1721. Swapping Nodes in a Linked List

You are given the head of a linked list, and an integer k.

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).

Example 1:
Input: head = [1,2,3,4,5], k = 2
Output: [1,4,3,2,5]

Example 2:
Input: head = [7,9,6,6,7,8,3,0,9,5], k = 5
Output: [7,9,6,6,8,7,3,0,9,5]

Example 3:
Input: head = [1], k = 1
Output: [1]

Example 4:
Input: head = [1,2], k = 1
Output: [2,1]

Example 5:
Input: head = [1,2,3], k = 2
Output: [1,2,3]

Constraints:
The number of nodes in the list is n.
1 <= k <= n <= 105
0 <= Node.val <= 100

Solution:
Two pointers

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# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def swapNodes(self, head: ListNode, k: int) -> ListNode:
        fast, slow = head, head
        for _ in range(k-1):
            fast = fast.next
        beginning = fast
        while fast.next:
            fast, slow = fast.next, slow.next
        beginning.val, slow.val = slow.val, beginning.val
        return head

references

• https://leetcode.com/problems/swapping-nodes-in-a-linked-list/