Map Inversion

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))

Flatten the Map into a collection of tuples. groupBy will create a new Map with the old values as the new keys. Then un-tuple the values by removing the key (previously value) elements.

Map of type Map[A, Seq[B]] to Map[B, Seq[A]], where each B in the new map is associated with every A in the old map for which the B was contained in A’s associated sequence.