ReversePolish.kt

import java.util.*

fun main() {
    println(evaluate("4 2 /"))
}

fun evaluate(expr: String): Float {
    val chars = expr.split(" ")
    val stack = Stack<Float>()
    val operator = operationForOperator()
    for (c in chars) {
        operator[c]?.let { stack.push(operate(it, stack)) } ?: stack.push(c.toFloat())
    }
    return stack.pop()
}

fun operationForOperator(): Map<String, (Float, Float) -> Float> {
    return mapOf(
        "+" to { a, b -> a + b },
        "-" to { a, b -> a - b },
        "*" to { a, b -> a * b },
        "/" to { a, b -> a / b }
    )
}

fun operate(function: (a: Float, b: Float) -> Float, stack: Stack<Float>): Float {
    val numberOne = stack.pop()
    val numberTwo = stack.pop()
    return function(numberTwo, numberOne)
}