inline fun CharSequence.reduceRightIndexedOrNull(
    operation: (index: Int, Char, acc: Char) -> Char
): Char?

Accumulates value starting with the last character and applying operation from right to left to each character with its index in the original char sequence and current accumulator value.

Returns null if the char sequence is empty.

import kotlin.test.*

fun main(args: Array<String>) {
//sampleStart
val strings = listOf("a", "b", "c", "d")
println(strings.reduceRightOrNull { string, acc -> acc + string }) // dcba
println(strings.reduceRightIndexedOrNull { index, string, acc -> acc + string + index }) // dc2b1a0

println(emptyList<String>().reduceRightOrNull { _, _ -> "" }) // null
//sampleEnd
}

Parameters

operation - function that takes the index of a character, the character itself and current accumulator value, and calculates the next accumulator value.