LeetCode 2. Add Two Numbers

    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 lnResult = new ListNode(0);
            ListNode currentNode = lnResult;
            ListNode pre = null;
            int carry = 0;
            while (l1 != null && l2 != null)
            {
                currentNode.val = (l1.val + l2.val + carry) % 10;
                carry = (l1.val + l2.val + carry) / 10;
                pre = currentNode;
                currentNode = new ListNode(0);
                pre.next = currentNode;
                l1 = l1.next;
                l2 = l2.next;
            }
            currentNode = AddWithCarry(l1, l2, carry);
            pre.next = currentNode;

            return lnResult;
        }

        public ListNode AddWithCarry(ListNode l1, ListNode l2, int carry)
        {
            ListNode cur = null;
            if (l1 == null && l2 == null)
            {
                if (carry != 0)
                {
                    cur = new ListNode(0);
                    cur.val = carry;
                    return cur;
                }
                return null;
            }
            if (l1 == null)
            {
                return AddWithCarry(l2, l1, carry);
            }
            ListNode result = new ListNode(0);
            cur = result;
            ListNode pre = null;
            while (l1 != null)
            {
                cur.val = (l1.val + carry) % 10;
                carry = (l1.val + carry) / 10;
                pre = cur;
                cur = new ListNode(carry);
                pre.next = cur;
                l1 = l1.next;
            }
            if (pre.next.val == 0)
            {
                pre.next = null;
            }
            return result;
        }
    }