JS converts from flat array to tree

Sometimes we need to construct a tree array structure from a flat array structure (flat array) based on id and pid. This situation often occurs in nested directory structures, as shown below:

Or the departmental structure of the enterprise will also have a similar structure above.

Similar to the above scenario, if we have the following data:

 let entries = [{
    "id": "12",
    "parentId": "0",
    "text": "Man",
    "level": "1",
    "children": null
  },
  {
    "id": "7",
    "parentId": "12",
    "text": "Other",
    "level": "2",
    "children": null
  },
  {
    "id": "8",
    "parentId": "7",
    "text": "Bird",
    "level": "3",
    "children": null
  },
  {
    "id": "9",
    "parentId": "0",
    "text": "Woman",
    "level": "1",
    "children": null
  },
  {
    "id": "11",
    "parentId": "9",
    "text": "Girl",
    "level": "2",
    "children": null
  }
];

We expect to construct the following structure through an algorithm:

 [
   {
      "id": "12",
      "parentId": "0",
      "text": "Man",
      "level": "1",
      "children": [
         {
            "id": "7",
            "parentId": "12",
            "text": "Other",
            "level": "2",
            "children": [
               {
                  "id": "8",
                  "parentId": "7",
                  "text": "Bird",
                  "level": "3",
                  "children": []
               }
            ]
         }
      ]
   },
   {
      "id": "9",
      "parentId": "0",
      "text": "Woman",
      "level": "1",
      "children": [
         {
            "id": "11",
            "parentId": "9",
            "text": "Girl",
            "level": "2",
            "children": []
         }
      ]
   }
]

Maybe we can think of using recursion when traversing the array. It's true that the problem can be solved, but it's not efficient this way.

Recently, I saw a method on Stack Overflow, I felt good, posted it and shared it:

 function list_to_tree(list) {
    var map = {}, node, roots = [], i;
    
    for (i = 0; i < list.length; i += 1) {
      map[list[i].id] = i;      // initialize the map
      list[i].children = [];    // initialize the children
    }
    
    for (i = 0; i < list.length; i += 1) {
      node = list[i];
      if (node.parentId !== "0") {
        // if you have dangling branches check that map[node.parentId] exists
        list[map[node.parentId]].children.push(node);
      } else {
        roots.push(node);
      }
    }
    return roots;
}

The complexity of the algorithm is Θ(n log(n)).

If recursion is used, the complexity is Θ(n^2). If the data set is relatively large, the execution time will be very long.