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.

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    /**
	 * This is a fundamental problem.<br>
	 * 
	 * @param head --The head node of the input linked list
	 * @return head of the rotated linked list
	 * @author Averill Zheng
	 * @version 2014-06-03
	 * @since JDK 1.7
	 */
	public ListNode swapPairs(ListNode head) {
		ListNode newHead = head;
		if(head != null && head.next != null){
			ListNode currentNode = head;
			ListNode nextNode = currentNode.next;
			newHead = nextNode;
			currentNode.next = nextNode.next;
			nextNode.next = currentNode;
			ListNode previous = currentNode;
			currentNode = currentNode.next;
			while(currentNode != null && currentNode.next != null){
				nextNode = currentNode.next;
				currentNode.next = nextNode.next;
				nextNode.next = currentNode;
				previous.next = nextNode;
				previous = currentNode;
				currentNode = currentNode.next;
			}
		}
		return newHead;
    }
}