24. Swap Nodes in Pairs Java solutions

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.

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 1 /**
 2  * Definition for singly-linked list.
 3  * public class ListNode {
 4  *     int val;
 5  *     ListNode next;
 6  *     ListNode(int x) { val = x; }
 7  * }
 8  */
 9 public class Solution {
10     public ListNode swapPairs(ListNode head) {
11         if(head == null || head.next == null) return head;
12         ListNode res = new ListNode(0);
13         ListNode pre = new ListNode(0);
14         res.next = head;
15         boolean isfirst = true;//标志是否第一次处理head，是的话将res指向第一个节点。
16         while(head != null && head.next != null){
17             pre.next = head.next;
18             head.next = pre.next.next;
19             pre.next.next = head;
20             if(isfirst){
21                 res.next = pre.next;
22                 isfirst = false;
23             }
24             pre = pre.next.next;
25             head = head.next;
26         }
27         return res.next;
28     }
29 }

题目貌似要求不能改变节点的值，所以只能通过改变next 的指向来处理，只要在纸上画好链的拆解过程即可。