24. 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.

代碼如下：

 1 /**
 2  * Definition for singly-linked list.
 3  * public class ListNode {
 4  *     int val;
 5  *     ListNode next;
 6  *     ListNode(int x) { val = x; }
 7  * }
 8  */
 9 public class Solution {
10     public ListNode swapPairs(ListNode head) {
11         
12         if(head==null||head.next==null)
13         return head;
14         ListNode first=head;
15         ListNode second=first.next;
16         ListNode temp=first;
17         head=second;
18         
19         while(first!=null&&second!=null)
20         {
21             temp.next=second;
22             first.next=second.next;
23             temp=first;
24             second.next=first;
25             first=first.next;
26             if(first!=null&&first.next!=null)
27             {
28                 second=first.next;
29             }
30             else break;
31         }
32         return head;
33     }
34 
35 }