Swap Nodes in Pairs

class Solution {
public:
    ListNode* swapPairs(ListNode* head) {
        if(head==NULL||head->next==NULL) return head;
        ListNode prehead(0);
        prehead.next=head;
        ListNode *pre=&prehead,*p;
        while(head&&head->next)
        {
            p=head->next;
            head->next=p->next;
           p->next=pre->next;
           pre->next=p;
           
           pre=head;
           head=head->next;
        }
        return prehead.next;
    }
};