# Definition for singly-linked list.
# class ListNode:
# def __init__(self, val=0, next=None):
# self.val = val
# self.next = next
class Solution:
def swapPairs(self, head: ListNode) -> ListNode:
sentinalNode = ListNode(next=head) #哨兵节点与head链表接头
pre = sentinalNode
while pre.next and pre.next.next: #循环条件 pre的下个和下下个节点存在才有交换的必要
cur = pre.next
post = pre.next.next
cur.next = post.next
post.next = cur
pre.next = post
pre = pre.next.next
return sentinalNode.next