206.reverse-linked-list.md

## 题目地址
https://leetcode.com/problems/reverse-linked-list/description/

## 题目描述
Reverse a singly linked list.

Example:

Input: 1->2->3->4->5->NULL
Output: 5->4->3->2->1->NULL
Follow up:

A linked list can be reversed either iteratively or recursively. Could you implement both?

## 思路
这个就是常规操作了，使用一个变量记录前驱pre，一个变量记录后继next.

不断更新`current.next = pre` 就好了
## 关键点解析

- 链表的基本操作（交换）
- 虚拟节点dummy 简化操作
- 注意更新current和pre的位置， 否则有可能出现溢出


## 代码

```js
/*
 * @lc app=leetcode id=206 lang=javascript
 *
 * [206] Reverse Linked List
 *
 * https://leetcode.com/problems/reverse-linked-list/description/
 *
 * algorithms
 * Easy (52.95%)
 * Total Accepted:    532.6K
 * Total Submissions: 1M
 * Testcase Example:  '[1,2,3,4,5]'
 *
 * Reverse a singly linked list.
 * 
 * Example:
 * 
 * 
 * Input: 1->2->3->4->5->NULL
 * Output: 5->4->3->2->1->NULL
 * 
 * 
 * Follow up:
 * 
 * A linked list can be reversed either iteratively or recursively. Could you
 * implement both?
 * 
 */
/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var reverseList = function(head) {
    const dummyHead = {
        next: head
    }
    let current = dummyHead.next;
    let pre = null;

    while(current) {
        const next = current.next;
        current.next = pre;
        pre = current;
        current = next;
    }
    return pre;
};

```