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.

1,

/**
 * 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) {
        if(head==null||head.next==null){
        	return head;
        }
        ListNode temp=new ListNode(0);
        temp.next=head;
        head=temp;
        while(temp.next!=null&&temp.next.next!=null){
        	ListNode sign=temp.next.next.next;
        	temp.next.next.next=temp.next;
        	temp.next=temp.next.next;
        	temp.next.next.next=sign;
        	temp=temp.next.next;        	
        }
        return head.next;
    }
}