Add Factorial algorithm using recursion

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

@@ -0,0 +1,32 @@
+package com.thealgorithms.recursion;
+
+/*
+    The Factorial of a non-negative integer n is the product of all positive integers less than or
+   equal to n. It is defined as: n! = n × (n - 1) × (n - 2) × ... × 1 with the base case: 0! = 1
+
+    Example:
+        5! = 5 × 4 × 3 × 2 × 1 = 120
+*/
+
+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 +1,35 @@
+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;
+
+public class FactorialTest {
+  @Test
+  public void testFactorialOfZero() {
+    assertEquals(1, Factorial.factorial(0));
+  }
+
+  @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));
+  }
+}