[206] 反转链表

难度：简单 题目来源：力扣（LeetCode） https://leetcode-cn.com/problems/reverse-linked-list

说明：
反转一个单链表。

示例：_

输入：1->2->3->4->5->NULL 输出：5->4->3->2->1->NULL

解法：
常规遍历

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
func reverseList(head *ListNode) *ListNode {
    var tmp *ListNode
    for head != nil {
        tmp = &ListNode{Val: head.Val, Next: tmp}
        head = head.Next
    }
    return tmp
}

递归**

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
func reverseList(head *ListNode) *ListNode {
    if head == nil || head.Next == nil {
        return head
    }
    last := reverseList(head.Next)
    head.Next.Next = head
    head.Next = nil
    return last
}