2. Add Two Numbers

leetcode2.cs

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)
    {
        int c = 0;
        ListNode l = new ListNode(0);
        ListNode head = l;
        while (l1.next != null | l2.next != null)
        {
            l.next = new ListNode((c + l1.val + l2.val) % 10);
            c = (c + l1.val + l2.val) / 10;
            l = l.next;
            if (l1.next == null)
            {
                l1.val = 0;
            }
            else
            {
                l1 = l1.next;
            }
            if (l2.next == null)
            {
                l2.val = 0;
            }
            else
            {
                l2 = l2.next;
            }
        }
        l.next = new ListNode((c + l1.val + l2.val) % 10);
        c = (c + l1.val + l2.val) / 10;
        l = l.next;
        if (c != 0)
        {
            l.next = new ListNode(c);
            l = l.next;
        }
        return head.next;
    }
}