S10, S11

S-99: Ninety-Nine Scala Problemsを試してみる。
勉強のためにListのAPIはなるべく使わない方向で。

// P10 (*) Run-length encoding of a list.
// Use the result of problem P09 to implement the so-called run-length encoding data compression method. Consecutive duplicates of elements are encoded as tuples (N, E) where N is the number of duplicates of the element E.
// Example:
// scala> encode(List('a, 'a, 'a, 'a, 'b, 'c, 'c, 'a, 'a, 'd, 'e, 'e, 'e, 'e))
// res0: List[(Int, Symbol)] = List((4,'a), (1,'b), (2,'c), (2,'a), (1,'d), (4,'e))

def pack[A](ls: List[A]): List[List[A]] = {
  if (ls.isEmpty) List(List())
  else {
    val (packed, next) = ls span { _ == ls.head }
    if (next == Nil) List(packed)
    else{
      packed :: pack(next)
    }
  }
}
def encode[T](xs: List[T]) = {
  pack(xs).map{ (x) => (x.length, x.head) }
}

def encodeModified[T](xs: List[T]) = {
  pack(xs).map{ (x) => if(x.length == 1) x.head else (x.length, x.head) }
}

解答例
同じだ。