the channel's name
     * @param user_id: the user who unsubscribes the channel
     * @return: nothing
     */
    public void unsubscribe(String channel, int user_id) {
        // Write your code here
        if (channelTable.containsKey(channel)) {
            Set<Integer> users = channelTable.get(channel);
            users.remove(user_id);
        }
    }

    /**
     * @param channel: the channel's name
     * @param message: the message need to be delivered to the channel's subscribers
     * @return: nothing
     */
    public void publish(String channel, String message) {
        // Write your code here
        if (channelTable.containsKey(channel)) {
            Set<Integer> users = channelTable.get(channel);
            for (Integer user : users) {
                PushNotification.notify(user, message);
            }
        }
    }
}