1.保存反转开始节点的前驱节点 2.断链&接链过程中注意顺序，不要丢失节点
https://leetcode.cn/problems/reverse-linked-list/description/ https://leetcode.cn/problems/reverse-linked-list-ii/description/ https://leetcode.cn/problems/reverse-nodes-in-k-group/description/
木ノ葉の黄色い閃光
