[leetcode]Swap Nodes in Pairs

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：https://oj.leetcode.com/problems/swap-nodes-in-pairs/

描述：链表中按对交换节点

solution by python:

# Definition for singly-linked list.# class ListNode:#     def __init__(self, x):#         self.val = x#         self.next = Noneclass Solution:    # @param a ListNode    # @return a ListNode    def swapPairs(self, head):        if head==None or head.next==None: return head        h = head.next        head.next = h.next        h.next = head        head.next = self.swapPairs(head.next)        return h