非科班出身，欢迎指正。

要实现的逻辑是，在一棵树中，给出任意一个节点，获取到该节点下的N个层级。

一.树型结构

下图的树中，节点上的字母代表节点的名字，字母下的代表该节点的下单金额。

二.数据准备

组装数据

package ATree;

import java.util.ArrayList;

import java.util.List;

public class PreData {

public static List getData(){

NodeOrder A = new NodeOrder("0", "A","-1","123");

NodeOrder B = new NodeOrder("1", "B","0","10");

NodeOrder C = new NodeOrder("2", "C","0","20");

NodeOrder D = new NodeOrder("3", "D","0","30");

NodeOrder E = new NodeOrder("4", "E","1","40");

NodeOrder F = new NodeOrder("5", "F","3","50");

NodeOrder G = new NodeOrder("6", "G","3","60");

NodeOrder H = new NodeOrder("7", "H","4","70");

NodeOrder I = new NodeOrder("8", "I","4","80");

NodeOrder J = new NodeOrder("9", "J","4","90");

NodeOrder K = new NodeOrder("10","K","6","100");

NodeOrder L = new NodeOrder("11","L","8","110");

NodeOrder M = new NodeOrder("12","M","10","120");

NodeOrder N = new NodeOrder("13","N","10","130");

NodeOrder O = new NodeOrder("14","O","12","140");

NodeOrder P = new NodeOrder("15","P","12","150");

NodeOrder Q = new NodeOrder("16","Q","13","160");

NodeOrder R = new NodeOrder("17","R","13","170");

NodeOrder S = new NodeOrder("18","S","13","180");

NodeOrder T = new NodeOrder("19","T","15","190");

NodeOrder U = new NodeOrder("20","U","15","200");

NodeOrder V = new NodeOrder("21","V","19","210");

NodeOrder W = new NodeOrder("22","W","19","220");

NodeOrder X = new NodeOrder("23","X","20","230");

NodeOrder Y = new NodeOrder("24","Y","20","240");

NodeOrder Z = new NodeOrder("25","Z","20","250");

List list = new ArrayList<>();

list.add(A);

list.add(B);

list.add(C);

list.add(D);

list.add(E);

list.add(F);

list.add(G);

list.add(H);

list.add(I);

list.add(J);

list.add(K);

list.add(L);

list.add(M);

list.add(N);

list.add(O);

list.add(P);

list.add(Q);

list.add(R);

list.add(S);

list.add(T);

list.add(U);

list.add(V);

list.add(W);

list.add(X);

list.add(Y);

list.add(Z);

return list;

}

}

package ATree;

import java.math.BigDecimal;

public class NodeOrder {

private String id;

private String tag;

private String parentId;

private BigDecimal orderAmount;

public NodeOrder(String id, String tag, String parentId, String orderAmount) {

this.id = id;

this.tag = tag;

this.parentId = parentId;

this.orderAmount = new BigDecimal(orderAmount);

}

public String getId() {

return id;

}

public void setId(String id) {

this.id = id;

}

public String getTag() {

return tag;

}

public void setTag(String tag) {

this.tag = tag;

}

public String getParentId() {

return parentId;

}

public void setParentId(String parentId) {

this.parentId = parentId;

}

public BigDecimal getOrderAmount() {

return orderAmount;

}

public void setOrderAmount(BigDecimal orderAmount) {

this.orderAmount = orderAmount;

}

}

编写逻辑

packageATree;importcom.alibaba.fastjson.JSON;importorg.apache.commons.collections.CollectionUtils;importjava.math.BigDecimal;importjava.util.ArrayList;importjava.util.HashMap;importjava.util.List;importjava.util.Map;importjava.util.stream.Collectors;public classTest {//计算层级的最大值

public static final int MAX_LEVEL = 3;public static voidmain(String[] args) {

Test test= newTest();

List list =PreData.getData();

test.start("10",list);

}public void start(String id, Listsources){

Map> parentMap =groupByParentId(sources);

Map> levelMap = new HashMap<>();

getLevel(id, parentMap, levelMap,1);

System.out.println(JSON.toJSONString(levelMap));

}public void getLevel(String parentId,Map> parentMap,Map> levelMap,intcount){//根据parentId获取节点

List nextLevel =parentMap.get(parentId);if(CollectionUtils.isEmpty(nextLevel))return;

String countStr=String.valueOf(count);

List thisLevel =levelMap.get(countStr);if(CollectionUtils.isEmpty(thisLevel)) {

levelMap.put(countStr, nextLevel);

}else{

thisLevel.addAll(nextLevel);

levelMap.put(countStr, thisLevel);

}

count++;if(count >MAX_LEVEL)return;for(NodeOrder nodeOrder : nextLevel) {

String tempParentId=nodeOrder.getId();

getLevel(tempParentId, parentMap, levelMap, count);

}

}//按父节点分组

public Map> groupByParentId(Listsources){

Map> listGroupby =sources.parallelStream().collect(Collectors.groupingBy(NodeOrder::getParentId));returnlistGroupby;

}

}

三.测试

按照图中的树形结构，以K节点举例，遍历得到K节点下的3个层级，输出的字符串为：

{"1":[{"id":"12","orderAmount":120,"parentId":"10","tag":"M"},{"id":"13","orderAmount":130,"parentId":"10","tag":"N"}],"2":[{"id":"14","orderAmount":140,"parentId":"12","tag":"O"},{"id":"15","orderAmount":150,"parentId":"12","tag":"P"},{"id":"16","orderAmount":160,"parentId":"13","tag":"Q"},{"id":"17","orderAmount":170,"parentId":"13","tag":"R"},{"id":"18","orderAmount":180,"parentId":"13","tag":"S"}],"3":[{"id":"19","orderAmount":190,"parentId":"15","tag":"T"},{"id":"20","orderAmount":200,"parentId":"15","tag":"U"}]}

JSON结构为：

1 {2 "1": [3 {4 "id": "12",5 "orderAmount": 120,6 "parentId": "10",7 "tag": "M"

8 },9 {10 "id": "13",11 "orderAmount": 130,12 "parentId": "10",13 "tag": "N"

14 }15 ],16 "2": [17 {18 "id": "14",19 "orderAmount": 140,20 "parentId": "12",21 "tag": "O"

22 },23 {24 "id": "15",25 "orderAmount": 150,26 "parentId": "12",27 "tag": "P"

28 },29 {30 "id": "16",31 "orderAmount": 160,32 "parentId": "13",33 "tag": "Q"

34 },35 {36 "id": "17",37 "orderAmount": 170,38 "parentId": "13",39 "tag": "R"

40 },41 {42 "id": "18",43 "orderAmount": 180,44 "parentId": "13",45 "tag": "S"

46 }47 ],48 "3": [49 {50 "id": "19",51 "orderAmount": 190,52 "parentId": "15",53 "tag": "T"

54 },55 {56 "id": "20",57 "orderAmount": 200,58 "parentId": "15",59 "tag": "U"

60 }61 ]62 }