[leetcode]Swap Nodes in Pairs @ Python

原题地址：http://oj.leetcode.com/problems/swap-nodes-in-pairs/

题意：将链表中的节点两两交换。Given 1->2->3->4, you should return the list as 2->1->4->3.

解题思路：这题主要涉及到链表的操作，没什么特别的技巧，注意不要出错就好。最好加一个头结点，操作起来会很方便。

代码：

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    # @param a ListNode
    # @return a ListNode
    def swapPairs(self, head):
        if head == None or head.next == None:
            return head
        dummy = ListNode(0); dummy.next = head
        p = dummy
        while p.next and p.next.next:
            tmp = p.next.next
            p.next.next = tmp.next
            tmp.next = p.next
            p.next = tmp
            p = p.next.next
        return dummy.next