fun main() {

    var num1 = listOf(1,2,3)
    var num2 = listOf(1,2,3)

    println(sum(num1, num2)) // [0, 2, 4, 6]

    num1 = listOf(9,9,9)
    num2 = listOf(1)
    println(sum(num1, num2)) // [1, 0, 0, 0]

    num1 = listOf(9,9,9)
    num2 = listOf(9,9,9)
    println(sum(num1, num2)) // [1, 9, 9, 8]

    num1 = listOf(5,4,4)
    num2 = listOf(4,5,6)
    println(sum(num1, num2)) // [1, 9, 9, 8]
}

// [9,9]
// [9,9]
// [0, 1, 9, 8]
fun sum(num1: List<Int>, num2: List<Int>): List<Int> {

    val resultSize = max(num1.size, num2.size) + 1
    val result = ArrayList<Int>(resultSize)
    (0 until resultSize).forEach { _ ->
        result.add(0)
    }

    var carry = 0
    var p1 = num1.size - 1
    var p2 = num2.size - 1

    var resultP = resultSize - 1

    while(p1 >= 0 || p2 >= 0 || carry != 0) {
        val firstLatestNum = if (p1 >= 0) num1[p1--] else 0
        val secondLatestNum = if (p2 >= 0) num2[p2--] else 0
        val sum = firstLatestNum + secondLatestNum + carry

        result[resultP--] = sum % 10
        carry = sum / 10
    }
    return result
}