二、 编写一个类，用两个栈实现队列，支持队列的基本操作（add,poll,peek）

请指教交流！

 package com.it.hxs.c01;

 import java.util.Stack;

 /*
  编写一个类，用两个栈实现队列，支持队列的基本操作（add,poll,peek）
  */
 public class HxsQueue {

     public static void main(String args[]) {
         HxsQueue demoQueue = new HxsQueue();
         demoQueue.add("111");
         demoQueue.add("222");
         demoQueue.add("333");
         demoQueue.poll();
         System.out.println(demoQueue.peek());
     }

     private Stack<String> pushStack;// 推入栈
     private Stack<String> popStack;// 推出栈

     public HxsQueue() {
         this.pushStack = new Stack<String>();
         this.popStack = new Stack<String>();
     }

     public void add(String content) {
         if ("".equals(content) || content == null) {
             throw new RuntimeException("添加的元素值不能为空！");
         } else {
             pushStack.push(content);
             this.popStack = new Stack<String>();
             for (int index = pushStack.size() - 1; index >= 0; index--) {
                 popStack.push(pushStack.elementAt(index));
             }
         }
     }

     public String poll() {
         String result = "无元素";
         if (!popStack.isEmpty()) {
             result = popStack.pop();
             this.pushStack = new Stack<String>();
             for (int index = popStack.size() - 1; index >= 0; index--) {
                 pushStack.push(popStack.elementAt(index));
             }
         }
         return result;
     }

     public String peek() {
         String result = "无元素";
         if (!popStack.isEmpty()) {
             result = popStack.peek();
         }
         return result;
     }

 }