Solution.kt

class Solution {
    fun multiply(num1: String, num2: String): String {
        if (num1 == "0" || num2 == "0") return "0"

        val chars_1 = num1.toCharArray().reversedArray()
        val chars_2 = num2.toCharArray().reversedArray()

        val result = mutableListOf<Int>()

        chars_1.forEachIndexed { i, c1 ->
            val multiplier_1 = c1 - '0'
            var over = 0
            var index = 0

            fun sum(product: Int = 0): Unit {
                while (index >= result.size) {
                    result.add(0)
                }
                val value = product + over + result[index]
                result[index] = value % 10
                over = value / 10
                return
            }

            chars_2.forEachIndexed { j, c2 ->
                index = i + j
                val multiplier_2 = c2 - '0'
                sum(multiplier_1 * multiplier_2)
            }

            while (over > 0) {
                index++
                sum()
            }
        }

        return result.reversed().joinToString("")
    }
}