用两个栈实现队列

package test;

import java.util.Stack;

public class StackQueneTest {

	private Stack<Integer> stackA = new Stack<Integer>();
	private Stack<Integer> stackB = new Stack<Integer>();
	
	/**
	 * 入队操作
	 * 
	 * @param element
	 *            入队的元素
	 */
	public void enQueue(int element) {
		stackA.push(element);
	}

	/**
	 * 出队操作
	 */
	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) throws Exception {
		StackQueneTest stackQueue = new StackQueneTest();
		stackQueue.enQueue(4);
		stackQueue.enQueue(2);
		stackQueue.enQueue(7);
		System.out.println(stackQueue.deQueue());
		System.out.println(stackQueue.deQueue());
		stackQueue.enQueue(9);
		System.out.println(stackQueue.deQueue());
		System.out.println(stackQueue.deQueue());
	}
}