24. 两两交换链表中的节点

给定 1->2->3->4, 你应该返回 2->1->4->3.

/**
 * 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 second = head.next;
        ListNode third = second.next;

        second.next = head;
        head.next = swapPairs(third);

        return second;
    }
}