[1,null,2,3] represents the serialized format of a binary tree using level order traversal, where
null signifies a path terminator where no node exists below.
We provided a Tree Visualizer tool to help you visualize the binary tree while you are solving problems. By opening the console panel, you should see a Tree Visualizer toggle switch under the TestCase tab. Click on it and it will show the test case's binary tree representation.
The root is a reference to