Design Hit Counter

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?

Note

基本的解法就是使用队列，去比较一下，但是这样删起来比较麻烦，比如一直都是hit

比较好一些可以使用bucket去存300个时间段，类似环形数组，这样更快

O(s) s is total seconds in given time interval, in this case 300.

basic ideal is using buckets. 1 bucket for every second because we only need to keep the recent hits info for 300 seconds. hit[] array is wrapped around by mod operation. Each hit bucket is associated with times[] bucket which record current time. If it is not current time, it means it is 300s or 600s... ago and need to reset to 1.

Code

public class HitCounter {
    private int[] times;
    private int[] hits;
    /** Initialize your data structure here. */
    public HitCounter() {
        times = new int[300];
        hits = new int[300];
    }

    /** Record a hit.
        @param timestamp - The current timestamp (in seconds granularity). */
    public void hit(int timestamp) {
        int index = timestamp % 300;
        if (times[index] != timestamp) {
            times[index] = timestamp;
            hits[index] = 1;
        } else {
            hits[index]++;
        }
    }

    /** Return the number of hits in the past 5 minutes.
        @param timestamp - The current timestamp (in seconds granularity). */
    public int getHits(int timestamp) {
        int total = 0;
        for (int i = 0; i < 300; i++) {
            if (timestamp - times[i] < 300) {
                total += hits[i];
            }
        }
        return total;
    }
}

class HitCounter {
    private final int TIME = 300; // five minutes
    Deque<Integer> dq;
    /** Initialize your data structure here. */
    public HitCounter() {
        dq = new LinkedList<>();
    }

    /** Record a hit.
        @param timestamp - The current timestamp (in seconds granularity). */
    public void hit(int timestamp) {
        dq.offer(timestamp);
    }

    /** Return the number of hits in the past 5 minutes.
        @param timestamp - The current timestamp (in seconds granularity). */
    public int getHits(int timestamp) {
        if (timestamp <= TIME) {
            return dq.size();
        }
        int start = timestamp - TIME;
        while (!dq.isEmpty() && dq.peek() <= start) {
            dq.poll();
        }

        return dq.size();
    }
}

/**
 * Your HitCounter object will be instantiated and called as such:
 * HitCounter obj = new HitCounter();
 * obj.hit(timestamp);
 * int param_2 = obj.getHits(timestamp);
 */