Given a linked list, swap every two adjacent nodes and return its head.
Example:
Given 1->2->3->4, you should return the list as 2->1->4->3.
Note:
本题是两个一组翻转链表，类似LeetCode 206. Reverse Linked List —— 反转链表。
(Java) LeetCode 24. Swap Nodes in Pairs —— 两两交换链表中的节点
原文：https://www.cnblogs.com/tengdai/p/9279427.html