Swap k-th from End Node with Head in a Singly Linked List

Given the head of a singly linked list and an integer k, implement a function that swaps the k-th node from the end of the list with the head node.

Requirements:

• The swap must be performed by changing pointers, not by modifying node values.
• The solution must use O(1) extra memory.
• Assume 1 <= k <= n, where n is the total number of nodes in the list.
• If the k-th node from the end is already the head node (i.e., k == n), the list should remain unchanged, and the original head should be returned.

Examples:

Consider a list: 1 -> 2 -> 3 -> 4 -> 5 -> 6

• If k = 1, the 1st node from the end is 6. The new list should be 6 -> 2 -> 3 -> 4 -> 5 -> 1.
• If k = 2, the 2nd node from the end is 5. The new list should be 5 -> 2 -> 3 -> 4 -> 1 -> 6.
• If k = 5, the 5th node from the end is 2. The new list should be 2 -> 1 -> 3 -> 4 -> 5 -> 6.

Return Value:

Return the new head of the modified linked list.