/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode swapNodes(ListNode head, int k) {
        ListNode fast = head;
        ListNode slow = head;
        ListNode first = head, second = head;
        
		// Put fast (k-1) nodes after slow
        for (int i = 1; i < k; i++)
            fast = fast.next;
            
		// Save the node for swapping
        first = fast;

		// Move until the end of the list
        while (fast.next != null) {
			slow = slow.next;
            fast = fast.next;
        }
        
        // Save the second node for swapping
		// Note that the pointer second isn't necessary: we could use slow for swapping as well
		// However, having second improves readability
        second = slow;
		
		// Swap values
        int temp = first.val;
        first.val = second.val;
        second.val = temp;
        
        return head;
    }
}