LeetCode/kt/evaluate-reverse-polish-notation.kt

class Solution {
    private fun getFunction(op: String): (Int, Int) -> Int =
        when (op) {
            "+" -> Int::plus
            "-" -> Int::minus
            "*" -> Int::times
            "/" -> Int::div
            else -> error("invalid operator")
        }

    fun evalRPN(tokens: Array<String>): Int =
        tokens.fold(mutableListOf<Int>()) { stack, token ->
            when (token) {
                "+", "-", "*", "/" -> {
                    check(stack.size >= 2)

                    val r = stack.removeLast()
                    val l = stack.removeLast()

                    stack.add(getFunction(token)(l, r))
                }
                else -> {
                    stack.add(token.toInt())
                }
            }

            stack
        }.last()
}