java build tree

BuildTree.java

import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.Optional;
import java.util.StringJoiner;
import java.util.stream.Collectors;

/**
 * BuildTree.
 *
 * @version 0.0.1
 * @serial 2021-07-19 : base version.
 */
public class BuildTree {

    public static void main(String[] args) {
        List<Node> nodeList = new ArrayList<Node>() {{
            add(new Node(1, null));
            add(new Node(2, 1));
            add(new Node(3, 1));
            add(new Node(4, 2));
            add(new Node(5, 3));
            add(new Node(6, 5));
        }};
        Map<Integer, Node> temp = nodeList.stream().collect(Collectors.toMap(i -> i.id, i -> i));
        for (Node value : temp.values()) {
            if (temp.containsKey(value.pid)) {
                final Node parent = temp.get(value.pid);
                parent.children.add(value);
            }
        }
        final Optional<Node> first = temp.values().stream().filter(i -> i.pid == null).findFirst();
        System.out.println(first.orElse(null));
    }

    static class Node {

        Integer id;
        List<Node> children;
        Integer pid;

        public Node(Integer id, Integer pid) {
            this.id = id;
            this.pid = pid;
            children = new LinkedList<>();
        }


        @Override
        public String toString() {

            return new StringJoiner(", ", Node.class.getSimpleName() + "[", "]")
                    .add("id=" + id)
                    .add("children=" + children)
                    .add("pid=" + pid)
                    .toString();
        }
    }
}