LeetCode (Swap Nodes in Pairs)

Problem:

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.
Solution:
/**
* 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) {
ListNode l(0), *p = &l;
p->next = head;
while (p->next && p->next->next){
ListNode* first = p->next;
ListNode* second = p->next->next;
first->next = second->next;
p->next = second;
p->next->next = first;
p = p->next->next;
}
return l.next;
}
};