链表的实现和数组的实现最大的不同在于链表的插入操作代价要低于数组，不过总体代价还是数组更低，因为链表的构造和连接部分代价其实很高。

基本结构

	private Node head = null;

	public void push(String str) {
		// create a new node and put the str into item
		Node newNode = new Node(str);
		// insert before the new node
		newNode.next = head;
		head = newNode;
	}

	public String pop() {

		String popItem; // store the pop String
		
		// if stack is not empty
		if (!isEmpty()) {
			popItem = head.item;
			head = head.next;
			return popItem;
		}

		// empty can not delete
		else {
			System.err.println("Stack is empty");
			return "";
		}
	}

	public boolean isEmpty() {
		return head == null;
	}

	public int size() {
		int size = 0;
		while (head != null) {
			head = head.next;
			size++;
		}
		return size;
	}