preorder.kt

/**
 * We declare a package-level function main which returns Unit and takes
 * an Array of strings as a parameter. Note that semicolons are optional.
 */

fun main() {
    val tree = Node(
        1,
        listOf(
            Node(
                2, listOf(
                    Node(3),
                    Node(4),
                    Node(5)
                )
            ),
            Node(6)
        )
    )
 
    val list = preorder({ x -> "name: " + x.toString() }, tree)
    println()
    println(list)
}
 
fun <A, B> preorder(transform: (A) -> B, node: Node<A>) : List<B>{
    return listOf(transform(node.rootLabel)) +
    	node.subForest.flatMap { preorder(transform, it) }
}
 
data class Node<T>(
    val rootLabel: T,
    val subForest: List<Node<T>> = emptyList()
)