Java树节点遍历变成迭代器

public class Node {
    final Map<String, Node> valueMap = new HashMap<>();
}
public Stream<Node> stream(Order order) {
        //valueMap is children
        Stream<Node> childrenStream = this.valueMap.values().stream().map(i -> i.stream(order)).reduce(Stream::concat).orElse(Stream.empty());
        //this is parent
        switch (order) {
            case PRE:
                return Stream.concat(Collections.singleton(this).stream(), childrenStream);
            case SUFFIX:
                return Stream.concat(childrenStream, Collections.singleton(this).stream());
            default:
                throw new UnsupportedOperationException();
        }
    }

因为children没有区分左右，是多个的情况，所以没有中缀遍历，可以自行根据情况实现

   public Stream<Node> stream(Order order){
      return   stream((i)->true,order);
    }
    public Stream<Node> stream(Predicate<Node> predicate,Order order) {
        //valueMap is children
        Stream<Node> childrenStream = this.valueMap.values().stream().filter(predicate).map(i -> i.stream(predicate,order)).reduce(Stream::concat).orElse(Stream.empty());
        //this is parent
        if (predicate.test(this)){
            switch (order) {
                case PREFIX:
                    return Stream.concat(Collections.singleton(this).stream(), childrenStream);
                case SUFFIX:
                    return Stream.concat(childrenStream, Collections.singleton(this).stream());
                default:
                    throw new UnsupportedOperationException();
            }
        }else {
            return childrenStream;
        }
    }
//过滤特定节点,以及后代