That's the solution for the DiamondPrint problem: http://exercism.io/exercises/kotlin/diamond/readme

DiamondPrinter.kt

class DiamondPrinter {
    fun printToList(char: Char): List<String> {
        val size = ('A'..char).count() * 2 - 1
        val half = size / 2

        return (0..half).map { lineIndex ->
            StringBuilder("".padStart(size))
                    .apply {
                        setCharAt(half + lineIndex, 'A' + lineIndex)
                        setCharAt(half - lineIndex, 'A' + lineIndex)
                    }.toString()
        }.let { it + it.reversed().drop(1) }
    }
}