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 {
    public ListNode swapPairs(ListNode head) {
        //if(head==null){return null;}
        ListNode temp ;
        if(head!=null&&head.next!=null){
            temp=head.next;
            head.next=temp.next;
            temp.next=head;
            head=temp;
        
            ListNode node0=head.next;
            while(node0.next!=null&&node0.next.next!=null){
                temp=node0.next;
                node0.next=temp.next;
                temp.next=node0.next.next;
                node0.next.next=temp;
                node0=node0.next.next;
            }
        }
        return head;
    }