实现LIFO（先进后出）的下压栈

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

/**
 * Created by ZYQ on 2016/7/26.
 * 实现LIFO（先进后出）的下压栈
 */
public class Stack<String> {

    private Node first; // 栈顶
    private int N;      // 元素数量

    // 定义接点的嵌套类
    private class Node{
        String string;
        Node next;
    }

    // 判断链表是否为空
    public boolean isEmpty() {
        return first == null;
    }

    // 统计链表结点数
    public int size() {
        return N;
    }

    // 把元素添加在表头
    public void push(String s) {
        Node oldfirst = first;
        first = new Node();
        first.string = s;
        first.next = oldfirst;
        N++;
    }

    // 将元素从表头删除
    public String pop() {
        String s = first.string;
        first = first.next;
        N--;
        return s;
    }

    // 测试用例
    public static void main(java.lang.String[] args ) throws IOException{

        Stack<java.lang.String> stack = new Stack<java.lang.String>();

        // 创建输入流对象，读取一行数据，并以空格为分隔转化为String数组
        System.out.println("Enter the statement:");
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
        java.lang.String intput = reader.readLine();
        java.lang.String[] item = intput.split(" ");
        for (java.lang.String s : item) {
            // 输入"-"代表出栈
            if (!s.equals("-")) {
                stack.push(s);
            } else if (!stack.isEmpty()) {
                System.out.println(stack.first.string + " left on stack");
                stack.pop();
            }
        }

        // 遍历链表输入下压堆栈的值
        System.out.println("The Stack:");
        int number = stack.size();
        while (number != 0) {
            System.out.print(stack.first.string + " ");
            stack.first = stack.first.next;
            number--;
        }
    }

}