Fix

Author: Hardvan

src/main/java/com/thealgorithms/stacks/PrefixEvaluator.java

@@ -1,5 +1,6 @@
 package com.thealgorithms.stacks;
 
+import java.util.Set;
 import java.util.Stack;
 
 /**
@@ -14,6 +15,8 @@ public final class PrefixEvaluator {
     private PrefixEvaluator() {
     }
 
+    private static final Set<String> OPERATORS = Set.of("+", "-", "*", "/");
+
     /**
      * Evaluates the given prefix expression and returns the result.
      *
@@ -32,7 +35,7 @@ public static int evaluatePrefix(String expression) {
                 int operand2 = stack.pop();
                 stack.push(applyOperator(token, operand1, operand2));
             } else {
-                stack.push(Integer.parseInt(token));
+                stack.push(Integer.valueOf(token));
             }
         }
 
@@ -50,7 +53,7 @@ public static int evaluatePrefix(String expression) {
      * @return true if the token is an operator, false otherwise.
      */
     private static boolean isOperator(String token) {
-        return token.equals("+") || token.equals("-") || token.equals("*") || token.equals("/");
+        return OPERATORS.contains(token);
     }
 
     /**