LeetCode 129 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.
分析:
これはReverse Node in k-Groupsの考え方と同じですが、ここのkは2になりました.

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode swapPairs(ListNode head) {
        if(head==null || head.next==null) 
            return head;
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode pre = dummy;
        while(pre.next != null && pre.next.next != null){
            ListNode first = pre.next;
            ListNode second = pre.next.next;
            //  
            first.next = second.next;
            second.next = first;
            pre.next = second;
            //
            pre = first;
        }
        return dummy.next;
    }
}