フラット配列リストからネストされた配列ツリーを効率的に作成するには?

# Create a new array indexed by parent ID
$new = [];
foreach ($arr as $a) {
    $new[$a['parentid']][] = $a;
}

# Start with the root node
$tree = createTree($new, [$arr[0]]);

# Recursive function to build the tree
function createTree(&$list, $parent) {
    $tree = [];
    foreach ($parent as $l) {
        # If there are children, create children tree
        if (isset($list[$l['id']])) {
            $l['children'] = createTree($list, $list[$l['id']]);
        }

        # Add parent to the tree
        $tree[] = $l;
    }
    return $tree;
}