Add Two Numbers with Linked Lists

AddTwoNumbers_LinkedLists.cs

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public int val;
 *     public ListNode next;
 *     public ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public void PrintNode(ListNode node)
    {
        while (node != null)
        {
            Console.Write(node.val + " -> ");
            node = node.next;
        }
        
        Console.Write("\n");
    }
    
    public ListNode AddTwoNumbers(ListNode l1, ListNode l2) {
        
        if (l1 is null || l2 is null)
            return null;
        
        bool overflow = false;
        int reminder = 0;
        ListNode firstNode = null;
        ListNode result = null;
        
        while(l1 != null || l2 != null || overflow)
        {
            int sum = (l1?.val ?? 0) + (l2?.val ?? 0) + (overflow ? 1 : 0);
            
            reminder = sum % 10;
            overflow = sum > 9;

            if (result is null)
                firstNode = result = new ListNode(reminder);
            else
            {
                result.next = new ListNode(reminder);
                result = result.next;
            }
            
            l1 = l1?.next;
            l2 = l2?.next;
            
            PrintNode(result);
        }
        
        return firstNode;
    }
}