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) {
    ListNode* res,*p,*q;
    if(head==NULL||head->next==NULL)
    return head;
    res=head->next;
    head->next=res->next;
    res->next=head;
    while(head->next!=NULL&&head->next->next!=NULL)
    { 
        q=head;
        head=head->next;
        p=head->next;
        q->next=p;
        head->next=p->next;
        p->next=head;
    }

    return res;
    }
};