2-add-two-numbers.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 ListNode AddTwoNumbers(ListNode l1, ListNode l2)
    {
        ListNode dummy = new ListNode(0);
        ListNode current = dummy;
        int carry = 0;

        while (l1 != null || l2 != null || carry != 0)
        {
            int sum = carry;
            if (l1 != null)
            {
                sum += l1.val;
                l1 = l1.next;
            }
            if (l2 != null)
            {
                sum += l2.val;
                l2 = l2.next;
            }

            carry = sum / 10;
            sum = sum % 10;
            current.next = new ListNode(sum);
            current = current.next;
        }

        return dummy.next;
    }
}