Swap kth-from-end node with head

Given the head of a singly linked list and an integer `k`, swap the **k-th node from the end** with the **head node**, using **O(1)** extra memory. Requirements: - Perform the swap by changing pointers (swap nodes), not by swapping values. - Assume `1 <= k <= n` where `n` is the list length. - If the k-th node from the end is already the head (i.e., `k == n`), return the list unchanged. Examples (list: `1 -> 2 -> 3 -> 4 -> 5 -> 6`): - `k = 1` => `6 -> 2 -> 3 -> 4 -> 5 -> 1` - `k = 2` => `5 -> 2 -> 3 -> 4 -> 1 -> 6` - `k = 5` => `2 -> 1 -> 3 -> 4 -> 5 -> 6` Return the new head of the linked list.