程序代码：

public class Count3Quit2 {
    public static void main(String[] args) {
        KidCircle kc = new KidCircle(500);
        int countNum = 0;
        Kid k = kc.first;
        while(kc.count > 1) {
            countNum ++;
            if(countNum == 3) {
                countNum = 0;
                kc.delete(k);
            }
            k = k.right;
        }
   
        System.out.println(kc.first.id);
    }
}
class Kid {
    int id;
    Kid left;
    Kid right;       //这里定义了left和right他们是kid类里面的变量
}
class KidCircle {
    int count = 0;
    Kid first, last;//这里定义first,last，他们是kidcircle类里面的变量

    KidCircle(int n) {
        for(int i=0; i<n; i++) {
            add();
        }
    }

    void add() {
        Kid k = new Kid();
        k.id = count;
        if(count <= 0) {
            first = k;
            last = k;
            k.left = k;
            k.right = k;
        } else {
            last.right = k;//这里看不明白是什么语法？请指导。。。
            k.left = last;
            k.right = first;
            first.left = k;
            last = k;
        }
        count ++;
    }

    void delete(Kid k) {
        if(count <= 0) {
            return;
        } else if (count == 1) {
            first = last = null;
        } else {
            k.left.right = k.right;//这里也不明白，left是kidcircle类里面的变量，为什么KID类的对象K，能用它？真的不懂，请指导。谢谢
            k.right.left = k.left;
       
            if(k == first) {
                first = k.right;
            } else if( k == last) {
                last = k.left;
            }
        }
        count --;
    }
}