Refactored ReverseStringUsingStack utility for reversing strings using stack

Author: navaneedan07

src/test/java/com/thealgorithms/stacks/ReverseStringUsingStackTest.java

@@ -0,0 +1,44 @@
+package com.thealgorithms.stacks;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import org.junit.jupiter.api.Test;
+
+class ReverseStringUsingStackTest {
+
+    @Test
+    void testRegularString() {
+        assertEquals("olleh", ReverseStringUsingStack.reverse("hello"));
+    }
+
+    @Test
+    void testEmptyString() {
+        assertEquals("", ReverseStringUsingStack.reverse(""));
+    }
+
+    @Test
+    void testPalindromeString() {
+        assertEquals("madam", ReverseStringUsingStack.reverse("madam"));
+    }
+
+    @Test
+    void testSpecialCharacters() {
+        assertEquals("#@!321cba", ReverseStringUsingStack.reverse("abc123!@#"));
+    }
+
+    @Test
+    void testSingleCharacter() {
+        assertEquals("x", ReverseStringUsingStack.reverse("x"));
+    }
+
+    @Test
+    void testWhitespaceHandling() {
+        assertEquals("dlroW olleH", ReverseStringUsingStack.reverse("Hello World"));
+    }
+
+    @Test
+    void testNullInput() {
+        assertThrows(IllegalArgumentException.class, () -> { ReverseStringUsingStack.reverse(null); });
+    }
+}