Leetcode 2: Add two numbers - prototype - add two linked lists with only one node

Leetcode2_AddTwoNumbers_Prototype.cs

/// <summary>
        /// add two linked list together, both have only one node. 
        /// </summary>
        /// <param name="l1"></param>
        /// <param name="l2"></param>
        /// <returns></returns>
        public ListNode AddTwoNumbers_prototype(ListNode l1, ListNode l2)
        {
            if (l1 == null || l2 == null)
            {
                return l1 == null ? l2 : l1;
            }

            int value = l1.val + l2.val;

            var result = new ListNode(value % 10);           

            if (value >= 10)
            {
                // make this carry as a linked list as well 
                result.next = AddTwoNumbers_prototype(new ListNode(value / 10), null);
            }

            return result;
        }