Project Euler: Problem 24

Euler0024.scala

package euler.zerosum

object Euler0024 {

  def main(args: Array[String]) {
    println(getArrangement(999999, (0 to 9).toList, Nil).mkString)
  }

  private def getArrangement(order: Int, objects: List[Int], arrangement: List[Int]): List[Int] = {
    val unit = fact(objects.length - 1, 1)

    val front = objects.zipWithIndex.takeWhile(unit*_._2 <= order)
    val tail = objects.drop(front.length)

    val next = front.last
    val init = front.init.unzip._1

    val left = order - unit * next._2
    if (left == 0) {
      arrangement ::: (next._1 :: init ::: tail)
    } else {
      getArrangement(left, init ::: tail, arrangement ::: next._1 :: Nil)
    }
  }

  private def fact(n: Int, prod: Int): Int = if (n <= 1)  prod else  fact (n-1, n * prod)
}