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;
}
};