结果以json格式输出，可以用json在线解析，方便查看
package com.xintone.demo;import cn.hutool.json.JSONUtil;import lombok.Data;import org.springframework.util.CollectionUtils;import java.util.ArrayList;import java.util.List;import java.util.stream.Collectors;@Datapublic class Recursion { // 主键id private Integer id; // 父级id private Integer parentId; // 子集 private List children; // 层级 private Integer level; public Recursion(Integer id, Integer parentId) { this.id = id; this.parentId = parentId; } public static void main(String[] args) { // 获取测试数据 List recursions1 = getList(); // 获取顶级父集 List parents = recursions1.stream().filter(item -> item.getParentId().equals(0)).collect(Collectors.toList()); // 设置层级 parents.forEach(item -> item.setLevel(0)); // 向下递归 downwardRecursion(parents, recursions1); System.out.println("向下递归：" + JSONUtil.toJsonStr(parents)); System.out.println("-------------分割线-------------"); // 获取测试数据 List recursions2 = getList(); // 获取测试数据中所有的 parentId List parentIds = recursions2.stream().map(Recursion::getParentId).collect(Collectors.toList()); // 判断 id 是否在 parentIds 中，不在则是最子级 List children = recursions2.stream().filter(item -> !parentIds.contains(item.getId())).collect(Collectors.toList()); // 向上递归 upwardRecursion(children, recursions2); // 递归完从测试数据中筛选出最顶级 List tree = recursions2.stream().filter(item -> item.getParentId().equals(0)).collect(Collectors.toList()); System.out.println("向上递归：" + JSONUtil.toJsonStr(tree)); } private static void upwardRecursion(List children, List all) { // 遍历子集 children.forEach(child -> { // 获取该子级的父级 Recursion parent = all.stream().filter(item -> child.getParentId().equals(item.getId())).findFirst().orElse(null); // 判断父级是否为空，如果为空则是最顶级 if (parent != null) { // 判断父级的子集是否为空 if (parent.getChildren() == null) { // 新建一个集合将子级添加到父级的子集中 parent.setChildren(new ArrayList() {{ add(child); }}); // 判断该子级是否在父级的子集中存在 // 因为在遍历子级时，会出现多个子级的父级相同，如果不加判断会导致数据重复 } else if (!parent.getChildren().contains(child)) { // 将该子级添加到父级的子集中 parent.getChildren().add(child); } // 将父级添加到集合并向上递归 upwardRecursion(new ArrayList() {{ add(parent); }}, all); child.setLevel(parent.getLevel() + 1); } else { child.setLevel(0); } }); } private static void downwardRecursion(List parents, List all) { // 遍历父集 parents.forEach(recursion -> { // 获取该父级的子集 List children = all.stream().filter(item -> item.getParentId().equals(recursion.getId())).collect(Collectors.toList()); // 判断子集是否为空 if (!CollectionUtils.isEmpty(children)) { // 设置子集的层级 children.forEach(item -> item.setLevel(recursion.getLevel() + 1)); // 将子集添加到该父级的子集去 recursion.setChildren(children); // 向下递归 downwardRecursion(children, all); } }); } private static List getList() { List list = new ArrayList(); list.add(new Recursion(1, 0)); list.add(new Recursion(2, 0)); list.add(new Recursion(3, 0)); list.add(new Recursion(4, 1)); list.add(new Recursion(5, 1)); list.add(new Recursion(6, 2)); list.add(new Recursion(7, 2)); list.add(new Recursion(8, 3)); list.add(new Recursion(9, 3)); list.add(new Recursion(10, 4)); list.add(new Recursion(11, 4)); list.add(new Recursion(12, 5)); list.add(new Recursion(13, 5)); list.add(new Recursion(14, 6)); list.add(new Recursion(15, 6)); list.add(new Recursion(16, 7)); list.add(new Recursion(17, 7)); list.add(new Recursion(18, 8)); list.add(new Recursion(19, 8)); list.add(new Recursion(20, 9)); list.add(new Recursion(21, 9)); return list; }}