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;
        }
        int temp;
        ListNode *p=head;
        while(p&&p->next)
        {
            temp=p->val;
            p->val=p->next->val;
            p->next->val=temp;
            p=p->next;
            p=p->next;
        }
        return head;
    }
};