leetcode : 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.

tag : dummy node .  类似题： reverse linked list

/**
 * 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;
        head = dummy;
        
        while(head.next != null && head.next.next != null){
            ListNode n1 = head.next;
            ListNode n2 = head.next.next;
            
            head.next = n2;
            n1.next = n2.next;
            n2.next = n1;
            
            head = n1;
        }
        
        return dummy.next;
    }
}