不用递归实现List转Tree

缘由：能不递归就不递归

代码实现Demo

import com.alibaba.fastjson.JSON;
import org.springframework.util.CollectionUtils;

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class ListToTreeDemo {

    private static class Node {
        private int id;
        private int pid;
        private String name;
        private List<Node> child;

        public Node(int id, int pid) {
            this.id = id;
            this.pid = pid;
            this.name = "测试:"+ pid + ":" + id;
        }

        public int getId() {
            return id;
        }

        public void setId(int id) {
            this.id = id;
        }

        public int getPid() {
            return pid;
        }

        public void setPid(int pid) {
            this.pid = pid;
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        public List<Node> getChild() {
            return child;
        }

        public void setChild(List<Node> child) {
            this.child = child;
        }
    }

    public static void main(String[] args) {

        List<Node> nodes = new ArrayList<>();
        nodes.add(new Node(1, 0));
        nodes.add(new Node(2, 0));
        nodes.add(new Node(3, 0));
        nodes.add(new Node(4, 0));
        nodes.add(new Node(5, 1));
        nodes.add(new Node(6, 1));
        nodes.add(new Node(7, 2));
        nodes.add(new Node(8, 5));
        Map<Integer, List<Node>> collect = nodes.stream().collect(Collectors.groupingBy(Node::getPid));

        nodes.forEach((item) -> {
            if (CollectionUtils.isEmpty(collect.get(item.getId()))) {
                item.setChild(new ArrayList<>());
            } else {
                item.setChild(collect.get(item.getId()));
            }
        });
        List<Node> list = nodes
                .stream()
                .filter((item) -> item.getPid() == 0)
                .collect(Collectors.toList());

        System.out.println(JSON.toJSONString(list));
    }
}