题目要求：
Implement the so-called run-length encoding data compression method directly. I.e. don’t use other methods you’ve written (like P09’s pack); do all the work directly.
Example:
scala> encodeDirect(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))
解法：
def encodeDirect[A](ls:List[A]):List[(Int,A)]={
if(ls==Nil) Nil
else {
val l = ls.dropWhile(_ == ls.head)
List((ls.length - l.length, ls.head)) ::: encodeDirect(l)
}
}
def main(args: Array[String])= {
println(encodeDirect(List('a, 'a, 'a, 'a, 'b, 'c, 'c, 'a, 'a, 'd, 'e, 'e, 'e)))
}
或者
def encodeDirect[A](ls: List[A]): List[(Int, A)] =
if (ls.isEmpty) Nil
else {
val (packed, next) = ls span { _ == ls.head }
(packed.length, packed.head) :: encodeDirect(next)
}
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