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.
思路：肯定是递归方便啦，我们做好了base case，一直调用自己就可以了。 要是想不出来base case是什么，多理解理解就熟悉了
[LeetCode]Swap Nodes in Pairs 成对交换
原文：http://blog.csdn.net/jhbxlx/article/details/39649957