Solution to two sums in kotlin (https://bit.ly/2GjpF4v)

SumTwoInts.kt

fun addTwoNumbers(l1: ListNode?, l2: ListNode?, nextNode: ListNode? = totalNode): ListNode? {
        val firstVal = l1?.`val` ?: 0
        val secondVal = l2?.`val` ?: 0

        var total = firstVal + secondVal + (nextNode?.`val` ?: 0)

        if (total >= 10) {
            total = total.rem(10)
            carryOn = 1
        }

        nextNode?.`val` = total
        nextNode?.next = ListNode(carryOn)

        return if (l1?.next == null && l2?.next == null) {
            if (carryOn == 0) nextNode?.next = null
            totalNode
        } else {
            carryOn = 0
            addTwoNumbers(l1?.next, l2?.next, nextNode?.next)
        }
    }