单链表是一种非常适合插入和删除的数据结构，但是查找和修改时间复杂度为O(N)

节点部分：

首先定义一个value来存放值，再定义一个指针来指向下一个节点

	int val;
	Node next;

然后定义构造函数并初始化节点的值

	Node(int val) {
		this.val = val;
		this.next = null;
	}

链表部分：

首先定义头指针和尾指针

	Node head;
	Node tail;

然后是基本操作：增删查

头插：

	public void insertBefore(Node newNode) {
		newNode.next = head;
		head = newNode;
	}

	public void insertAfter(Node newNode) {
		if (head == null)
			head = tail = newNode;
		else {
			tail.next = newNode;
			tail = newNode;
		}
	}

因为如果tail为空，tail.next必定出现空指针错误。而头插法没有出现head.xxx这样的情况

头删：

	public void deleteBefore() {
		if (head == null) {
			System.out.println("linkedlist is empty");
			return;
		}		
		else
			head = head.next;
	}

	public void deleteAfter() {
		if (head == null) {
			System.out.println("linkedlist is empty");
			return;
		}
		if (head.next == null)
			head = null;
		else {
			Node runner = head;
			while (runner.next.next != null)
				runner = runner.next;
			runner.next = runner.next.next;
		}
	}

我们可以看出，尾删的时间复杂度是最高的，因为要一个while循环来找最后一个元素，所以尽量避免用尾删，后面的栈和队列操作我们会更深入的理解。

	public int searchIndex(int value) {
		int counter = 1;
		Node runner = head;

		while (runner != null) {
			if (runner.val == value)
				return counter;
			else {
				runner = runner.next;
				counter++;
			}
		}
		return -1;
	}

	public String searchValue(int index) {
		int counter = 1;
		Node runner = head;

		while (runner != null) {
			if (index == counter)
				return runner.item;
			else {
				runner = runner.next;
				counter++;
			}
		}
		return "out of index!";
	}