leetcode: merge two sorted linklist

key: dummy node; how to copy link node(p-> next = l)

ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {
        ListNode dummy(INT_MIN);
        ListNode *p = &dummy;
        while(l1 && l2){
            if(l1->val < l2->val){
                 p->next = l1;
                l1 = l1->next;
            }
            else{
                p->next= l2;
                l2 = l2->next;
            }
            p = p->next;
        }
        p->next = l1 ? l1 : l2;
        return dummy.next;
    }