BST.kt

val numberOfBSTs = hashMapOf(
        0 to 1.0,
        1 to 1.0,
        2 to 2.0
)

fun numberOfBSTs(nodes: Int): Double =
        when (nodes) {
            in numberOfBSTs -> numberOfBSTs[nodes]!!
            else -> ((1..nodes)
                    .sumByDouble { numberOfBSTs(it - 1) * numberOfBSTs(nodes - it) })
                    .apply { numberOfBSTs[nodes] = this }
        }

fun main(args: Array<String>) {
    (1..100)
            .asSequence()
            .map { "There are ${numberOfBSTs(it)} BSTs for $it nodes" }
            .forEach { println(it) }
}