import java.util.*

fun verticalWriting(txt:String, offset:Int) =

    txt.mapIndexed { i, c -> Pair(i, c) }

        .groupByTo(TreeMap(), { it.first % offset }, { it.second.toString() })

        .forEach { println(it.value.reversed().joinToString("|")) }

fun main(args: Array<String>) =

    verticalWriting("床前明月光疑是地上霜举头望明月低头思故乡", 5)

/*

低|举|疑|床

头|头|是|前

思|望|地|明

故|明|上|月

乡|月|霜|光

*/