链式栈 java实现

来自thinking In java 的链式栈简单实现[主要是添加末端哨兵，用于判断栈空]

这个跟LinkedList存入一个多余的header节点是一样的道理

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/**
 *链式栈 thinking in java chapter15
 * @author gaoxingliang
 */
public class LinkedStack<T> {
    
    private static class Node<T>{
        T data;
        Node<T> next;
        Node(){
            data=null;
            next=null;
        }
        Node(T data,Node<T> next){
            this.data=data;
            this.next=next;
        }
        boolean hasNext(){
            return !(data==null&&next==null);
        }
    }
    
    private Node<T> top=new Node<T>();
    
    /**
     * 压入栈
     * @param data 
     */
    public void push(T data){
        top=new Node<T>(data,top);
    }
 
    public T pop(){
        T result=top.data;
        if(top.hasNext()==true){
            top=top.next;
        }else{
            //System.out.println("no node for pop");
        }
        return result;
    }
    
    //test
    public static void main(String[] args){
        //java version 1.7
        LinkedStack<String> stack=new LinkedStack<>();
        stack.push("123");
        stack.push("456");
        System.out.println(stack.pop());
        System.out.println(stack.pop());
        System.out.println(stack.pop());//null
        System.out.println(stack.pop());//null
    }
}