public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode dummy = new ListNode(0);
        ListNode p = dummy;

        while(l1 != null && l2 != null){
            if(l1.val < l2.val){
                p.next = l1;
                l1 = l1.next;
            }else{
                p.next = l2;
                l2 = l2.next;
            }
            p = p.next;
        }
        if(l1 != null) p.next = l1;
        if(l2 != null) p.next = l2;
        return dummy.next;
    }