/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode swapPairs(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }

        ListNode left = head;
        ListNode right = head.next;
        left.next = swapPairs(right.next);
        right.next = left;
        return right;
    }
}