用栈实现队列

/**
 * @Author: subd
 * @Date: 2019/9/5 8:38
 * 用栈来实现队列
 */
public class StackImplementQueue {
    private Stack<Integer> stackA = new Stack<>();
    private Stack<Integer> stackB = new Stack<>();

    /**
     * 入栈操作
     *
     * @param element 入队元素
     */
    public void enQueue(int element) {
        stackA.push(element);
    }

    /**
     * 出队操作
     *
     * @return 出队元素
     */
    public Integer deQueue() {
        if (stackB.isEmpty()) {
            if (stackA.isEmpty()) {
                return null;

            }
            transfer();
        }
        return stackB.pop();
    }

    /**
     * 栈A元素转移到栈B
     */
    private void transfer() {
        while (!stackA.isEmpty()) {
            stackB.push(stackA.pop());
        }
    }

    public static void main(String[] args) {
        StackImplementQueue stackImplementQueue = new StackImplementQueue();
        stackImplementQueue.enQueue(1);
        stackImplementQueue.enQueue(2);
        stackImplementQueue.enQueue(3);
        System.out.println(stackImplementQueue.deQueue());
        System.out.println(stackImplementQueue.deQueue());
        stackImplementQueue.enQueue(4);
        System.out.println(stackImplementQueue.deQueue());
        System.out.println(stackImplementQueue.deQueue());
    }
}