Leetcode-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.

/**
* Definition for singly-linked list.
* struct ListNode {
*     int val;
*     ListNode *next;
*     ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* swapPairs(ListNode* head) {
if(head == NULL)
return NULL;
ListNode *p = head, *q = head;
q = q->next;
while(q)
{
int tmp = p->val;
p->val = q->val;
q->val = tmp;
if(q->next)
{
p = q->next;
q = p->next;
}
else
break;
}
return head;
}
};