feat: Add PrefixEvaluator new algorithm with Junit tests (#5755)

Author: Hardvan

DIRECTORY.md

@@ -600,6 +600,7 @@
             * [NextSmallerElement](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/NextSmallerElement.java)
             * [PostfixEvaluator](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/PostfixEvaluator.java)
             * [PostfixToInfix](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/PostfixToInfix.java)
+            * [PrefixEvaluator](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/PrefixEvaluator.java)
             * [PrefixToInfix](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/PrefixToInfix.java)
             * [SortStack](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/SortStack.java)
             * [StackPostfixNotation](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/StackPostfixNotation.java)
@@ -1131,6 +1132,7 @@
             * [NextSmallerElementTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/NextSmallerElementTest.java)
             * [PostfixEvaluatorTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/PostfixEvaluatorTest.java)
             * [PostfixToInfixTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/PostfixToInfixTest.java)
+            * [PrefixEvaluatorTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/PrefixEvaluatorTest.java)
             * [PrefixToInfixTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/PrefixToInfixTest.java)
             * [SortStackTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/SortStackTest.java)
             * [StackPostfixNotationTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/StackPostfixNotationTest.java)

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

@@ -0,0 +1,76 @@
+package com.thealgorithms.stacks;
+
+import java.util.Set;
+import java.util.Stack;
+
+/**
+ * Evaluate a prefix (Polish) expression using a stack.
+ *
+ * <p>Example: Expression "+ * 2 3 4" results in 10.
+ * <p>Applications: Useful for implementing compilers and interpreters.
+ *
+ * @author Hardvan
+ */
+public final class PrefixEvaluator {
+    private PrefixEvaluator() {
+    }
+
+    private static final Set<String> OPERATORS = Set.of("+", "-", "*", "/");
+
+    /**
+     * Evaluates the given prefix expression and returns the result.
+     *
+     * @param expression The prefix expression as a string with operands and operators separated by spaces.
+     * @return The result of evaluating the prefix expression.
+     * @throws IllegalArgumentException if the expression is invalid.
+     */
+    public static int evaluatePrefix(String expression) {
+        Stack<Integer> stack = new Stack<>();
+        String[] tokens = expression.split("\\s+");
+
+        for (int i = tokens.length - 1; i >= 0; i--) {
+            String token = tokens[i];
+            if (isOperator(token)) {
+                int operand1 = stack.pop();
+                int operand2 = 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");
+        };
+    }
+}

src/test/java/com/thealgorithms/stacks/PrefixEvaluatorTest.java

@@ -0,0 +1,27 @@
+package com.thealgorithms.stacks;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.util.EmptyStackException;
+import org.junit.jupiter.api.Test;
+
+public class PrefixEvaluatorTest {
+
+    @Test
+    public void testValidExpressions() {
+        assertEquals(10, PrefixEvaluator.evaluatePrefix("+ * 2 3 4"));
+        assertEquals(5, PrefixEvaluator.evaluatePrefix("- + 7 3 5"));
+        assertEquals(6, PrefixEvaluator.evaluatePrefix("/ * 3 2 1"));
+    }
+
+    @Test
+    public void testInvalidExpression() {
+        assertThrows(EmptyStackException.class, () -> PrefixEvaluator.evaluatePrefix("+ 3"));
+    }
+
+    @Test
+    public void testMoreThanOneStackSizeAfterEvaluation() {
+        assertThrows(IllegalArgumentException.class, () -> PrefixEvaluator.evaluatePrefix("+ 3 4 5"));
+    }
+}