midterm.kt

import kotlinx.coroutines.launch
import kotlinx.coroutines.runBlocking
import java.util.*

fun main() {
    val a = Stack<Int>().also {
        it.push(9)
        it.push(8)
        it.push(7)
        it.push(6)
        it.push(5)
        it.push(4)
        it.push(3)
        it.push(2)
        it.push(1)
    }

    val b = Stack<Int>()
    val c = Stack<Int>()


    runBlocking {
        launch {
            solve(a, b, c, a.size)
            println(a)
            println(b)
            println(c)
        }
    }
}

fun solve(from: Stack<Int>, into: Stack<Int>, temp: Stack<Int>, n: Int) {
    if (n == 0) {
        return
    }
    if (n == 1) {
        into.push(from.pop())
        return
    }
    solve(from, temp, into, n - 1) // solving for n - 1, but into temporary stack
    into.push(from.pop())
    solve(
        temp,
        into,
        from,
        n - 1
    ) // solving for n - 1, but the temporary stack (the which is the solution of line 40) into 'into' as 'into' now is the 'temporary' stack
}