Nice stuff to get your skills up to shape. It helped me to improve my understanding of folds, maps, flatMaps, zips, etc. Also I learned new pattern matching techniques (tuples and guards).

You must really give the exercises a shot!

Here are my solutions:

```
package exercises
object NinetyNine {
//P01
def last[A](xs: List[A]): A = xs match {
case x :: Nil => x
case x :: xs => last(xs)
case Nil => throw new NoSuchElementException
}
//P02
def penultimate[A](xs: List[A]): A = xs match {
case x :: y :: Nil => x
case x :: xs => penultimate(xs)
case _ => throw new NoSuchElementException
}
//P03
def nth[A](i: Int, xs: List[A]): A = xs match {
case x :: xs => if (i == 0) x else nth(i - 1, xs)
case _ => throw new NoSuchElementException
}
def nth2[A](i: Int, xs: List[A]): A = xs match {
case x :: xs if (i == 0) => x
case x :: xs if (i > 0) => nth(i - 1, xs)
case _ => throw new NoSuchElementException
}
//P04
def length[A](xs: List[A]): Int = xs match {
case x :: xs => 1 + length(xs)
case Nil => 0
}
//Although straight recursion is fun, using folds is more effective
// folds are abstractions for recursion over data structures
//P04
def length1[A](xs: List[A]): Int = xs.foldLeft(0)((a, b) => a + 1)
//P05
def reverse[A](xs: List[A]): List[A] = xs.foldLeft(Nil: List[A])((a, b) => b :: a)
//P06
def isPalindrome[A](xs: List[A]): Boolean = reverse(xs) == xs
//P07
def flatten[A](xs: List[List[A]]): List[A] = xs.foldRight(Nil: List[A])(
(xs, acc) => xs match {
case ys: List[A] => ys ::: acc
case y: A => y :: acc
})
//P08
def compress[A](xs: List[A]): List[A] =
xs.foldRight(List[A]())((y, ys) =>
if (ys != Nil && ys.head == y) ys else y :: ys)
//P09
def pack[A](xs: List[A]): List[List[A]] = xs match {
case Nil => Nil
case _ => xs.takeWhile(_ == xs.head) :: pack(xs.dropWhile(_ == xs.head)) //can use span too, see P13
}
//P10
def encode[A](xs: List[A]): List[(Int, A)] =
pack(xs).map(y => (y.length, y.head))
//P11
def encodeModified[A](xs: List[A]): List[Any] =
pack(xs).map(
y => if (y.length == 1) y.head else (y.length, y.head))
//Type safe version
def encodeModifiedTypSafe[A](xs: List[A]): List[Either[A, (Int, A)]] =
pack(xs).map(
y =>
if (y.length == 1) Left(y.head)
else Right(y.length, y.head))
//P12
def decode[A](xs: List[(Int, A)]): List[A] = {
def expand(i: Int, a: A): List[A] = if (i == 0) Nil else a :: expand(i - 1, a)
flatten(xs.foldRight(List[List[A]]())((y, ys) => expand(y._1, y._2) :: ys))
}
//is better with flatMap
def decode1[A](xs: List[(Int, A)]): List[A] =
xs.flatMap(ny => List.make(ny._1, ny._2))
//P13
def encodeDirect[A](xs: List[A]): List[(Int, A)] = xs match {
case Nil => Nil
case _ => {
val ys = xs.span(_ == xs.head)
(ys._1.length, xs.head) :: encodeDirect(ys._2)
}
}
//P14
def duplicate[A](xs: List[A]): List[A] =
xs.foldRight(Nil: List[A])((y, ys) => y :: y :: ys)
//is better with flatMap
def duplicate1[A](xs: List[A]): List[A] = xs.flatMap(y => y :: y :: Nil)
//P15
def duplicateN[A](n: Int, xs: List[A]): List[A] =
xs.foldRight(Nil: List[A])((y, ys) => (1 to n).toList.map(_ => y) ::: ys)
//is better with flatMap
def duplicateN1[A](n: Int, xs: List[A]): List[A] =
xs.flatMap(y => List.make(n, y))
//P16
def drop[A](n: Int, xs: List[A]): List[A] =
xs.zipWithIndex.filter(_._2 % n != 0).map(_._1)
//P17
def split[A](n: Int, xs: List[A]): (List[A], List[A]) =
xs.splitAt(n) // or (xs.take(n),xs.drop(n))
//ok, that was too easy, let's try recursion
def splitRec[A](n: Int, xs: List[A]): (List[A], List[A]) =
(n, xs) match {
case (0, _) => (Nil, xs)
case (_, y :: ys) => {
val rec = splitRec(n - 1, ys)
(y :: rec._1, rec._2)
}
case (_, Nil) => (xs, Nil)
}
//P18
def slice[A](s: Int, e: Int, xs: List[A]): List[A] =
xs.slice(s, e) // or xs.drop(s).take(e-s)
//with recursion
def sliceRec[A](s: Int, e: Int, xs: List[A]): List[A] =
(s, e, xs) match {
case (0, 0, xs) => Nil
case (0, _, y :: ys) => y :: sliceRec(0, e - 1, ys)
case (_, _, y :: ys) => sliceRec(s - 1, e - 1, ys)
}
//P19
def rotate[A](n: Int, xs: List[A]): List[A] = {
val s = split((if (n > 0) n else n + xs.length), xs)
s._2 ::: s._1
}
//P20
def removeAt[A](n: Int, xs: List[A]): (List[A], A) = {
val (heads, tails) = split(n, xs)
(heads ::: tails.tail, tails.head)
}
//P21
def insertAt[A](e: A, n: Int, xs: List[A]): List[A] = {
val (heads, tails) = split(n, xs)
heads ::: e :: tails
}
//with recursion
def insertRecAt[A](e: A, n: Int, xs: List[A]): List[A] =
(n, xs) match {
case (0, ys) => e :: ys
case (_, y :: ys) => y :: insertRecAt(e, n - 1, ys)
}
//P22
def range[A](s: Int, e: Int): List[Int] = (s to e).toList
//I don't think is the purpose of the exercise! Again, let's try recursion
def rangeRec(s: Int, e: Int): List[Int] = if (e - s == 0) e :: Nil else s :: rangeRec(s + 1, e)
//recursion and pattern matching with guards
//a little more readable
def rangeRecPm(s: Int, e: Int): List[Int] = (e - s) match {
case x if (x > 0) => s :: rangeRecPm(s + 1, e)
case x if (x == 0) => e :: Nil
case _ => error("Invalid range")
}
```

` }`