aoc-2020-1.scala

import $ivy.`org.typelevel::cats-core:2.3.0`

def find2Addends(numbers: List[Long], sum: Long): Option[(Long, Long)] = {
  numbers.collectFirstSome { x =>
    val y = sum - x
    if (numbers.contains(y)) Some((x, y))
    else None
  }
}

def findNAddends(numbers: List[Long], sum: Long, n: Long): Option[List[Long]] = {
  n match {
    case 2 => find2Addends(numbers, sum).map { case (x, y) => List(x, y) }
    case _ =>
      def loop(l: List[Long]): Option[List[Long]] = {
        l match {
          case head :: rest =>
            findNAddends(rest, sum - head, n - 1) match {
              case None => loop(rest)
              case Some(addends) => Some(head :: addends)
            }
          case Nil => None
        }
      }
      loop(numbers)
  }
}