If left is empty and the parent node is not empty, the bookmark will move to the first child node of the parent node.
If parent is empty and left is not empty, the bookmark moves to the front of the left node.
-- If both left and parent are empty, the bookmark moves to the first child node under the root node.
If both left and parent are not empty, then left has the highest priority, parent is invalid, and the bookmark is moved to the front of the left node.
Add a bookmark node.