给定一个链表，两两交换其中相邻的节点，并返回交换后的链表。
你不能只是单纯的改变节点内部的值，而是需要实际的进行节点交换。
示例:
给定 1->2->3->4, 你应该返回 2->1->4->3.
题解：
head = 1 2 3 4
post = 2 3 4
head.next = post.next, head = 1 3 4
post.next = head, post = 2 1 3 4
pre = -1 1 3 4, pre.next = post, pre = -1 2 1 3 4
# Definition for singly-linked list.
# class ListNode:
# def __init__(self, x):
# self.val = x
# self.next = None
class Solution:
def swapPairs(self, head: ListNode) -> ListNode:
if not head or not head.next:
return head
dummy = ListNode(-1)
dummy.next = head
pre = dummy
while head and head.next:
post = head.next
head.next = post.next
post.next = head
pre.next = post
pre = head
head = head.next
return dummy.next
