请把下面的对象数组变成对象嵌套。 规则：每个对象的parentId就是父级对象的id。

原数组：

[
  {id: 1, name: 'a', parentId: null},
  {id: 2, name: 'b', parentId: 1},
  {id: 3, name: 'c', parentId: 2},
  {id: 4, name: 'd', parentId: 1},
  {id: 5, name: 'e', parentId: 4},
  {id: 6, name: 'f', parentId: 2},
  {id: 7, name: 'g', parentId: null},
  {id: 8, name: 'h', parentId: 7},
  {id: 9, name: 'i', parentId: 8}
]

转换后的对象嵌套：

[
  {
    id: 1,
    name: 'a',
    parentId: null,
    children: [
      {
        id: 2,
        name: 'b',
        parentId: 1,
        children: [
          {
            id: 3,
            name: 'c',
            parentId: 2,
            children: []
          },
          {
            id: 6,
            name: 'f',
            parentId: 2,
            children: []
          }
        ]
      },
      {
        id: 4,
        name: 'd',
        parentId: 1,
        children: [
          {
            id: 5,
            name: 'e',
            parentId: 4,
            children: []
          }
        ]
      }
    ]
  },
  {
    id: 7,
    name: 'g',
    parentId: null,
    children: [
      {
        id: 8,
        name: 'h',
        parentId: 7,
        children: [
          {
            id: 9,
            name: 'i',
            parentId: 8,
            children: []
          }
        ]
      }
    ]
  }
]