[LwwtCode]24. Swap Nodes in Pairs

Given a linked list, swap every two adjacent nodes and return its head.

For example,
Given 1->2->3->4, you should return the list as 2->1->4->3.

Your algorithm should use only constant space. You may not modify the values in the list, only nodes itself can be changed.

思路：交换链表的前两个节点 p1,p2,保存pnext = p2.next；交换后为p2,p1,p1.next = swap(pnext)；即递归调用交换链表前两个节点的方法

/** * 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;        ListNode res = new ListNode(0);        res.next = head;        ListNode p1 = res.next;        ListNode p2 = res.next.next;        ListNode pnext = res.next.next.next;        res.next = p2;        p2.next = p1;        p1.next = swapPairs(pnext);        return res.next;    }    }