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 || head->next == NULL)
	    {
		    return head;
	    }

	    ListNode* result = head->next;
	    head->next = result->next;
	    result->next = head;

	    ListNode *p = head;
	    ListNode *q = p->next;
	    while (q && q->next)
	    {
		    ListNode *r = q->next;
		    q->next = r->next;
		    r->next = q;
		    p->next = r;
		    p = q;
		    q = q->next;
	    }

	    return result;
    }
};