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 head;
   }
   int count = 1;
   ListNode *p = head;
   int temp;
   while(p != NULL)
   {
	   if (count % 2 == 1)
	   {
		  if (p->next != NULL)
		  {
			  temp = p->val;
			  p->val = p->next->val;
			  p->next->val = temp;
		  }  
	   }
	   p = p->next;
	   count++;
   }
   return head;  
    }
};