leetcode -- Swap Nodes in Pairs

双指针互换，要考虑一些边界条件，比如链表为空，链表长度为1，链表长度为2.
加一个safeGuard可以避开链表长度为2的检测。

 1 /**
 2  * Definition for singly-linked list.
 3  * public class ListNode {
 4  *     int val;
 5  *     ListNode next;
 6  *     ListNode(int x) {
 7  *         val = x;
 8  *         next = null;
 9  *     }
10  * }
11  */
12 public class Solution {
13     public ListNode swapPairs(ListNode head) {
14         // Start typing your Java solution below
15         // DO NOT write main() function
16         
17         if(head == null)
18             return null;
19             
20         ListNode safeG = new ListNode(-1);
21         safeG.next = head;
22         ListNode before = safeG;
23         ListNode cur = safeG.next;
24         ListNode after = cur.next;
25         
26         while(after != null){
27             ListNode tmp = after.next;
28             after.next = cur;
29             cur.next = tmp;
30             before.next = after;
31             before = cur;
32             if(before.next == null)
33                 break;
34                 
35             cur = before.next;
36             after = cur.next;
37         }
38         
39         head = safeG.next;
40         return head;
41         
42     }
43 }