Kotlin sort Array<Int>

sort.kt

fun sort(array: Array<Int>): Array<Int> {
    when {
        array.size == 1 -> {
            return array
        }
        array.size <= 2 -> {
            val left = array[0]
            val right = array[1]
            return if (right < left) {
                arrayOf(right, left)
            } else {
                arrayOf(left, right)
            }
        }
        else -> {
            val middle = array.size / 2
            val left = sort(array.sliceArray(0 until middle))
            val right = sort(array.sliceArray(middle until array.size))
            val last = left[left.size - 1]
            val first = right[0]
            if (last > first) {
                left[left.size - 1] = first
                right[0] = last
                return (sort(left.plus(right)))
            }
            return left.plus(right)
        }
    }
}