suma de dos nodos enlazados

public class ListNode {
     public int val;
      public ListNode next;
       public ListNode(int x) { val = x; }
  }

public static ListNode AddTwoNumbers(ListNode l1, ListNode l2) 
{
   var result = new ListNode(0);
   var current = result;
   var sum = 0;

   while (l1 != null || l2 != null)
   {
       sum = sum / 10;
       if (l1 != null)
       {
           sum = sum + l1.val;
           l1 = l1.next;
       }
       if (l2 != null)
       {
           sum = sum + l2.val;
           l2 = l2.next;
       }
       current.next = new ListNode(sum % 10);
       current = current.next;
   }

   if (sum / 10 == 1)
   {
       current.next = new ListNode(1);
   }

   return result.next;