Heart Beat

https://github.com/zxqiu/leetcode-lintcode/blob/master/system%20design/Heart_Beat.java
Heart Beat
In the Master-Slave architecture, slave server will ping master in every k seconds to tell master server he is alive. If a master server didn't receive any ping request from a slave server in 2 * k seconds, the master will trigger an alarm (for example send an email) to administrator.
Let's mock the master server, you need to implement the following three methods:
    initialize(slaves_ip_list, k). salves_ip_list is a list of slaves' ip addresses. k is define above.
    ping(timestamp, slave_ip). This method will be called every time master received a ping request from one of the slave server. timestamp is the current timestamp in seconds. slave_ip is the ip address of the slave server who pinged master.
    getDiedSlaves(timestamp). This method will be called periodically (it's not guaranteed how long between two calls). timestamp is the current timestamp in seconds, and you need to return a list of slaves' ip addresses that died. Return an empty list if no died slaves found.
You can assume that when the master started, the timestamp is 0, and every method will be called with an global increasing timestamp.
 Example
initialize(["10.173.0.2", "10.173.0.3"], 10)
ping(1, "10.173.0.2")
getDiedSlaves(20)
>> ["10.173.0.3"]
getDiedSlaves(21)
>> ["10.173.0.2", "10.173.0.3"]
ping(22, "10.173.0.2")
ping(23, "10.173.0.3")
getDiedSlaves(24)
>> []
getDiedSlaves(42)
>> ["10.173.0.2"]
解：
用一个HashMap维护每个slave和对应的最后一次出现的timestamp。
当有新的ping时，更新这个map。
当取dead slave时，对比当前timestamp和map中每个slave的timestamp，找出超时的即可。
    Map<String, Integer> slaves;
    int interval;

    public HeartBeat() {
        slaves = new HashMap<>();
    }

    // @param slaves_ip_list a list of slaves'ip addresses
    // @param k an integer
    // @return void
    public void initialize(List<String> slaves_ip_list, int k) {
        if (slaves_ip_list == null) {
            return;
        }
       
        for (String ip : slaves_ip_list) {
            slaves.put(ip, 0);
        }
        interval = k;
    }

    // @param timestamp current timestamp in seconds
    // @param slave_ip the ip address of the slave server
    // @return nothing
    public void ping(int timestamp, String slave_ip) {
        if (!slaves.containsKey(slave_ip)) {
            return;
        }
       
        slaves.put(slave_ip, timestamp);
    }

    // @param timestamp current timestamp in seconds
    // @return a list of slaves'ip addresses that died
    public List<String> getDiedSlaves(int timestamp) {
        List<String> ret = new ArrayList<>();
       
        for (Map.Entry entry : slaves.entrySet()) {
            if (timestamp - (Integer)entry.getValue() >= 2 * interval) {
                ret.add((String)entry.getKey());
            }
        }
       
        return ret;
    }