LeetCode 24. Swap Nodes in Pairs （Python）

题目描述：

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.

大致思路：
因为要求不能只交换节点的值，所以在遍历的时候通过三个指针进行节点的交换，注意开始时添加一个任意节点指向头结点以便于交换。

AC代码：

class Solution(object):    def swapPairs(self, head):        """        :type head: ListNode        :rtype: ListNode        """        if not head:            return head        dummy = ListNode(0)        dummy.next = head        copy = dummy        while copy.next and copy.next.next:            p = copy.next            q = p.next            copy.next = q            p.next = q.next            q.next = p            copy = p        return dummy.next