Swap Nodes in Pairs

题目：Swap Nodes in Pairs

描述：

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.

题意：链表的基础操作，元素两两交换，记住头结点防止边界情况就可以了。