/**
 * 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 pHead=new ListNode(0);
        ListNode p;
        ListNode first;
        ListNode second;
        ListNode pFirst;
        ListNode pSecond;
        pHead.next=head;
        first=head;
        pFirst=first;
        second=first.next;
        pSecond=first.next;
        for(p=pSecond.next;p!=null;)
        {
            pFirst.next=p;
            pFirst=p;
            p=p.next;
            if(p!=null)
            {
                pSecond.next=p;
                pSecond=p;
                p=p.next;
            }
            else
            {
                break;
            }
        }
        if(pFirst.next!=null)
        {
            pFirst.next=null;
        }
        if(pSecond.next!=null)
        {
            pSecond.next=null;
        }
        pFirst=first;
        pSecond=second;
        for(p=pHead;pSecond!=null;)
        {
            p.next=pSecond;
            pSecond=pSecond.next;
            p=p.next;
            if(p!=null)
            {
                p.next=pFirst;
                pFirst=pFirst.next;
                p=p.next;
            }
        }
        return pHead.next;
    }
}