static <T> Stream<T> levelOrder(Tree<T> tree) {
Stream<T> result = Stream.empty();
final java.util.Queue<Tree<T>> queue = new java.util.LinkedList<>();
queue.add(tree);
while (!queue.isEmpty()) {
final Tree<T> next = queue.remove();
result = result.prepend(next.getValue());
queue.addAll(next.getChildren().toJavaList());
}
return result.reverse();
}
