实现 convert 方法，把原始 list 转换成树形结构，要求尽可能降低时间复杂度

[guyuezhai]

以下数据结构中，id 代表部门编号，name 是部门名称，parentId 是父部门编号，为 0 代表一级部门，现在要求实现一个 convert 方法，把原始 list 转换成树形结构，parentId 为多少就挂载在该 id 的属性 children 数组下，结构如下：

// 原始 list 如下
let list =[
{id:1,name:'部门A',parentId:0},
{id:2,name:'部门B',parentId:0},
{id:3,name:'部门C',parentId:1},
{id:4,name:'部门D',parentId:1},
{id:5,name:'部门E',parentId:2},
{id:6,name:'部门F',parentId:3},
{id:7,name:'部门G',parentId:2},
{id:8,name:'部门H',parentId:4}
];
const result = convert(list, ...);

// 转换后的结果如下 let result = [ { id: 1, name: '部门A', parentId: 0, children: [ { id: 3, name: '部门C', parentId: 1, children: [ { id: 6, name: '部门F', parentId: 3 }, { id: 16, name: '部门L', parentId: 3 } ] }, { id: 4, name: '部门D', parentId: 1, children: [ { id: 8, name: '部门H', parentId: 4 } ] } ] }, ··· ];

[guyuezhai]

let list =[
    {id:1,name:'部门A',parentId:0},
    {id:2,name:'部门B',parentId:0},
    {id:3,name:'部门C',parentId:1},
    {id:4,name:'部门D',parentId:1},
    {id:5,name:'部门E',parentId:2},
    {id:6,name:'部门F',parentId:3},
    {id:7,name:'部门G',parentId:2},
    {id:8,name:'部门H',parentId:4}
];
function convert(list){
  const res=[];
  const map=list.reduce((res,v)=>(res[v.id]=v,res),{})
  for(const item of list){
    if(item.parentId==0){
      res.push(item)
      continue
    }
    if(item.parentId in map){
       const parent=map[item.parentId];
       parent.children=parent.children || [];
       parent.children.push(item)
    }
  }
  return res
}
const result = convert(list);
console.log(JSON.stringify(result));

[{"id":1,"name":"部门A","parentId":0,"children":[{"id":3,"name":"部门C","parentId":1,"children":[{"id":6,"name":"部门F","parentId":3}]},{"id":4,"name":"部门D","parentId":1,"children":[{"id":8,"name":"部门H","parentId":4}]}]},{"id":2,"name":"部门B","parentId":0,"children":[{"id":5,"name":"部门E","parentId":2},{"id":7,"name":"部门G","parentId":2}]}]