帮忙做一道数据结构的题目

public static void main(String args[]) {
        List<Integer> a = new ArrayList<Integer>();
        for (int i = 1; i <= 500; i++) {
            a.add(i);// 把每个小朋友的号码依次放入.
        }
        int num=0,currentIndex=0;
        while (a.size() > 1) {
            num++;
            if(num==3){
                a.remove(currentIndex);
                currentIndex--;
                num=0;
            }
            currentIndex++;
            if(currentIndex>=a.size())
                currentIndex=0;
        }
        System.out.println("最后一个小朋友的号码是: " + a.get(0));
    }