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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，一直调用自己就能够了。 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
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原文地址：http://www.cnblogs.com/bhlsheji/p/4299504.html
