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.

class Solution {
public:
    ListNode *swapPairs(ListNode *head) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        ListNode dummynode(0), *h = &dummynode, *p = head, *n,*temp;
        h->next = head;
        while(p){
            n = p->next;
            if (n){
                h->next = n;
                temp = n->next;
                n->next = p;
                p->next = NULL;
                p = temp;
                h = h->next;
                h = h->next; 
            }else{
             h->next = p;
             break;
            }
        }
        return dummynode.next;
    }
};