Swap Nodes in Pairs

Q:
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.

T:
Simple linked list operation.

A:

/**
 * 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 || head->next == NULL) {
            return head;
        }
        ListNode * p = head;
        head = head->next;
        p->next = head->next;
        head->next = p;
        ListNode * q = p->next;
        while(q && q->next) {
            p->next = q->next;
            q->next = q->next->next;
            p->next->next = q;
            p = q;
            q = q->next;
        }
        return head;
    }
};