Add basic calculator implementation using stack

Author: itsKomal1508

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

@@ -0,0 +1,90 @@
+package com.thealgorithms.stacks;
+
+import java.util.Stack;
+
+/**
+ * Utility class to evaluate a basic arithmetic expression containing
+ * non-negative integers, '+', '-', parentheses '(', ')', and spaces.
+ *
+ * <p>The evaluation is done using two stacks:
+ * one for operands and one for operators.
+ *
+ * <p>This class cannot be instantiated.</p>
+ */
+public final class BasicCalculatorUsingStack {
+
+    private BasicCalculatorUsingStack() {
+    }
+
+    /**
+     * Evaluates a mathematical expression.
+     *
+     * @param expression the input expression
+     * @return the evaluated result
+     * @throws IllegalArgumentException if the expression is null or empty
+     */
+    public static int evaluate(String expression) {
+        if (expression == null || expression.isEmpty()) {
+            throw new IllegalArgumentException("Expression must not be null or empty.");
+        }
+
+        Stack<Integer> operands = new Stack<>();
+        Stack<Character> operators = new Stack<>();
+
+        for (int i = 0; i < expression.length(); i++) {
+            char current = expression.charAt(i);
+
+            if (current == ' ') {
+                continue;
+            }
+
+            if (Character.isDigit(current)) {
+                int number = 0;
+                while (i < expression.length() && Character.isDigit(expression.charAt(i))) {
+                    number = number * 10 + (expression.charAt(i) - '0');
+                    i++;
+                }
+                i--;
+                operands.push(number);
+            } else if (current == '(') {
+                operators.push(current);
+            } else if (current == ')') {
+                while (!operators.isEmpty() && operators.peek() != '(') {
+                    applyOperation(operands, operators);
+                }
+                operators.pop(); // remove '('
+            } else if (current == '+' || current == '-') {
+                if (isUnaryMinus(expression, i)) {
+                    operands.push(0);
+                }
+                while (!operators.isEmpty() && operators.peek() != '(') {
+                    applyOperation(operands, operators);
+                }
+                operators.push(current);
+            }
+        }
+
+        while (!operators.isEmpty()) {
+            applyOperation(operands, operators);
+        }
+
+        return operands.pop();
+    }
+
+    private static void applyOperation(Stack<Integer> operands, Stack<Character> operators) {
+        int b = operands.pop();
+        int a = operands.pop();
+        char operator = operators.pop();
+        operands.push(operator == '+' ? a + b : a - b);
+    }
+
+    private static boolean isUnaryMinus(String expression, int index) {
+        int j = index - 1;
+        while (j >= 0 && expression.charAt(j) == ' ') {
+            j--;
+        }
+        return j < 0 || expression.charAt(j) == '('
+            || expression.charAt(j) == '+'
+            || expression.charAt(j) == '-';
+    }
+}

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

@@ -0,0 +1,42 @@
+package com.thealgorithms.stacks;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import org.junit.jupiter.api.Test;
+
+class BasicCalculatorUsingStackTest {
+
+    @Test
+    void testSimpleAddition() {
+        assertEquals(5, BasicCalculatorUsingStack.evaluate("2 + 3"));
+    }
+
+    @Test
+    void testSimpleSubtraction() {
+        assertEquals(1, BasicCalculatorUsingStack.evaluate("3 - 2"));
+    }
+
+    @Test
+    void testWithParentheses() {
+        assertEquals(23, BasicCalculatorUsingStack.evaluate("(1 + (4 + 5 + 2) - 3) + (6 + 8)"));
+    }
+
+    @Test
+    void testUnaryMinus() {
+        assertEquals(-2, BasicCalculatorUsingStack.evaluate("-2"));
+        assertEquals(1, BasicCalculatorUsingStack.evaluate("1 - (-2)"));
+    }
+
+    @Test
+    void testSpacesInExpression() {
+        assertEquals(3, BasicCalculatorUsingStack.evaluate(" 2 + 1 "));
+    }
+
+    @Test
+    void testInvalidExpression() {
+        IllegalArgumentException exception =
+            assertThrows(IllegalArgumentException.class, () -> BasicCalculatorUsingStack.evaluate(""));
+        assertEquals("Expression must not be null or empty.", exception.getMessage());
+    }
+}