反转链表 https://leetcode-cn.com/problems/fan-zhuan-lian-biao-lcof/ https://www.cnblogs.com/zhbeii/p/15422709.html 两两交换相邻 https://leetcode-cn.com/problems/swap-nodes-in-pairs/ https://www.cnblogs.com/zhbeii/p/15685172.html