Enhance docs, remove main, add tests in ReverseStack (#6018)

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

@@ -0,0 +1,70 @@
+package com.thealgorithms.datastructures.stacks;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.util.Stack;
+import org.junit.jupiter.api.Test;
+
+class ReverseStackTest {
+
+    @Test
+    void testReverseEmptyStack() {
+        Stack<Integer> stack = new Stack<>();
+        ReverseStack.reverseStack(stack);
+        assertTrue(stack.isEmpty(), "Reversing an empty stack should result in an empty stack.");
+    }
+
+    @Test
+    void testReverseSingleElementStack() {
+        Stack<Integer> stack = new Stack<>();
+        stack.push(1);
+        ReverseStack.reverseStack(stack);
+        assertEquals(1, stack.peek(), "Reversing a single-element stack should have the same element on top.");
+    }
+
+    @Test
+    void testReverseTwoElementStack() {
+        Stack<Integer> stack = new Stack<>();
+        stack.push(1);
+        stack.push(2);
+        ReverseStack.reverseStack(stack);
+
+        assertEquals(1, stack.pop(), "After reversal, the stack's top element should be the first inserted element.");
+        assertEquals(2, stack.pop(), "After reversal, the stack's next element should be the second inserted element.");
+    }
+
+    @Test
+    void testReverseMultipleElementsStack() {
+        Stack<Integer> stack = new Stack<>();
+        stack.push(1);
+        stack.push(2);
+        stack.push(3);
+        stack.push(4);
+
+        ReverseStack.reverseStack(stack);
+
+        assertEquals(1, stack.pop(), "Stack order after reversal should match the initial push order.");
+        assertEquals(2, stack.pop());
+        assertEquals(3, stack.pop());
+        assertEquals(4, stack.pop());
+    }
+
+    @Test
+    void testReverseStackAndVerifySize() {
+        Stack<Integer> stack = new Stack<>();
+        stack.push(10);
+        stack.push(20);
+        stack.push(30);
+        stack.push(40);
+        int originalSize = stack.size();
+
+        ReverseStack.reverseStack(stack);
+
+        assertEquals(originalSize, stack.size(), "Stack size should remain unchanged after reversal.");
+        assertEquals(10, stack.pop(), "Reversal should place the first inserted element on top.");
+        assertEquals(20, stack.pop());
+        assertEquals(30, stack.pop());
+        assertEquals(40, stack.pop());
+    }
+}