Swapping Nodes in a Linked List

For LeetCode 1721, the simplest safe solution swaps the values of the kth node from the start and kth node from the end. Walk first from head k-1 steps. Set lead=first and second=head; advance both until lead.next is null. At that moment second is kth from the end. Exchange first.val and second.val. This is one pass after the initial positioning, O(n) time and O(1) extra space. Do not rewire next pointers when only values must be swapped: adjacent nodes, head/tail, and identical positions then need no special-case pointer logic. Clarify that k is valid and one-indexed. Test k=1, k=n, k in the middle, and odd-length lists where both references can be the same node.