kt: add «150. Evaluate Reverse Polish Notation»

URL:	https://leetcode.com/problems/evaluate-reverse-polish-notation/
Signed-off-by: Matej Focko <me@mfocko.xyz>

kt/evaluate-reverse-polish-notation.kt

@@ -0,0 +1,29 @@
+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()
+}