// linear time and linear space
def twoSum(list: Array[Int], k: Int): Option[Result] = {
  val cache = mutable.Map.empty[Int, Int]
  for (i <- list.indices) {
    val elem = list(i)
    val dif = k - elem
    val cacheValue = cache.get(dif)
    if (cacheValue.isDefined) {
      return Some(Result(cacheValue.get, i))
    } else {
      cache.put(elem, i)
    }
  }
  None
}

case class Result (
    aIdx: Int,
    bIdx: Int
)