24. Swap Nodes in Pairs leetcode Python 2016 new Season

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.

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# Definition for singly-linked list.# class ListNode(object):#     def __init__(self, x):#         self.val = x#         self.next = Noneclass Solution(object):    def swapPairs(self, head):        """        :type head: ListNode        :rtype: ListNode        """        if not head or not head.next:            return head        dummyHead = pNode = ListNode(None)        pNode.next = head        while pNode.next and pNode.next.next:            tmp = pNode.next.next            pNode.next.next = tmp.next            tmp.next = pNode.next            pNode.next = tmp            pNode = pNode.next.next        return dummyHead.next