BAEL-4864: Fix traverseInOrderWithoutRecursion method (#10588)

Co-authored-by: Krzysztof Woyke <krzysztof.woyke.sp@lhsystems.com>

data-structures/src/test/java/com/baeldung/tree/BinaryTreeUnitTest.java

@@ -13,7 +13,7 @@ public class BinaryTreeUnitTest {
 
         BinaryTree bt = createBinaryTree();
 
-        assertTrue(!bt.isEmpty());
+        assertFalse(bt.isEmpty());
     }
 
     @Test
@@ -72,6 +72,7 @@ public class BinaryTreeUnitTest {
 
     @Test
     public void it_deletes_the_root() {
+
         int value = 12;
         BinaryTree bt = new BinaryTree();
         bt.add(value);
@@ -91,6 +92,14 @@ public class BinaryTreeUnitTest {
         bt.traverseInOrderWithoutRecursion();
     }
 
+    @Test
+    public void givenAnEmptyBinaryTree_WhenTraversingInOrderWithoutRecursion_ThenNoException() {
+
+        BinaryTree empty = new BinaryTree();
+
+        empty.traverseInOrderWithoutRecursion();
+    }
+
     @Test
     public void givenABinaryTree_WhenTraversingPreOrder_ThenPrintValues() {
 
@@ -101,6 +110,14 @@ public class BinaryTreeUnitTest {
         bt.traversePreOrderWithoutRecursion();
     }
 
+    @Test
+    public void givenAnEmptyBinaryTree_WhenTraversingPreOrderWithoutRecursion_ThenNoException() {
+
+        BinaryTree empty = new BinaryTree();
+
+        empty.traversePreOrderWithoutRecursion();
+    }
+
     @Test
     public void givenABinaryTree_WhenTraversingPostOrder_ThenPrintValues() {
 
@@ -111,6 +128,14 @@ public class BinaryTreeUnitTest {
         bt.traversePostOrderWithoutRecursion();
     }
 
+    @Test
+    public void givenAnEmptyBinaryTree_WhenTraversingPostOrderWithoutRecursion_ThenNoException() {
+
+        BinaryTree empty = new BinaryTree();
+
+        empty.traversePostOrderWithoutRecursion();
+    }
+
     @Test
     public void givenABinaryTree_WhenTraversingLevelOrder_ThenPrintValues() {