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; }
* }
*/
public class Solution {
public ListNode swapPairs(ListNode head)
{
ListNode p=new ListNode(0);
ListNode p1,p2;
p.next=head;
while(p.next!=null&&p.next.next!=null)
{
p1=p.next;
p2=p.next.next;
p1.next=p2.next;
p2.next=p1;
if(p.next==head)
head=p2;
p.next=p2;
p=p1;
}
return head;
}
}