Given two non-negative numbers, num1 and num2 represented as strings, sum the integers together and return the result as a string.

AddStrings.kt

/*
Given two non-negative numbers, num1 and num2 represented as strings, sum the integers together and return the result as a string.

Ex: Given the following values for num1 and num2...

num1 = “2”, num2 = “5”, return “7”.
Ex: Given the following values for num1 and num2...

num1 = “7”, num2 = “95”, return “102”.
* */

fun addStrings(num1: String, num2: String): String {
    var i: Int = num1.lastIndex
    var j: Int = num2.lastIndex
    var carry: Int = 0
    val sb: StringBuilder = StringBuilder()
    while (i >= 0 || j >= 0 || carry != 0) {
        var sum: Int = carry
        // if we have a character
        if (i >= 0) {
            // retrieve digit by ASCII code at the current index
            val digit: Int = num1[i--] - '0'
            sum += digit
        }
        if (j >= 0) {
            val digit: Int = num2[j--] - '0'
            sum += digit
        }
        // getting the last digit if number > 10
        sb.insert(0, sum % 10)
        // remainder (if number bigger than 10, it will give 1)
        carry = sum / 10
    }
    return sb.toString()
}