diff --git a/src/main/kotlin/com_github_leetcode/Node.kt b/src/main/kotlin/com_github_leetcode/Node.kt
index 4d435c9b3..f65b9473f 100644
--- a/src/main/kotlin/com_github_leetcode/Node.kt
+++ b/src/main/kotlin/com_github_leetcode/Node.kt
@@ -1,8 +1,5 @@
 package com_github_leetcode
 
-import java.util.StringJoiner
-import kotlin.collections.ArrayList
-
 class Node {
     var `val`: Int
     var neighbors: List<Node>
@@ -23,18 +20,14 @@ class Node {
     }
 
     override fun toString(): String {
-        val result = StringJoiner(",", "[", "]")
-        for (node in neighbors) {
+        return neighbors.joinToString(separator = ",", prefix = "[", postfix = "]") { node ->
             if (node.neighbors.isEmpty()) {
-                result.add(node.`val`.toString())
+                node.`val`.toString()
             } else {
-                val result2 = StringJoiner(",", "[", "]")
-                for (nodeItem in node.neighbors) {
-                    result2.add(nodeItem.`val`.toString())
+                node.neighbors.joinToString(separator = ",", prefix = "[", postfix = "]") { nodeItem ->
+                    nodeItem.`val`.toString()
                 }
-                result.add(result2.toString())
             }
         }
-        return result.toString()
     }
 }
diff --git a/src/main/kotlin/g0101_0200/s0150_evaluate_reverse_polish_notation/Solution.kt b/src/main/kotlin/g0101_0200/s0150_evaluate_reverse_polish_notation/Solution.kt
index 36a637edc..ced8f2045 100644
--- a/src/main/kotlin/g0101_0200/s0150_evaluate_reverse_polish_notation/Solution.kt
+++ b/src/main/kotlin/g0101_0200/s0150_evaluate_reverse_polish_notation/Solution.kt
@@ -3,14 +3,12 @@ package g0101_0200.s0150_evaluate_reverse_polish_notation
 // #Medium #Top_Interview_Questions #Array #Math #Stack #Programming_Skills_II_Day_3
 // #2022_10_09_Time_233_ms_(88.82%)_Space_36.7_MB_(91.45%)
 
-import java.util.function.BiFunction
-
 class Solution {
-    val op = mapOf<String, BiFunction<Int, Int, Int>>(
-        "/" to BiFunction { a, b -> a / b },
-        "*" to BiFunction { a, b -> a * b },
-        "+" to BiFunction { a, b -> a + b },
-        "-" to BiFunction { a, b -> a - b }
+    val op = mapOf<String, (Int, Int) -> Int>(
+        "/" to { a, b -> a / b },
+        "*" to { a, b -> a * b },
+        "+" to { a, b -> a + b },
+        "-" to { a, b -> a - b }
     )
     fun evalRPN(tokens: Array<String>): Int {
         val stack = ArrayDeque<String>()
@@ -18,7 +16,7 @@ class Solution {
             if (op.contains(t)) {
                 val b = stack.removeFirst().toInt()
                 val a = stack.removeFirst().toInt()
-                val c = op.getValue(t).apply(a, b)
+                val c = op.getValue(t).invoke(a, b)
                 stack.addFirst(c.toString())
             } else {
                 stack.addFirst(t)