用两个stack设计一个队列

import java.util.Stack;

//用两个stack设计一个队列 p142

public class MyQueue<T> {
    Stack<T> in;
    Stack<T> out;

    public MyQueue() {
        in = new Stack<T>();
        out = new Stack<T>();
    }

    public void enqueue(T a) {
        in.push(a);
    }

    public T dequeue() {
        shiftItems();
        return out.pop();
    }

    public T peek() {
        shiftItems();
        return out.peek();
    }

    private void shiftItems() {
        if (!out.isEmpty())
            return;
        while (!in.isEmpty()) {
            out.push(in.pop());
        }
    }

    public static void main(String[] args) {
        MyQueue<Integer> queue = new MyQueue<Integer>();
        queue.enqueue(1);
        queue.enqueue(2);
        queue.enqueue(3);

        System.out.println(queue.dequeue());

        queue.enqueue(4);
        System.out.println(queue.dequeue());
        System.out.println(queue.dequeue());
        System.out.println(queue.dequeue());

    }

}