// 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
  }