Swap_nodes_in_pairs

/*
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 *dummyHead = new ListNode(INT_MIN);
	dummyHead->next=head;
	ListNode *p=dummyHead, *q=dummyHead;
	while(p->next!=NULL)
	{
	  int counter=2;
	  //first move pointer q to the place
	  while((counter--)>0)
	  {
	    q=q->next;
	    if (q==NULL) return dummyHead->next;
	  }
	  p->next->next=q->next;
	  q->next=p->next;
	  p->next=q;
	  counter=2;
	  //move pointer q to next place
	  while((counter--)>1) q=q->next;
	  p=q;
	}
	return dummyHead->next;
    }
};

REMARK:
1.Special version of the problem "reverse nodes in k-Group."refer to that problem.