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.

Solution:

/**
 * 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) {
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        
        ListNode previous = dummy;
        while(previous.next!=null && previous.next.next != null)
        {
            ListNode node1 = previous.next;
            ListNode node2 = previous.next.next;
            previous.next = node2;
            node1.next = node2.next;
            node2.next = node1;
            previous = node1;
        }
        return dummy.next;
    }
}