如何将扁平化的数组转换成树形结构？

const arr = [  
  {id: 4, pid: 3},  
  {id: 'aa',pid:'a'},
  {id: 1, pid: null},  
  {id: 3, pid: 2},
  {id: 'a',pid: 'a0'},
  {id: 2, pid: 1},
  {id: 'a0',pid: null}  
];  

[
    {
        "id": 1,
        "pid": null,
        "children": [
            {
                "id": 2,
                "pid": 1,
                "children": [
                    {
                        "id": 3,
                        "pid": 2,
                        "children": [
                            {
                                "id": 4,
                                "pid": 3
                            }
                        ]
                    }
                ]
            }
        ]
    },
    {
        "id": "a0",
        "pid": null,
        "children": [
            {
                "id": "a",
                "pid": "a0",
                "children": [
                    {
                        "id": "aa",
                        "pid": "a"
                    }
                ]
            }
        ]
    }
]

arr.reduce((o, i) => {
    i = object.assign(o[i.id] ??= {}, i);
    ((o[i.pid ?? ''] ??= {}).children ??= []).push(i);
    return o;
}, {})['']?.children

[
  {
    "id": 1,
    "pid": null,
    "children": [
      {
        "id": 2,
        "pid": 1,
        "children": [
          {
            "id": 3,
            "pid": 2,
            "children": [
              {
                "id": 4,
                "pid": 3
              }
            ]
          }
        ]
      }
    ]
  },
  {
    "id": "a0",
    "pid": null,
    "children": [
      {
        "id": "a",
        "pid": "a0",
        "children": [
          {
            "id": "aa",
            "pid": "a"
          }
        ]
      }
    ]
  }
]