flatMap
Returns a single list of all elements yielded from results of transform function being invoked on each character of original char sequence.
import kotlin.test.*
fun main(args: Array<String>) {
//sampleStart
val list = listOf("123", "45")
println(list.flatMap { it.toList() }) // [1, 2, 3, 4, 5]
//sampleEnd
}