Three Stacks implements a Deque

import java.util.*;

public class TwoStackImplementDeque {
    public static void main(String[] args) {
        DequeStructure d = new DequeStructure();
        d.leftPush(4);
        d.leftPush(3);
        d.leftPush(2);
        d.leftPush(1);
        d.rightPush(9);
        d.rightPush(8);
        d.rightPush(7);
        d.rightPush(6);
        System.out.println(d.leftPop());
        System.out.println(d.leftPop());
        System.out.println(d.leftPop());
        System.out.println(d.leftPop());
        System.out.println(d.leftPop());
        System.out.println(d.leftPop());
        System.out.println(d.leftPop());
        System.out.println(d.leftPop());
    }
}

class DequeStructure {
    Deque<Integer> stack1;
    Deque<Integer> stack2;
    Deque<Integer> stack3;
    public DequeStructure () {
        stack1 = new LinkedList<>();
        stack2 = new LinkedList<>();
        stack3 = new LinkedList<>();
    }

    public void leftPush(int x) {
        stack1.push(x);
    }

    public void rightPush(int x) {
        stack2.push(x);
    }

    public int leftPop() {
        if (stack1.isEmpty() && stack2.isEmpty()) {
            return -1;
        }
        if (stack1.isEmpty()){
            shuffle();
        }
        return stack1.pop();
    }

    public int rightPop() {
        if (stack1.isEmpty() && stack2.isEmpty()) {
            return -1;
        }
        if (stack2.isEmpty()) {
            shuffle();
        }
        return stack2.pop();
    }
    
    private void shuffle() {
        //用两个指针，可以省去写多余的代码块！！！
        Deque<Integer> nonEmpty = stack1;
        Deque<Integer> empty = stack2;
        if (stack1.isEmpty()) {
            nonEmpty = stack2;
            empty = stack1;
        }

        int halfSize = nonEmpty.size() / 2;
        for (int i = 0; i < halfSize; i++) {
            stack3.push(nonEmpty.pop());
        }
        while (!nonEmpty.isEmpty()) {
            empty.push(nonEmpty.pop());
        }

        while (!stack3.isEmpty()) {
            nonEmpty.push(stack3.pop());
        }
    }
}