362. Design Hit Counter

package LeetCode_362

import java.util.*
import kotlin.collections.HashMap

/**
 * 362. Design Hit Counter
 * (Prime)
 * Design a hit counter which counts the number of hits received in the past 5 minutes.
Each function accepts a timestamp parameter (in seconds granularity) and you may assume that calls are being made to the system in chronological order (ie, the timestamp is monotonically increasing).
You may assume that the earliest timestamp starts at 1.
It is possible that several hits arrive roughly at the same time.
Example:
HitCounter counter = new HitCounter();
// hit at timestamp 1.
counter.hit(1);
// hit at timestamp 2.
counter.hit(2);
// hit at timestamp 3.
counter.hit(3);
// get hits at timestamp 4, should return 3.
counter.getHits(4);
// hit at timestamp 300.
counter.hit(300);
// get hits at timestamp 300, should return 4.
counter.getHits(300);
// get hits at timestamp 301, should return 3.
counter.getHits(301);
Follow up:
What if the number of hits per second could be very large? Does your design scale?
 * */
class Solution {
    /*
    * solution, HashMap + Queue
    * */

    //key: timestamp, value: Node that contains timestamp and hit count with same timestamp
    val map = HashMap<Int, Node>()
    val queue = LinkedList<Node>()

    /**
     * Record a hit.
    @param timestamp - The current timestamp (in seconds granularity).
     */
    fun hit(timestamp: Int) {
        if (map.containsKey(timestamp)) {
            val node = map.get(timestamp)
            node!!.count++
        } else {
            val node = Node()
            node.timestamp = timestamp
            node.count = 1
            queue.add(node)
            map.put(timestamp, node)
        }
        //check if need remove
        removeOld(timestamp)
    }

    /**
     * Return the number of hits in the past 5 minutes.
    @param timestamp - The current timestamp (in seconds granularity).
     */
    fun getHits(timestamp: Int): Int {
        //check if need remove
        removeOld(timestamp)
        var sum = 0
        for (node in queue) {
            sum += node.count
        }
        return sum
    }

    /**
     * check timestamp in queue if > 300s (5 minutes), if true, remove it
     * */
    private fun removeOld(timestamp: Int) {
        while (queue.isNotEmpty()) {
            val cur = queue.peek()
            if (timestamp - cur.timestamp >= 300) {
                queue.poll()
                map.remove(cur.timestamp)
            } else {
                break
            }
        }
    }
}

class Node {
    var timestamp = 0
    var count = 0
}