362. Design Hit Counter

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Description

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?

Problem URL

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Solution

设计一个hit counter，计算300秒以内的hit数。传入timestamp分析实现。

Using two arrays to perform as two round array. The two arrays, time stores timestamp, hits stores corresponding hit time of this timestamp. The two array all have length 300.

For hit(), when timestamp is sended, mod 300 to get the index in array. If times have this timestamp, hits[index]++, or add it in times and set hits to 1;

For getHits(), iterative traverse hits, find the timestamp less than 300 to the new timestamp and add its hit up.

Code

1. heap

class HitCounter {

    /** Initialize your data structure here. */
    int[] times;
    int[] hits;
    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 res = 0;
        for (int i = 0; i < 300; i++){
            if (timestamp - times[i] < 300){
                res += hits[i];
            }
        }
        return res;
    }
}

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

Time Complexity: O(n)
Space Complexity: O(300)

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