题目：
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.
链接： http://leetcode.com/problems/swap-nodes-in-pairs/
题解：
原文：http://www.cnblogs.com/yrbbest/p/4434861.html