feat: Add PostfixEvaluator new algorithm with Junit tests (#5754)

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

@@ -0,0 +1,74 @@
+package com.thealgorithms.stacks;
+
+import java.util.Set;
+import java.util.Stack;
+
+/**
+ * Evaluate a postfix (Reverse Polish) expression using a stack.
+ *
+ * <p>Example: Expression "5 6 + 2 *" results in 22.
+ * <p>Applications: Used in calculators and expression evaluation in compilers.
+ *
+ * @author Hardvan
+ */
+public final class PostfixEvaluator {
+    private PostfixEvaluator() {
+    }
+
+    private static final Set<String> OPERATORS = Set.of("+", "-", "*", "/");
+
+    /**
+     * Evaluates the given postfix expression and returns the result.
+     *
+     * @param expression The postfix expression as a string with operands and operators separated by spaces.
+     * @return The result of evaluating the postfix expression.
+     * @throws IllegalArgumentException if the expression is invalid.
+     */
+    public static int evaluatePostfix(String expression) {
+        Stack<Integer> stack = new Stack<>();
+
+        for (String token : expression.split("\\s+")) {
+            if (isOperator(token)) {
+                int operand2 = stack.pop();
+                int operand1 = stack.pop();
+                stack.push(applyOperator(token, operand1, operand2));
+            } else {
+                stack.push(Integer.valueOf(token));
+            }
+        }
+
+        if (stack.size() != 1) {
+            throw new IllegalArgumentException("Invalid expression");
+        }
+
+        return stack.pop();
+    }
+
+    /**
+     * Checks if the given token is an operator.
+     *
+     * @param token The token to check.
+     * @return true if the token is an operator, false otherwise.
+     */
+    private static boolean isOperator(String token) {
+        return OPERATORS.contains(token);
+    }
+
+    /**
+     * Applies the given operator to the two operands.
+     *
+     * @param operator The operator to apply.
+     * @param a The first operand.
+     * @param b The second operand.
+     * @return The result of applying the operator to the operands.
+     */
+    private static int applyOperator(String operator, int a, int b) {
+        return switch (operator) {
+            case "+" -> a + b;
+            case "-" -> a - b;
+            case "*" -> a * b;
+            case "/" -> a / b;
+            default -> throw new IllegalArgumentException("Invalid operator");
+        };
+    }
+}