added factorial using recursion

Author: Ayush41

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

@@ -0,0 +1,36 @@
+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);
+    }
+}