Java用递归之外的办法组装树形结构
1.Java代码：
package com.hello.test;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
public class ArrToTree {
public static void main(String[] args){
//设置初始数据(对顺序没有要求)
List<Node> list = new ArrayList<Node>();
list.add(new Node(1L, null));//pid可以是null也可以是-1等不存在的id
list.add(new Node(2L, 1L));
list.add(new Node(3L, 1L));
list.add(new Node(4L, 2L));
list.add(new Node(5L, 2L));
list.add(new Node(6L, 3L));
//创建JSON处理器
GsonBuilder gsonbuilder = new GsonBuilder().serializeNulls();
Gson gson = gsonbuilder.create();
Node root = buildTree(list, gson);
System.out.println("tree: "+ gson.toJson(root));
}
static Node buildTree(List<Node> list, Gson gson){
Map<Long, Node> map = new HashMap<Long, Node>();
for(Node node : list){
//把所有节点放进map中， id作为key
map.put(node.getId(), node);
}
System.out.println("map1: "+ gson.toJson(map));
//将节点加入对应父节点的children中
//因为每个节点的父节点有且只有一个
for(Entry<Long, Node> entry : map.entrySet()){
Node currNode = entry.getValue();
Node parentNode = map.get(currNode.getPid());
if(parentNode != null){
parentNode.addToChildren(currNode);
}
}
System.out.println("map2: "+ gson.toJson(map));
return map.get(1L);
}
//节点定义
static class Node{
private Long id;//id
private Long pid;//父id
private List<Node> children;//子节点集合
public Node(){}
public Node(Long id, Long pid){
this.id = id;
this.pid = pid;
}
public Long getId(){
return this.id;
}
public Long getPid(){
return this.pid;
}
public void addToChildren(Node node){
if(this.children==null){
this.children = new ArrayList<Node>();
}
this.children.add(node);
}
}
}
2.输出结果：
map1：
{
"1":{
"id":1,
"pid":null,
"children":null
},
"2":{
"id":2,
"pid":1,
"children":null
},
"3":{
"id":3,
"pid":1,
"children":null
},
"4":{
"id":4,
"pid":2,
"children":null
},
"5":{
"id":5,
"pid":2,
"children":null
},
"6":{
"id":6,
"pid":3,
"children":null
}
}
map2：
{
"1":{
"id":1,
"pid":null,
"children":[
{
"id":2,
"pid":1,
"children":[
{
"id":4,
"pid":2,
"children":null
},
{
"id":5,
"pid":2,
"children":null
}
]
},
{
"id":3,
"pid":1,
"children":[
{
"id":6,
"pid":3,
"children":null
}
]
}
]
},
"2":{
"id":2,
"pid":1,
"children":[
{
"id":4,
"pid":2,
"children":null
},
{
"id":5,
"pid":2,
"children":null
}
]
},
"3":{
"id":3,
"pid":1,
"children":[
{
"id":6,
"pid":3,
"children":null
}
]
},
"4":{
"id":4,
"pid":2,
"children":null
},
"5":{
"id":5,
"pid":2,
"children":null
},
"6":{
"id":6,
"pid":3,
"children":null
}
}
tree：
{
"id":1,
"pid":null,
"children":[
{
"id":2,
"pid":1,
"children":[
{
"id":4,
"pid":2,
"children":null
},
{
"id":5,
"pid":2,
"children":null
}
]
},
{
"id":3,
"pid":1,
"children":[
{
"id":6,
"pid":3,
"children":null
}
]
}
]
}
3.另一种写法（单次遍历）
package com.hello.test;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
public class ArrToTreeV2 {
public static void main(String[] args){
//设置初始数据(对顺序没有要求)
List<Item> list = new ArrayList<Item>();
list.add(new Item(6L, 3L));
list.add(new Item(5L, 2L));
list.add(new Item(4L, 2L));
list.add(new Item(3L, 1L));
list.add(new Item(2L, 1L));
list.add(new Item(1L, null));//pid可以是null也可以是-1等不存在的id
//创建JSON处理器
GsonBuilder gsonbuilder = new GsonBuilder().serializeNulls();
Gson gson = gsonbuilder.create();
Node root = list2tree(list);
System.out.println(gson.toJson(root));
}
static Node list2tree(List<Item> list){
if(list==null || list.isEmpty()){
return null;
}
Map<Long, Node> map = new HashMap<Long, Node>();
for(Item item : list){
//1.加入当前节点
Long id = item.getId();
Node currNode = map.get(id);
if(currNode == null){
currNode = new Node(item);
map.put(id, currNode);
}else{//被别的节点以父节点加入
currNode.setByItem(item);
}
//2.把当前节点加入父节点的children
Long pid = item.getPid();
Node parentNode = map.get(pid);
if(parentNode == null){//父节点不存在则临时创建一个
parentNode = new Node(pid);
parentNode.addToChildren(currNode);
map.put(pid, parentNode);
}else{//已存在父节点，直接加入他的children
parentNode.addToChildren(currNode);
}
}
return map.get(null);
}
static class Item{
private Long id;//id
private Long pid;//父id
//other fields...
public Item(){}
public Item(Long id, Long pid){
this.id = id;
this.pid = pid;
}
public void setId(Long id) {
this.id = id;
}
public void setPid(Long pid) {
this.pid = pid;
}
public Long getId() {
return id;
}
public Long getPid() {
return pid;
}
}
//节点定义
static class Node{
private Long id;//id
private Long pid;//父id
private List<Node> children;//子节点集合
public Node(){}
public Node(Item item){
this.id = item.getId();
this.pid = item.getPid();
}
public Node(Long id){
this.id = id;
}
public void setByItem(Item item) {
//在这里复制id，children以外的所有属性值
this.pid = item.getPid();
}
public Long getId(){
return this.id;
}
public Long getPid(){
return this.pid;
}
public void addToChildren(Node node){
if(this.children==null){
this.children = new ArrayList<Node>();
}
this.children.add(node);
}
}
}
输出结果：
{
"id":null,
"pid":null,
"children":[
{
"id":1,
"pid":null,
"children":[
{
"id":3,
"pid":1,
"children":[
{
"id":6,
"pid":3,
"children":null
}
]
},
{
"id":2,
"pid":1,
"children":[
{
"id":5,
"pid":2,
"children":null
},
{
"id":4,
"pid":2,
"children":null
}
]
}
]
}
]
}