public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode head = new ListNode(-1);
        ListNode p = head;

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

        if (l1 != null) {
            p.next = l1;
        }
        if (l2 != null) {
            p.next = l2;
        }
        return head.next;
    }