public ListNode mergeTwoLists(ListNode l1, ListNode l2) {

        if (l1 == null) {
            return l2;
        } else if (l2 == null) {
            return l1;
        }

        ListNode root = new ListNode(0);
        ListNode cur = root;
        while (l1 != null && l2 != null) {
            if (l1.val > l2.val) {
                cur.next = l2;
                l2 = l2.next;
                cur = cur.next;
            } else {
                cur.next = l1;
                l1 = l1.next;
                cur = cur.next;
            }
        }
        if (l2 != null) {
            cur.next = l2;
        } else {
            cur.next = l1;
        }
        return root.next;
    }