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.

Swap kth-from-end node with head

Given a singly linked list (represented head-to-tail as an array of node values) and an integer k, swap the k-th node from the end with the head node, then return the resulting list.

The k-th node from the end (1-indexed) sits at 0-based position n - k, where n is the list length. Conceptually you perform the swap by relinking pointers (swapping nodes, not copying values); on the array representation this is exactly swapping the value at index 0 with the value at index n - k.

Rules:

• Use O(1) extra memory.
• Assume 1 <= k <= n.
• If the k-th node from the end is already the head (k == n), return the list unchanged.

Example ([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 list order from head to tail.

Constraints

• 1 <= k <= n where n is the list length
• 0 <= n (the list may be empty)
• Node values fit in a 32-bit signed integer
• Only O(1) extra memory may be used

Hints

1. The k-th node from the end of an n-element list (1-indexed) is at 0-based index n - k. Use the two-pointer technique to reach it in one pass without storing the list.
2. Swapping the head with the k-th-from-end node by relinking pointers reorders the chain so that, read head-to-tail, those two nodes trade positions.
3. Watch the no-op case: when k == n, n - k == 0, so the target node IS the head and you must return the list unchanged.