Leetcode: Swap Nodes in Pairs

1 Given a linked list, swap every two adjacent nodes and return its head.
2 
3 For example,
4 Given 1->2->3->4, you should return the list as 2->1->4->3.
5 
6 Your algorithm should use only constant space. You may not modify the values in the list, only nodes itself can be changed.

 1 /**
 2  * Definition for singly-linked list.
 3  * public class ListNode {
 4  *     int val;
 5  *     ListNode next;
 6  *     ListNode(int x) {
 7  *         val = x;
 8  *         next = null;
 9  *     }
10  * }
11  */
12 public class Solution {
13     public ListNode swapPairs(ListNode head) {
14         // Start typing your Java solution below
15         // DO NOT write main() function
16         ListNode dummy = new ListNode(0);
17         dummy.next = head;
18         ListNode curr = dummy;
19         ListNode node1 = null;
20         ListNode node2 = null;
21         
22         while(curr.next!=null && curr.next.next!=null){
23             node1 = curr.next;            
24             node2 = node1.next;
25             ListNode next =node2.next;            
26             curr.next = node2;
27             node2.next = node1;
28             node1.next = next;            
29             curr=node1;
30         }
31         return dummy.next;
32     }
33 }