Scala Reversing Strings

reverse.scala

def reverse_tail2(s: String): String = {
 @scala.annotation.tailrec
 def impl(ss: String, r: String): String = {
   if (ss == null) return null
   if (ss.tail.isEmpty) return ss.head + r
   impl(ss.tail, ss.head + r)
 }
 impl(s, "");
}

reverse2.scala

def reverseString(s:String) = (for(i<-s.length-1 to 0 by -1) yield s(i)).mkString