groupBy
Groups characters of the original char sequence by the key returned by the given keySelector function applied to each character and returns a map where each group key is associated with a list of corresponding characters.
inline fun <K> CharSequence.groupBy(keySelector: (Char) -> K): Map<K, List<Char>>(source)
val text = "banana"
val grouped = text.groupBy { it }
println(grouped) // {b=[b], a=[a, a, a], n=[n, n]}