Refactored ReverseStringUsingStack utility for reversing strings using stack

Author: navaneedan07

src/test/java/com/thealgorithms/strings/ReverseStringTest.java

@@ -1,8 +1,11 @@
 package com.thealgorithms.strings;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
 
 import java.util.stream.Stream;
+import org.junit.jupiter.api.Test;
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.Arguments;
 import org.junit.jupiter.params.provider.MethodSource;
@@ -31,4 +34,18 @@ public void testReverseString2(String input, String expectedOutput) {
     public void testReverseString3(String input, String expectedOutput) {
         assertEquals(expectedOutput, ReverseString.reverse3(input));
     }
+
+    @ParameterizedTest
+    @MethodSource("testCases")
+    public void testReverseString4(String input, String expectedOutput) {
+        assertEquals(expectedOutput, ReverseString.reverse4(input));
+    }
+
+    @Test
+    public void testReverseString4WithNull() {
+        Exception exception = assertThrows(IllegalArgumentException.class, () -> { ReverseString.reverse4(null); });
+        String expectedMessage = "Input string cannot be null";
+        String actualMessage = exception.getMessage();
+        assertTrue(actualMessage.contains(expectedMessage));
+    }
 }