title
给你链表的头节点 head 和一个整数 k 。
交换 链表正数第 k 个节点和倒数第 k 个节点的值后，返回链表的头节点（链表 从 1 开始索引）。
示例 1：
输入：head = [1,2,3,4,5], k = 2
输出：[1,4,3,2,5]
示例 2：
输入：head = [7,9,6,6,7,8,3,0,9,5], k = 5
输出：[7,9,6,6,8,7,3,0,9,5]
示例 3：
输入：head = [1], k = 1
输出：[1]
示例 4：
输入：head = [1,2], k = 1
输出：[2,1]
示例 5：
输入：head = [1,2,3], k = 2
输出：[1,2,3]
提示：
- 链表中节点的数目是
n 1 <= k <= n <= 1050 <= Node.val <= 100
solution
class Solution:
def swapNodes(self, head: ListNode, k: int) -> ListNode:
newhead = ListNode(0)
newhead.next = head
# k<=n
pre1 = newhead
for _ in range(k - 1):
pre1 = pre1.next
slow = newhead
fast = newhead
for _ in range(k + 1):
fast = fast.next
while fast:
slow = slow.next
fast = fast.next
pre2 = slow
p1 = pre1.next
p2 = pre2.next
p1.val, p2.val = p2.val, p1.val
return newhead.next