99 Scala problems — a few solutions

99problems.scala

// 1

def last[A](xs: List[A]): A =
  if(xs.tail.isEmpty) xs.head
  else last(xs.tail)

// 2

def penultimate[A](xs: List[A]): A =
  xs match {
    case List(x,_) => x
    case _ => penultimate(xs.tail)
  }

// 3

def nth[A](n: Int, xs: List[A]): A =
  if(n == 0) xs.head
  else nth(n - 1, xs.tail)

// 4

def length[A](xs: List[A]): Int =
  if(xs.isEmpty) 0
  else 1 + length(xs.tail)

// 5

def reverse[A](xs: List[A]): List[A] =
  xs.foldLeft[List[A]](Nil)((x, y) => y :: x)

// 6

def isPalindrome[A](xs: List[A]): Boolean =
  xs == reverse(xs)

// 7

def flatten(x: Any): List[Any] = x match {
  case xs:List[_] => xs.flatMap(flatten)
  case _ => List(x)
}