第 88 题：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

[DreamLee1997]

题目：
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
            }
          ]
        }
      ]
    },
  ···
];

解答：时间复杂度O(n)

function convert(list) {
    let res = [];
    let map = list.reduce((res, v) => (res[v.id] = v, res),{});
    console.log(map);
    for(const item of list){
        if(item.parentId === 0){
            res.push(item);
            continue;
        }
        for(item.parentId in map){
            const parent = map[item.parentId];
            parent.children = parent.children || [];
            parent.children.push(item);
        }
    }
    return res;
}

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}
];
let result =convert(list);
console.log(result);