Algorithms—24.Swap Nodes in Pairs

思路：递归，然后交换相邻的两个值。

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

耗时：300ms，中下游