Leetcode Swap Nodes in Pairs

    ListNode *swapPairs(ListNode *head) {        ListNode dummy(-1);        dummy.next = head;        ListNode *cur = head;        ListNode *prev = &dummy;        ListNode *p;        while(prev -> next != nullptr && prev -> next -> next != nullptr){            cur = prev -> next;            p = cur -> next;            // take out            cur -> next = p -> next;            // insert            p -> next = cur;            prev -> next = p;            prev = cur;        }        return dummy.next;    }