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:
Basic linked list implementation question.
Be care of null pointers.

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public static ListNode swapPairs(ListNode head) {
       if(head == null) return null;
       if(head.next == null) return head;
       ListNode first = head;
       ListNode second = head.next;
       ListNode third = null;
       head = second;
       if(second.next != null){
           third = second.next;
           first.next = third.next == null ? third : third.next;
       }
       else first.next = null;
       second.next = first;
       
       ListNode currentNode = third;
       while(currentNode != null){
            first = currentNode;
            second = null;
            third = null;
            if(currentNode.next != null){ 
                second = currentNode.next;
            }
            if(second != null && second.next != null){
                third = second.next;
                first.next = third.next == null ? third : third.next;
            }
            else first.next = null;
            if(second != null){
                second.next = first;
            }
            currentNode = third;
       }
       return head;
    }
}