fun lengthLeft(polymer: String): Int {
    val arr = CharArray(polymer.length)
    var i = 0
    var j = 0

    while (j <= polymer.lastIndex) {
        arr[i] = polymer[j++]

        if (i > 0 && arr[i - 1].isOpposite(arr[i]))
            i--
        else
            i++
    }

    return i
}

fun Char.isOpposite(other: Char) =
        this != other && this.toLowerCase() == other.toLowerCase()