leetcode 24.Swap Nodes in Pairs

https://leetcode.com/problems/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.

题意：

      以两个为一组，交换链表中的每组中的结点位置，

比如题中给出的例子，以1和2为一组交换位置，交换后为 2 -> 1,

接下来以3和4为一组交换，交换后的位置为 4 -> 3.

/**
 * 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) {
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        
        ListNode temp = dummy;
        
        while(temp.next != null && temp.next.next != null){
            ListNode n1 = temp.next;
            ListNode n2 = temp.next.next;
            
            temp.next = n2;
            n1.next = n2.next;
            n2.next = n1;
            
            //move to the next pair
            temp = n1;
        }
        return dummy.next;
        
    }
}