LeetCode: Swapping Nodes In A Linked List Solution

1/**
2 * Definition for singly-linked list.
3 * function ListNode(val, next) {
4 *     this.val = (val===undefined ? 0 : val)
5 *     this.next = (next===undefined ? null : next)
6 * }
7 */
8/**
9 * @param {ListNode} head
10 * @param {number} k
11 * @return {ListNode}
12 */
13var swapNodes = function (head, k) {
14  let first = head
15  let second = head
16  let temp = head
17  let len = 0
18  while (temp) {
19    len++
20    temp = temp.next
21  }
22  let firstIndex = k - 1
23  let secondIndex = len - k
24  while (firstIndex--) {
25    first = first.next
26  }
27  while (secondIndex--) {
28    second = second.next
29  }
30  let tempVal = first.val
31  first.val = second.val
32  second.val = tempVal
33  return head
34}