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. 

 1 public class LogDataStructure {
 2     private int maxSize;
 3     private int[] circularBuffer;
 4     private
 
 int currIdx;
 5         
 6     public LogDataStructure(int n) {
 7         this.maxSize = n;
 8         this.circularBuffer = new int[n];
 9         this.currIdx = 0;
10     }
11 
12     public void record(int orderId) {
13         circularBuffer[currIdx] = orderId;
14         currIdx = (currIdx + 1) % maxSize;
 
15     }
16     
17     public int getLast(int i) {
18         return circularBuffer[(currIdx - i + maxSize) % maxSize];
19     }
20 }