[LC] 24. Swap Nodes in Pairs

/**
 * 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 nxt = head.next;
        ListNode newHead = swapPairs(nxt.next);
        nxt.next = head;
        head.next = newHead;
        return nxt;
    }
}