Check the `left` property in order to understand if a node is root or not:
Check the `left` property in order to understand if a node is root or not:

In order to be able to use `id` (and so `parentId`) as string I would like
to check the `left` property to discover if a node is root, because 0 ==
'an-id-string' is true in php.
Also the parentId can be null.

bartko-s/stefano-tree