24. Swap Nodes in Pairs
题目：
https://leetcode.com/problems/swap-nodes-in-pairs/
难度 : Easy
看了hint，用loop做，每个node关系要弄清楚
class Solution(object):
def swapPairs(self, head):
"""
:type head: ListNode
:rtype: ListNode
"""
if head == None or head.next == None:
return head
dummy = ListNode(-1)
dummy.next = head
cur = dummy
while cur.next and cur.next.next:
next_one, next_two, next_three = cur.next, cur.next.next, cur.next.next.next
cur.next = next_two
next_two.next = next_one
next_one.next = next_three
cur = next_one
return dummy.next
