P19 (**) Rotate a list N places to the left.

Examples:

scala> rotate(3, List('a, 'b, 'c, 'd, 'e, 'f, 'g, 'h, 'i, 'j, 'k))res0: List[Symbol] = List('d, 'e, 'f, 'g, 'h, 'i, 'j, 'k, 'a, 'b, 'c)scala> rotate(-2, List('a, 'b, 'c, 'd, 'e, 'f, 'g, 'h, 'i, 'j, 'k))res1: List[Symbol] = List('j, 'k, 'a, 'b, 'c, 'd, 'e, 'f, 'g, 'h, 'i)

//19def rotate[A](n:Int,ls:List[A]):List[A]={  var list=ls  if(n>0){    for(i<-0 to n-1) {      list = list ::: List(list.head)      list = list.drop(1)    }    list  }else{    for(i<- n+1 to 0) {      list = List(list.last):::list      list = list.dropRight(1)    }    list  }}

当然直接用drop和take好一些

def rotate2[A](n:Int,ls:List[A]):List[A]={   if(n>0){     ls.drop(n):::ls.take(n)   }  else{     ls.takeRight(-n):::ls.dropRight(-n)   }}

参考答案

object P19 {  def rotate[A](n: Int, ls: List[A]): List[A] = {    val nBounded = if (ls.isEmpty) 0 else n % ls.length    if (nBounded < 0) rotate(nBounded + ls.length, ls)    else (ls drop nBounded) ::: (ls take nBounded)  }