Add Factorial algorithm using recursion

src/main/java/com/thealgorithms/recursion/Factorial.java

@@ -0,0 +1,23 @@
+public final class Factorial {
+    private Factorial() {
+        throw new UnsupportedOperationException("Utility class");
+    }
+
+    /**
+     * Computes the factorial of a non-negative integer using recursion.
+     *
+     * @param n the number for which factorial is to be calculated
+     * @return factorial of n
+     * @throws IllegalArgumentException if n is negative
+     */
+    public static long factorial(int n) {
+        if (n < 0) {
+            throw new IllegalArgumentException("Factorial is not defined for negative numbers.");
+        }
+
+        if (n == 0 || n == 1) {
+            return 1;
+        }
+        return n * factorial(n - 1);
+    }
+}

src/test/java/com/thealgorithms/recursion/FactorialTest.java

@@ -0,0 +1,29 @@
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import org.junit.jupiter.api.Test;
+
+public class FactorialTest {
+
+    @Test
+    public void testFactorialOfOne() {
+        assertEquals(1, Factorial.factorial(1));
+    }
+
+    @Test
+    public void testFactorialOfPositiveNumbers() {
+        assertEquals(120, Factorial.factorial(5));
+        assertEquals(720, Factorial.factorial(6));
+        assertEquals(5040, Factorial.factorial(7));
+    }
+
+    @Test
+    public void testFactorialOfTen() {
+        assertEquals(3628800, Factorial.factorial(10));
+    }
+
+    @Test
+    public void testNegativeNumberThrowsException() {
+        assertThrows(IllegalArgumentException.class, () -> Factorial.factorial(-1));
+    }
+}