Use BigInteger to prevent overflow in factorial calculation (#7358)

* Use BigInteger to prevent overflow in factorial calculation

* chore: remove unnecessary comment

* update

* Fix: improve factorial implementation and formatting

* test: final fix for FactorialTest logic and formatting

* chore: final formatting and test fix for BigInteger

* chore: final formatting and test fix for BigInteger

Author: VanshSharma3

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

@@ -1,23 +1,19 @@
 package com.thealgorithms.maths;
 
+import java.math.BigInteger;
+
 public final class Factorial {
     private Factorial() {
     }
 
-    /**
-     * Calculate factorial N using iteration
-     *
-     * @param n the number
-     * @return the factorial of {@code n}
-     */
-    public static long factorial(int n) {
+    public static BigInteger factorial(int n) {
         if (n < 0) {
             throw new IllegalArgumentException("Input number cannot be negative");
         }
-        long factorial = 1;
-        for (int i = 1; i <= n; ++i) {
-            factorial *= i;
+        BigInteger result = BigInteger.ONE;
+        for (int i = 1; i <= n; i++) {
+            result = result.multiply(BigInteger.valueOf(i));
         }
-        return factorial;
+        return result;
     }
 }

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

@@ -3,6 +3,7 @@
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 
+import java.math.BigInteger;
 import org.junit.jupiter.api.Test;
 
 public class FactorialTest {
@@ -16,9 +17,9 @@ public void testWhenInvalidInoutProvidedShouldThrowException() {
 
     @Test
     public void testCorrectFactorialCalculation() {
-        assertEquals(1, Factorial.factorial(0));
-        assertEquals(1, Factorial.factorial(1));
-        assertEquals(120, Factorial.factorial(5));
-        assertEquals(3628800, Factorial.factorial(10));
+        assertEquals(BigInteger.ONE, Factorial.factorial(0));
+        assertEquals(BigInteger.ONE, Factorial.factorial(1));
+        assertEquals(BigInteger.valueOf(120), Factorial.factorial(5));
+        assertEquals(BigInteger.valueOf(3628800), Factorial.factorial(10));
     }
 }