add tree display

app.py

@@ -143,7 +143,9 @@ def display_tree(self):
     if self.left:
         self.left.display_tree()
     
-    print(f'value: {self.val}')
+    print(
+        f'value: {current_node}'
+    )
     
     if self.right:
         self.right.display_tree()