P20 (*) P21 (*)Remove the Kth element from a list；Insert an element at a given position into a list.

Return the list and the removed element in a Tuple. Elements are numbered from 0.

Example:

scala> removeAt(1, List('a, 'b, 'c, 'd))
res0: (List[Symbol], Symbol) = (List('a, 'c, 'd),'b)

def removeAt[A](n:Int,ls:List[A]):(List[A],A)=(n,ls) match{
  case (n,ls) if(n<0) => throw new NoSuchElementException
  case (_,Nil) => throw new NoSuchElementException
  case (_,_)   =>(ls.take(n):::ls.drop(n+1),ls.drop(n).head)
}

也可以用splitAt函数

参考答案

Example:
scala> insertAt('new, 1, List('a, 'b, 'c, 'd))
res0: List[Symbol] = List('a, 'new, 'b, 'c, 'd
def insertAt[A](e:A,n:Int,ls:List[A]):List[A]={
  ls.take(n):::List(e):::ls.drop(n)
}

参考答案

def insertAt[A](e: A, n: Int, ls: List[A]): List[A] = ls.splitAt(n) match {
    case (pre, post) => pre ::: e :: post
  }