【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：

 struct ListNode
 {
     int val;
     ListNode *next;
     ListNode(int x) : val(x), next(NULL) {}

 };
 ListNode *swapPairs(ListNode *head)
 {
     if (head == NULL || head->next == NULL)
         return head;
     ListNode *p = head;
     ListNode *q = p->next;
     while (q)
     {
         int temp;
         temp = p->val;
         p->val = q->val;
         q->val = temp;
         if (q->next != NULL)
         {
             p = q->next;
             if (p->next != NULL)
                 q = p->next;
             else
                 break;

         }
             else
                 break;
         }
         return head;
     }