Add recursive factorial implementation with tests

Author: Koushik Sai

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

@@ -0,0 +1,37 @@
+package com.thealgorithms.recursion;
+
+/*
+ * Factorial of a number n is the product of all positive integers less than
+ * or equal to n.
+ *
+ * n! = n × (n - 1) × (n - 2) × ... × 1
+ *
+ * Examples:
+ * 0! = 1
+ * 1! = 1
+ * 5! = 120
+ */
+
+public final class FactorialRecursion {
+
+    private FactorialRecursion() {
+        throw new UnsupportedOperationException("Utility class");
+    }
+
+    /**
+     * Computes the factorial of a non-negative integer using recursion.
+     *
+     * @param n the number whose factorial is to be computed
+     * @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 <= 1) {
+            return 1;
+        }
+        return n * factorial(n - 1);
+    }
+}

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

@@ -0,0 +1,34 @@
+package com.thealgorithms.recursion;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import org.junit.jupiter.api.Test;
+
+class FactorialRecursionTest {
+
+    @Test
+    void testFactorialOfZero() {
+        assertEquals(1, FactorialRecursion.factorial(0));
+    }
+
+    @Test
+    void testFactorialOfOne() {
+        assertEquals(1, FactorialRecursion.factorial(1));
+    }
+
+    @Test
+    void testFactorialOfPositiveNumber() {
+        assertEquals(120, FactorialRecursion.factorial(5));
+    }
+
+    @Test
+    void testFactorialOfLargerNumber() {
+        assertEquals(3628800, FactorialRecursion.factorial(10));
+    }
+
+    @Test
+    void testFactorialOfNegativeNumber() {
+        assertThrows(IllegalArgumentException.class,
+                () -> FactorialRecursion.factorial(-1));
+    }
+}