在Scala中反转Map(String - > List)

Question

我有一个Map [String,List [String]],我想反转它.例如,如果我有类似的东西

  "1" -> List("a","b","c")
  "2" -> List("a","j","k")
  "3" -> List("a","c")

结果应该是

  "a" -> List("1","2","3")
  "b" -> List("1")
  "c" -> List("1","3")
  "j" -> List("2")
  "k" -> List("2")

我试过这个:

  m.map(_.swap)

但它返回一个Map [List [String],String]:

  List("a","b","c") -> "1"
  List("a","j","k") -> "2" 
  List("a","c") -> "3"

Answer 1

地图反演有点复杂.

val m = Map("1" -> List("a","b","c")
           ,"2" -> List("a","j","k")
           ,"3" -> List("a","c"))

m flatten {case(k, vs) => vs.map((_, k))} groupBy (_._1) mapValues {_.map(_._2)}
//res0: Map[String,Iterable[String]] = Map(j -> List(2), a -> List(1, 2, 3), b -> List(1), c -> List(1, 3), k -> List(2))

将其展平Map为一组元组.groupBy将Map使用旧值作为新键创建一个新的.然后通过删除键(先前值)元素来取消元组值.