java list 转树 tree 的三种写法

package com.lzk;

import java.util.List;

public class TreeNode {
    private String id;

    private String name;

    private String pid;

    private List<TreeNode> childrens;

    public TreeNode(String id,String name,String pid){
        this.id = id;
        this.name = name;
        this.pid = pid;
    }

    public String getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

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

    public String getPid() {
        return pid;
    }

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

    public List<TreeNode> getChildrens() {
        return childrens;
    }

    public void setChildrens(List<TreeNode> childrens) {
        this.childrens = childrens;
    }

    @Override
    public String toString() {
        return "TreeNode{" +
                "id='" + id + '\'' +
                ", name='" + name + '\'' +
                ", pid='" + pid + '\'' +
                ", childrens=" + childrens +
                '}';
    }
}

package com.lzk;

import java.util.ArrayList;
import java.util.List;

public class TreeBuild {

    /**
     * 方式一：使用两层循环实现
     * @param treeList
     * @return
     */
    public static List<TreeNode> toTree01(List<TreeNode> treeList,String pid) {
        List<TreeNode> retList = new ArrayList<TreeNode>();
        for (TreeNode parent : treeList) {
            if (pid.equals(parent.getPid())) {
                retList.add(parent);
            }
            for (TreeNode child : treeList) {
                if (child.getPid() == parent.getId()) {
                    if (parent.getChildrens() == null) {
                        parent.setChildrens(new ArrayList<TreeNode>());
                    }
                    parent.getChildrens().add(child);
                }
            }
        }
        return retList;
    }

    /**
     * 方式二：使用递归实现
     * @param treeList
     * @return
    **/
    public static List<TreeNode> toTree02(List<TreeNode> treeList,String pid) {
        List<TreeNode> retList = new ArrayList<TreeNode>();
        for (TreeNode parent : treeList) {
            if (pid.equals(parent.getPid())) {
                retList.add(findChildren(parent, treeList));
            }
        }
        return retList;
    }

    /*
    * 查询父节点下的所有子节点
    * */
    private static TreeNode findChildren(TreeNode parent, List<TreeNode> treeList) {
        for (TreeNode child : treeList) {
            if (parent.getId().equals(child.getPid())) {
                if (parent.getChildrens() == null) {
                    parent.setChildrens(new ArrayList<TreeNode>());
                }
                parent.getChildrens().add(findChildren(child, treeList));
            }
        }
        return parent;
    }

    /**
     *方法三
     * @param list
     * @return
     */
    public static List<TreeNode> toTree03(List<TreeNode> list,String pid) {
        List<TreeNode> treeList = new ArrayList<TreeNode>();
        for (TreeNode tree : list) {
            if(tree.getPid().equals(pid)){
                treeList.add(tree);
            }
        }
        for (TreeNode tree : list) {
            toTreeChildren(treeList,tree);
        }
        return treeList;
    }

    public static void toTreeChildren(List<TreeNode> treeList, TreeNode tree) {
        for (TreeNode node : treeList) {
            if(tree.getPid() == node.getId()){
                if(node.getChildrens() == null){
                    node.setChildrens(new ArrayList<TreeNode>());
                }
                node.getChildrens().add(tree);
            }
            if(node.getChildrens() != null){
                toTreeChildren(node.getChildrens(),tree);
            }
        }
    }
}

package com.lzk;

import java.util.ArrayList;
import java.util.List;

public class test {
    public static void main(String[] args) {
        List<TreeNode> list = new ArrayList<TreeNode>();
        list.add(new TreeNode("1", "顶层节点1", "0"));
        list.add(new TreeNode("2", "顶层节点2", "0"));
        list.add(new TreeNode("3", "顶层节点3", "0"));
        list.add(new TreeNode("4", "二级节点4", "1"));
        list.add(new TreeNode("5", "二级节点5", "2"));
        list.add(new TreeNode("6", "二级节点6", "3"));
        list.add(new TreeNode("7", "三级节点7", "4"));
        list.add(new TreeNode("8", "三级节点8", "4"));
        list.add(new TreeNode("9", "三级节点9", "5"));
//        List<TreeNode> list1 = TreeBuild.toTree02(list,"0");
//        System.out.println(list1);
        List<TreeNode> list2 = TreeBuild.toTree03(list,"0");
        System.out.println(list2);
    }
}