【LeetCode OJ】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 struct ListNode
 2 {
 3     int val;
 4     ListNode *next;
 5     ListNode(int x) : val(x), next(NULL) {}
 6     
 7 };
 8 ListNode *swapPairs(ListNode *head)
 9 {
10     if (head == NULL || head->next == NULL)
11         return head;
12     ListNode *p = head;
13     ListNode *q = p->next;
14     while (q)
15     {
16         int temp;
17         temp = p->val;
18         p->val = q->val;
19         q->val = temp;
20         if (q->next != NULL)
21         {
22             p = q->next;
23             if (p->next != NULL)
24                 q = p->next;
25             else
26                 break;
27             
28         }
29             else
30                 break;
31         }
32         return head;
33     }