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o(n) time and o(1) space using two pointers.

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	`1`	`+"""`
	`2`	`+You are given the head of a linked list, and an integer k.`
	`3`	`+`
	`4`	`+Return the head of the linked list after swapping the values of the kth node from the beginning and the kth node from the end (the list is 1-indexed).`
	`5`	`+`
	`6`	`+`
	`7`	`+`
	`8`	`+Example 1:`
	`9`	`+`
	`10`	`+`
	`11`	`+Input: head = [1,2,3,4,5], k = 2`
	`12`	`+Output: [1,4,3,2,5]`
	`13`	`+Example 2:`
	`14`	`+`
	`15`	`+Input: head = [7,9,6,6,7,8,3,0,9,5], k = 5`
	`16`	`+Output: [7,9,6,6,8,7,3,0,9,5]`
	`17`	`+`
	`18`	`+`
	`19`	`+Constraints:`
	`20`	`+`
	`21`	`+The number of nodes in the list is n.`
	`22`	`+1 <= k <= n <= 105`
	`23`	`+0 <= Node.val <= 100`
	`24`	`+"""`
	`25`	`+# Definition for singly-linked list.`
	`26`	`+# class ListNode:`
	`27`	`+# def __init__(self, val=0, next=None):`
	`28`	`+# self.val = val`
	`29`	`+# self.next = next`
	`30`	`+class Solution:`
	`31`	`+ def swapNodes(self, head: Optional[ListNode], k: int) -> Optional[ListNode]:`
	`32`	`+ left,right = head,head`
	`33`	`+ for _ in range(k-1):`
	`34`	`+ right = right.next`
	`35`	`+ r = right`
	`36`	`+ while right.next:`
	`37`	`+ left = left.next`
	`38`	`+ right = right.next`
	`39`	`+ r.val,left.val = left.val,r.val`
	`40`	`+ return head`

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