题目要求：
Given a run-length code list generated as specified in problem P10, construct its uncompressed version.
Example:
scala> decode(List((4, 'a), (1, 'b), (2, 'c), (2, 'a), (1, 'd), (4, 'e)))
res0: List[Symbol] = List('a, 'a, 'a, 'a, 'b, 'c, 'c, 'a, 'a, 'd, 'e, 'e, 'e, 'e)
解法：
List进行遍历的时候将每个元组的字母n次添加到list中
def decode[A](ls:List[(Int,Symbol)]):List[Symbol]={
var list:List[Symbol]=Nil
ls.foreach{e=>
for(i<-0 to e._1-1) {
list=list:::List(e._2)
}
}
list
}
def main(args: Array[String])= {
println(decode(List((1,'a),(2,'b),(3,'c),(2,'d))))
}
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