itemsTree method
Gets a tree structure (as a nested list) corresponding to the structure of the items and nodes in this tree.
The returned {@link List}s contain either {@link Object} items, or Lists which correspond to subtrees of the tree Subtrees which do not contain any items are not included.
Builds the tree if necessary.
@return a List of items and/or Lists
Implementation
List itemsTree() {
build();
List? valuesTree = itemsTreeWithNode(root);
if (valuesTree == null) return [];
return valuesTree;
}