P16 (**)-P17 (*) Drop every Nth element from a list；Split a list into two parts.

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scala> drop(3, List('a, 'b, 'c, 'd, 'e, 'f, 'g, 'h, 'i, 'j, 'k))
res0: List[Symbol] = List('a, 'b, 'd, 'e, 'g, 'h, 'j, 'k)

参考答案：

其中关于zip 和filter的用法见http://blog.youkuaiyun.com/springlustre/article/details/48804911

object P16 {
  // Simple recursion.
  def dropRecursive[A](n: Int, ls: List[A]): List[A] = {
    def dropR(c: Int, curList: List[A]): List[A] = (c, curList) match {
      case (_, Nil)       => Nil
      case (1, _ :: tail) => dropR(n, tail)
      case (_, h :: tail) => h :: dropR(c - 1, tail)
    }
    dropR(n, ls)
  }

  // Tail recursive.
  def dropTailRecursive[A](n: Int, ls: List[A]): List[A] = {
    def dropR(c: Int, curList: List[A], result: List[A]): List[A] = (c, curList) match {
      case (_, Nil)       => result.reverse
      case (1, _ :: tail) => dropR(n, tail, result)
      case (_, h :: tail) => dropR(c - 1, tail, h :: result)
    }
    dropR(n, ls, Nil)
  }

  // Functional.
  def dropFunctional[A](n: Int, ls: List[A]): List[A] = 
    ls.zipWithIndex filter { v => (v._2 + 1) % n != 0 } map { _._1 }
}

17.

scala> split(3, List('a, 'b, 'c, 'd, 'e, 'f, 'g, 'h, 'i, 'j, 'k))
res0: (List[Symbol], List[Symbol]) = (List('a, 'b, 'c),List('d, 'e, 'f, 'g, 'h, 'i, 'j, 'k))

//17
def split[A](N:Int,ls:List[A])={
  val(a,b)=ls.zipWithIndex span(_._2<N)
  (a.map(_._1),b.map(_._1))
}

除此之外，也可以用take和drop更加方便，还可以直接用splitAt函数

参考答案