visit method
Applies the function f
to all child nodes rooted from this node in
breadth first order.
Implementation
void visit(void Function(TreeNode<T> node) f) {
final queue = Queue<TreeNode<T>>()..add(this);
while (queue.isNotEmpty) {
final node = queue.removeFirst();
f(node);
queue.addAll(node.children);
}
}