How to Efficiently Convert an Array List into a Hierarchical Tree Structure?

Creating Array Trees from Array Lists

In situations where you have a list of elements with hierarchical relationships, converting it into a tree-like structure can be crucial. This transformation is particularly useful when dealing with complex data that requires efficient organization and retrieval.

Consider the following list as an example:

[
  {id: 100, parentId: 0, name: 'a'},
  {id: 101, parentId: 100, name: 'a'},
  {id: 102, parentId: 101, name: 'a'},
  {id: 103, parentId: 101, name: 'a'},
]

The goal is to reorganize this list into a hierarchical tree structure, where each element has a reference to its parent and its children (if any).

One effective approach to accomplish this conversion is demonstrated by the following code snippet:

<?php

$arr = [
    ['id' => 100, 'parentId' => 0, 'name' => 'a'],
    ['id' => 101, 'parentId' => 100, 'name' => 'a'],
    ['id' => 102, 'parentId' => 101, 'name' => 'a'],
    ['id' => 103, 'parentId' => 101, 'name' => 'a'],
];

$new = [];
foreach ($arr as $a) {
    $new[$a['parentId']][] = $a;
}
$tree = createTree($new, [$arr[0]]);
print_r($tree);

function createTree(&$list, $parent) {
    $tree = [];
    foreach ($parent as $k => $l) {
        if (isset($list[$l['id']])) {
            $l['children'] = createTree($list, $list[$l['id']]);
        }
        $tree[] = $l;
    }
    return $tree;
}

This code efficiently processes the input list by grouping elements based on their parentId. It then recursively builds the tree structure, populating each element with its children. The resulting tree provides a hierarchical representation of the original list, making it easier to navigate and retrieve data based on parent-child relationships.

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