Kotlin parseInt

main.kt

fun parseInt(s: String): Int {
    var num = 0
    var j = 1
    var i = s.length - 1
    
    while (i >= 0) {
        val c = s[i].toInt()

        if (c < 48 || c > 58) {
            i -= 1
            continue
        }
        
        num += powBase10(j - 1) * (c - 48)
        i -= 1
        j += 1
    }
    
    return num
}

fun powBase10(a: Int): Int {
    if (a < 0) {
        return 10
    }

    if (a == 0) {
        return 1
    }

    return 10 * powBase10(a - 1)
}

Didn't code negative powers of 10 but it works for positive powers of 10 😄