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Given a linked list, swap every two adjacent nodes and return its head.

You may not modify the values in the list's nodes, only nodes itself may be changed.

Example:

Given 1->2->3->4, you should return the list as 2->1->4->3

既然是以两个为一组，就应该每次设置一个头节点，在这两个节点前。这样每一轮才方便更新指针。

尽量少设置指针，容易把自己搞混

# 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                dummy = ListNode(0)        dummy.next = head        cur = dummy                while cur.next and cur.next.next:            first=cur.next            second=cur.next.next            first.next = second.next            second.next = first            cur.next = second            cur = cur.next.next        return dummy.next

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