Add DisariumNumbers with test

Author: iamcodinghere22

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

@@ -0,0 +1,37 @@
+package com.thealgorithms.maths;
+
+/**
+ * Disarium number is a number where the sum of its digits powered
+ * with their respective positions is equal to the number itself.
+ * Example: 135 = 1^1 + 3^2 + 5^3 = 1 + 9 + 125 = 135
+ *
+ * @see <a href="https://en.wikipedia.org/wiki/Disarium_number">Disarium Number</a>
+ */
+public final class DisariumNumber {
+
+    private DisariumNumber() {
+    }
+
+    /**
+     * Checks if a number is a Disarium number.
+     *
+     * @param number the number to check (must be positive)
+     * @return true if number is Disarium, false otherwise
+     * @throws IllegalArgumentException if number is not positive
+     */
+    public static boolean isDisarium(int number) {
+        if (number <= 0) {
+            throw new IllegalArgumentException("Input must be a positive integer.");
+        }
+        int digits = String.valueOf(number).length();
+        int temp = number;
+        int sum = 0;
+        while (temp > 0) {
+            int lastDigit = temp % 10;
+            sum += (int) Math.pow(lastDigit, digits);
+            digits--;
+            temp /= 10;
+        }
+        return sum == number;
+    }
+}

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

@@ -0,0 +1,32 @@
+package com.thealgorithms.maths;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import org.junit.jupiter.api.Test;
+
+class DisariumNumberTest {
+
+    @Test
+    void testDisariumNumbers() {
+        assertTrue(DisariumNumber.isDisarium(1));
+        assertTrue(DisariumNumber.isDisarium(89));
+        assertTrue(DisariumNumber.isDisarium(135));
+        assertTrue(DisariumNumber.isDisarium(175));
+        assertTrue(DisariumNumber.isDisarium(518));
+    }
+
+    @Test
+    void testNonDisariumNumbers() {
+        assertFalse(DisariumNumber.isDisarium(10));
+        assertFalse(DisariumNumber.isDisarium(100));
+        assertFalse(DisariumNumber.isDisarium(200));
+    }
+
+    @Test
+    void testInvalidInput() {
+        assertThrows(IllegalArgumentException.class, () -> DisariumNumber.isDisarium(0));
+        assertThrows(IllegalArgumentException.class, () -> DisariumNumber.isDisarium(-5));
+    }
+}