两个栈实现队列

题目很简单，今天看了下Java泛型的东西，用泛型改写了一下：

package com.wy.oj;

import java.util.Stack;

/**
 * Created by wy on 2016/12/23 023.
 */
public class TwoStackQueue<T> {
    private Stack<T> s1 = new Stack<>(); // 入队
    private Stack<T> s2 = new Stack<>(); // 出队

    public T pop() {
        if (isEmpty()) return null;
        if (!s2.isEmpty()) return s2.pop();
        else {
            while (!s1.isEmpty())
                s2.push(s1.pop());
            return s2.pop();
        }
    }

    public void push(T item) {
        s1.push(item);
    }

    public int size() {
        return s1.size() + s2.size();
    }

    public boolean isEmpty() {
        return (s1.isEmpty() && s2.isEmpty());
    }

    // 测试
    public static void main(String[] args) {
        TwoStackQueue<String> qs = new TwoStackQueue<>();
        for (String s : "qing li si nian chun".split(" "))
            qs.push(s);
        for (int i=0; i<3; ++i)
            System.out.println(qs.pop());
        System.out.println();
        for (String s : "ten zi jing".split(" "))
            qs.push(s);
        while (!qs.isEmpty())
            System.out.println(qs.pop());

        TwoStackQueue<Integer> qi = new TwoStackQueue<>();
        for (int i=10; i<100; i=2*i)
            qi.push(i);
        while (!qi.isEmpty())
            System.out.println(qi.pop());
    }
}