leetcode 题解 ||　Swap Nodes in Pairs 问题

ｐｒｏｂｌｅｍ：

Given a linked list, swap every two adjacent nodes and return its head.

For example,
Given 1->2->3->4, you should return the list as 2->1->4->3.

Your algorithm should use only constant space. You may not modify the values in the list,
 only nodes itself can be changed.Given a linked list, swap every two adja