/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
//帅气的递归
class Solution {
    public ListNode reverseTwo(ListNode head){
        if(head == null || head.next==null)return head;
        ListNode a = head, b = head.next, c = b.next;
        b.next = a;
        a.next = c;
        a.next = reverseTwo(c);
        return b;
    }
    public ListNode swapPairs(ListNode head) {
        return reverseTwo(head);
    }
}