scala 学习之：list span 用法

Pack consecutive duplicates of list elements into sublists.
If a list contains repeated elements they should be placed in separate sublists.
Example:

scala> pack(List(‘a, ‘a, ‘a, ‘a, ‘b, ‘c, ‘c, ‘a, ‘a, ‘d, ‘e, ‘e, ‘e, ‘e))
res0: List[List[Symbol]] = List(List(‘a, ‘a, ‘a, ‘a), List(‘b), List(‘c, ‘c), List(‘a, ‘a), List(‘d), List(‘e, ‘e, ‘e, ‘e))

题目描述： 如果一个list中有相同的元素，则将相同的元素放到一个新的list中，最后返回list[list]

scala List span 函数：

定义：

final def span(p: (A) ? Boolean): (List[A], List[A])
Splits this list into a prefix/suffix pair according to a predicate.

Note: c span p is equivalent to (but possibly more efficient than) (c takeWhile p, c dropWhile p), provided the evaluation of the predicate p does not cause any side-effects.
returns
a pair consisting of the longest prefix of this list whose elements all satisfy p, and the rest of this list.
Definition Classes
List → LinearSeqOptimized → TraversableLike → GenTraversableLike
Annotations
@inline()

即span 根据输入的bool表达式，将list进行分割。返回一个list集合。但是碰到第一个不满足的元素，即返回。如：

list 的partition： 会遍历所有元素。

思路： 题目的要求是，连续的相等的元素放到同一个 list中，因此

使用span 进行分割。

 def packList[T](a:List[T]): List[List[T]] = {
          def _pack(res:List[List[T]], tmp:List[T]):List[List[T]] = {
              tmp match{
                case Nil => res
                case ls =>{
                    val (s:List[T], r:List[T]) = tmp span{_ == tmp.head}
                    _pack(res:::List(s), r)
                }
              }
          }
          _pack(List(), a)
    }