Add Armstrong Number algorithm with comprehensive tests

- Implemented ArmstrongNumber.isArmstrong() method
- Added comprehensive unit tests for Armstrong numbers
- Includes documentation with examples
- Handles edge cases (negative numbers, zero)
- Tests cover known Armstrong numbers: 0, 1, 153, 370, 371, 407, 1634

Signed-off-by: duvvuvenkataramana <[email protected]>

Author: duvvuvenkataramana

src/main/java/com/thealgorithms/maths/ArmstrongNumber.java

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+package com.thealgorithms.maths;
+
+/**
+ * Armstrong Number (also known as Narcissistic Number)
+ * A number is called an Armstrong number if the sum of cubes of its digits equals the number itself.
+ * For example: 153 = 1³ + 5³ + 3³ = 1 + 125 + 27 = 153
+ *
+ * @author duvvuvenkataramana
+ */
+public final class ArmstrongNumber {
+    private ArmstrongNumber() {
+    }
+
+    /**
+     * Check if a number is an Armstrong number
+     *
+     * @param number the number to check
+     * @return true if the number is an Armstrong number, false otherwise
+     */
+    public static boolean isArmstrong(int number) {
+        if (number < 0) {
+            return false;
+        }
+
+        int original = number;
+        int sum = 0;
+        int digits = String.valueOf(number).length();
+
+        while (number > 0) {
+            int digit = number % 10;
+            sum += Math.pow(digit, digits);
+            number /= 10;
+        }
+
+        return sum == original;
+    }
+}

src/test/java/com/thealgorithms/maths/ArmstrongNumberTest.java

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+package com.thealgorithms.maths;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import org.junit.jupiter.api.Test;
+
+class ArmstrongNumberTest {
+
+    @Test
+    void testArmstrongNumbers() {
+        // Test known Armstrong numbers
+        assertTrue(ArmstrongNumber.isArmstrong(0));
+        assertTrue(ArmstrongNumber.isArmstrong(1));
+        assertTrue(ArmstrongNumber.isArmstrong(153));
+        assertTrue(ArmstrongNumber.isArmstrong(370));
+        assertTrue(ArmstrongNumber.isArmstrong(371));
+        assertTrue(ArmstrongNumber.isArmstrong(407));
+        assertTrue(ArmstrongNumber.isArmstrong(1634));
+    }
+
+    @Test
+    void testNonArmstrongNumbers() {
+        // Test numbers that are not Armstrong numbers
+        assertFalse(ArmstrongNumber.isArmstrong(10));
+        assertFalse(ArmstrongNumber.isArmstrong(100));
+        assertFalse(ArmstrongNumber.isArmstrong(152));
+        assertFalse(ArmstrongNumber.isArmstrong(200));
+    }
+
+    @Test
+    void testNegativeNumbers() {
+        // Negative numbers cannot be Armstrong numbers
+        assertFalse(ArmstrongNumber.isArmstrong(-1));
+        assertFalse(ArmstrongNumber.isArmstrong(-153));
+    }
+}