445. Add Two Numbers II

    func addTwoNumbers(_ l1: ListNode?, _ l2: ListNode?) -> ListNode? {
        if l1 == nil && l2 == nil {return nil}
        var stack1 = [ListNode]()
        var runner = l1
        while runner != nil {
            stack1.append(runner!)
            runner = runner!.next
        }
        var stack2 = [ListNode]()
        runner = l2
        while runner != nil {
            stack2.append(runner!)
            runner = runner!.next
        }

        var carry = 0
        runner = nil
        while stack1.count > 0 || stack2.count > 0 || carry > 0{
            let n1 = stack1.count > 0 ? stack1.removeLast().val : 0
            let n2 = stack2.count > 0 ? stack2.removeLast().val : 0
            let sum = carry + n1 + n2
            let node = ListNode(sum % 10)
            carry = sum / 10
            node.next = runner
            runner = node
        }
        return runner
    }