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def toNGrams1(lst: List[String], n: Int): List[List[String]] = {
    List.range(0, lst.length - n).map(i => lst.slice(i, i+n))
  }

  def toNGrams2(lst: List[String], n: Int): List[List[String]] = {
    @tailrec
    def toNGramAcc(acc: List[List[String]], lst: List[String]): List[List[String]] = {
      if (lst.length <= n) acc
      else toNGramAcc(lst.take(n) :: acc, lst.drop(1))
    }
    toNGramAcc(Nil, lst)
  }

  def toNGrams3(lst: List[String], n: Int) = {
    for (i <- List.range(0, lst.length - n) ) yield lst.slice(i, i+5)
  }