0916-decoded-string-at-index.kt

classSolution {
fundecodeAtIndex(s:String, k:Int): String {
val string =if (s.last().isLetter()) s +"2"else s

val intervalSizes =mutableListOf(0L)
val coefficients =mutableListOf(0L)
val tokens =mutableListOf("")

var isDigitBefore =false
var nextB =1L
val accumulated = mutableListOf<Char>()
for (char in string) {
if (char.isDigit() != isDigitBefore) {
if (!isDigitBefore) {
 tokens.add(accumulated.joinToString(""))
 coefficients.add(nextB)
 intervalSizes.add(intervalSizes.last() * nextB + accumulated.size)
 } else {
 nextB = accumulated.fold(1L) { acc:Long, c:Char-> acc * c.digitToInt() }
 }

 isDigitBefore =!isDigitBefore
 accumulated.clear()
 }

 accumulated.add(char)
 }

var remainder = k -1L
var current = intervalSizes.size -1
while (current >0) {
 remainder %= intervalSizes[current]
val additionalPartStartsAt = intervalSizes[current -1] * coefficients[current]
if (remainder >= additionalPartStartsAt) {
return tokens[current][(remainder - additionalPartStartsAt).toInt()].toString()
 }
--current
 }

return""
 }
}