fix: revert ReverseStack deletions and ensure lint compliance

Author: VickySource

src/main/java/com/thealgorithms/datastructures/stacks/ReverseStack.java

@@ -37,15 +37,19 @@ private ReverseStack() {
      *
      * @param stack the stack to reverse; should not be null
      */
-    public static void reverseStack(Stack<Integer> stack) {
-        if (stack.isEmpty()) {
-            return;
-        }
-
-        int element = stack.pop();
-        reverseStack(stack);
-        insertAtBottom(stack, element);
+public static void reverseStack(Stack<Integer> stack) {
+    if (stack == null) {
+        throw new IllegalArgumentException("Stack cannot be null");
     }
+    if (stack.isEmpty()) {
+        return;
+    }
+
+    int element = stack.pop();
+    reverseStack(stack);
+    insertAtBottom(stack, element);
+}
+
 
     /**
      * Inserts the specified element at the bottom of the stack.

src/test/java/com/thealgorithms/datastructures/stacks/ReverseStackTest.java

@@ -1,13 +1,22 @@
 package com.thealgorithms.datastructures.stacks;
 
 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.Stack;
 import org.junit.jupiter.api.Test;
 
 class ReverseStackTest {
 
+    @Test
+void testReverseNullStack() {
+    assertThrows(IllegalArgumentException.class,
+        () -> ReverseStack.reverseStack(null),
+        "Reversing a null stack should throw an IllegalArgumentException.");
+}
+
+
     @Test
     void testReverseEmptyStack() {
         Stack<Integer> stack = new Stack<>();