[Daily Coding Problem] 16. Last N order ids implementation

This problem was asked by Twitter.

You run an e-commerce website and want to record the last N order ids in a log. Implement a data structure to accomplish this, with the following API:

• record(order_id): adds the order_id to the log
• get_last(i): gets the ith last element from the log. i is guaranteed to be smaller than or equal to N.

You should be as efficient with time and space as possible.

 

Implementing a circular buffer suffices the requirement. It takes O(1) to record and get last ith. 

 

public class LogDataStructure {
    private int maxSize;
    private int[] circularBuffer;
    private int currIdx;
        
    public LogDataStructure(int n) {
        this.maxSize = n;
        this.circularBuffer = new int[n];
        this.currIdx = 0;
    }

    public void record(int orderId) {
        circularBuffer[currIdx] = orderId;
        currIdx = (currIdx + 1) % maxSize;
    }
    
    public int getLast(int i) {
        return circularBuffer[(currIdx - i + maxSize) % maxSize];
    }
}