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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/**
 * 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);
        ListNode pre = new ListNode(0);
        res.next = head;
        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。
        while(head != null && head.next != null){
            pre.next = head.next;
            head.next = pre.next.next;
            pre.next.next = head;
            if(isfirst){
                res.next = pre.next;
                isfirst = false;
            }
            pre = pre.next.next;
            head = head.next;
        }
        return res.next;
    }
}

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。