递归树实现 Java

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原创 2024-07-02 08:55:14 685 阅读 AI 写同款· GEO 优化 ›

前言

        不管是传统项目还是互联网电商项目，都会用到递归调用，比如传统的部门  等级，或者说商品的菜单目录，递归是循环调用，
    自己调用自己，两个问题比较明显，一个是效率问题，一个是一不小心就会导致OOM(Out Of  Memory)

思路

      递归分两种，一个是内存递归，一个是数据库递归，当然练习时，可以两种都写写比较一下，但是不用多说内存的效率那是没得说，
   接下来咱们就用代码实现一个递归

代码

/**
 * 实体类
 * @Classname Recursion
 * @Created by windBird
 */
public class Recursion implements Serializable {

    private static final long serialVersionUID = -8285127290179079661L;
  
    private   Integer  id; // 主键

    private  Integer pid;  //父id  与主键关联

    private  Integer icon; //递归对应的图标

    private Integer seq; //递归对应的顺序

    private Integer level;//递归的等级

    private String  title;  //显示的内容

    private List<Recursion> childrenList; //子类

    public Integer getId() {
        return id;
    }

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

    public Integer getPid() {
        return pid;
    }

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

    public Integer getIcon() {
        return icon;
    }

    public void setIcon(Integer icon) {
        this.icon = icon;
    }

    public Integer getSeq() {
        return seq;
    }

    public void setSeq(Integer seq) {
        this.seq = seq;
    }

    public Integer getLevel() {
        return level;
    }

    public void setLevel(Integer level) {
        this.level = level;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public List<Recursion> getChildrenList() {
        return childrenList;
    }

    public void setChildrenList(List<Recursion> childrenList) {
        this.childrenList = childrenList;
    }
}

统计递归关联次数
select 
t.id,
t.pid,t.name,
sum(case when t.id =n.name_id then 1 else 0 end) as total   from tb_name t 
left join tb_name_shop n on t.id =n.name_id
group by t.id
################ 具体书写##################
/**
 * @Description  具体实现
 * @Date 2020/11/28 11:02
 * @Created by windBird
 */
public class RecursionPractise {
    public static void main(String[] args) {
        List<Recursion> resultData = new ArrayList<>(); //最终返回的结果
        List<Recursion> recursions = new ArrayList<>(); //递归的单个对象
        getData(recursions);
        //递归树逻辑编写 
        resultData = recursions.stream().filter(recursion -> recursion.getPid() == -1)
                .map(recursion -> covertRecursionCode(recursion, recursions)).collect(Collectors.toList());

        resultData.forEach(r-> System.out.println("r = " + r));
    }

    private static Recursion covertRecursionCode(Recursion recursion, List<Recursion> recursions) {
        Recursion node = new Recursion();
        BeanUtils.copyProperties(recursion,node);
        List<Recursion> childNode = recursions.stream().filter(childrenRecursion -> childrenRecursion.getPid() == recursion.getId())
                .map(childrenRecursion -> covertRecursionCode(childrenRecursion, recursions)).collect(Collectors.toList());
        node.setChildrenList(childNode);
        return node;
    };

    public static void getData(List<Recursion> recursions) {
        //查询数据库获取数据 这里自己构造
        Recursion recursion = new Recursion();
        recursion.setId(1);
        recursion.setPid(-1);
        recursion.setIcon(1);
        recursion.setTitle("递归树顶层");
        recursion.setLevel(1);
        recursion.setSeq(0);
        recursions.add(recursion);
        Recursion recursion1 = new Recursion();
        recursion1.setId(2);
        recursion1.setPid(1);
        recursion1.setIcon(2);
        recursion1.setTitle("递归树次顶层");
        recursion1.setLevel(2);
        recursion1.setSeq(1);
        recursions.add(recursion1);
        Recursion recursion2 = new Recursion();
        recursion2.setId(3);
        recursion2.setPid(1);
        recursion2.setIcon(1);
        recursion2.setTitle("递归树次顶层");
        recursion2.setLevel(2);
        recursion2.setSeq(2);
        recursions.add(recursion2);

        Recursion recursion3 = new Recursion();
        recursion3.setId(4);
        recursion3.setPid(2);
        recursion3.setIcon(1);
        recursion3.setTitle("递归树次次顶层");
        recursion3.setLevel(3);
        recursion3.setSeq(0);
        recursions.add(recursion3);
    }
}

### 树转集合
 private static List<Recursion> treeToList(List<Recursion> messageList) {
        List<Recursion> result = new ArrayList<>();
        messageList.forEach(m->{
            result.add(m);
            List<Recursion> childrenList = m.getChildrenList();
            if(!CollectionUtils.isEmpty(childrenList)){
                List<Recursion> childrenListAgain = treeToList(childrenList);
                result.addAll(childrenListAgain);
            }
        });
        if(result.size() > 0) result.forEach(r-> r.setChildrenList(null));
        return result;
    }