static <T> Stream<T> inOrder(Tree<T> tree) {
if (tree.isLeaf()) {
return Stream.of(tree.getValue());
} else {
final List<Node<T>> children = tree.getChildren();
return children
.tail()
.foldLeft(Stream.<T>empty(), (acc, child) -> acc.appendAll(inOrder(child)))
.prepend(tree.getValue())
.prependAll(inOrder(children.head()));
}
}
private static String toLispString(Tree<?> tree) {
final String value = String.valueOf(tree.getValue());
if (tree.isLeaf()) {
return value;
} else {
return String.format(
"(%s %s)", value, tree.getChildren().map(Node::toLispString).mkString(" "));
}
}
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();
}
@SuppressWarnings("unchecked")
static <T, U> Tree<U> apply(
Node<T> node, Function<? super T, ? extends Iterable<? extends U>> mapper) {
final Tree<U> mapped = Tree.ofAll(mapper.apply(node.getValue()));
if (mapped.isEmpty()) {
return Tree.empty();
} else {
final List<Node<U>> children =
(List<Node<U>>)
(Object)
node.getChildren()
.map(child -> FlatMap.apply(child, mapper))
.filter(Tree::isDefined);
return Tree.of(mapped.getValue(), children.prependAll(mapped.getChildren()));
}
}
static <T> Stream<T> postOrder(Tree<T> tree) {
return tree.getChildren()
.foldLeft(Stream.<T>empty(), (acc, child) -> acc.appendAll(postOrder(child)))
.append(tree.getValue());
}
static <T> Stream<T> preOrder(Tree<T> tree) {
return tree.getChildren()
.foldLeft(Stream.of(tree.getValue()), (acc, child) -> acc.appendAll(preOrder(child)));
}