Swap Nodes in Pairs

题目：

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.

代码：

 1 public class Solution {
 2     public ListNode swapPairs(ListNode head) 
 3     {
 4         if(head == null || head.next == null)return head;
 5         ListNode fakehead = new ListNode(0);
 6         fakehead.next = head;
 7         
 8         ListNode ptr0 = fakehead;
 9         ListNode ptr1 = fakehead.next;
10         
11         while(ptr1!=null&&ptr1.next!=null)
12         {
13             ListNode ptr2 = ptr1.next;
14             ListNode ptr3 = ptr1.next.next;
15             ptr0.next=ptr2;
16             ptr2.next=ptr1;
17             ptr1.next=ptr3;
18             ptr0=ptr1;
19             ptr1=ptr3;
20             
21         }
22         
23         return fakehead.next;
24     }
25 }