Category navigation (list-sub-categories-lsc Moded)

Hi!
Its plugin-defoult (list-sub-categories-lsc) function for get subcategories:

global $cat;

// Get ancestors/parents (if there is any).
$ancestors = get_ancestors( $cat, 'category' );

// Get the first ancestor/parent (if there is one).
$ancestor = $ancestors[0];

// If we found an ancestor/parent (then we must be in a sub category).
if($ancestor){

// Get "children" to "ancestor/parent" (here we use $ancestor, since we are in a sub category).
// Note: $ancestor is the main category id of the sub category id.
foreach(get_categories("parent=$ancestor") as $category){
// echo categories.
echo 'term_id ) .'">'.$category->cat_name.'
';
}

}

// If we did NOT find an ancestor (then we must be in a main category).
if(!$ancestor){

// Get "children" to "ancestor/parent" (here we use $cat, since we are in a main category).
foreach(get_categories("parent=$cat") as $category){
// echo cateogries.
echo 'term_id ) .'">'.$category->cat_name.'
';
}

}

all work, but here:
1. not ID for curent category
2. there was no link to the parent category
I am not a programmer, but after week reading I get a work function, which I arranged for:

global $cat;

// Get ancestors/parents (if there is any).
$ancestors = get_ancestors( $cat, 'category' );

// Get the first ancestor/parent (if there is one).
$ancestor = $ancestors[0];

$child_cat = get_categories("parent=$cat");

// If we found an ancestor/parent (then we must be in a sub category).
if($ancestor){
echo '<h1>'.get_cat_name($ancestor).'</h1>';
?>
<nav>

<?php
foreach(get_categories("parent=$ancestor") as $category){
if($cat == $category->term_id) $current = '-current'; else $current = '';
// echo categories.
echo '<li class="cat-item'.$current.'">term_id ) .'">'.$category->cat_name.'';
}
}

// If we did NOT find an ancestor (then we must be in a main category).
if(!$ancestor && $child_cat){
?>
<h1><?php printf( single_cat_title( '', false ) . '' );?></h1>
<nav>

<?php
// Get "children" to "ancestor/parent" (here we use $cat, since we are in a main category).
foreach($child_cat as $category){
// echo cateogries.
echo '
• term_id ) .'">'.$category->cat_name.'

';
}
?>

Function is used in the template “Category” and can provide additional navigation for subcategories. like Apple ??
But it remains no certainty with respect to the code. All the same I do not have to do with PHP or other programming languages??, this is my first attempt.
In the end, I raised two questions:
1. My code is correct with respect to PHP and WP?
2. How well chosen the function with respect to performance?

Thanks.

https://www.ads-software.com/extend/plugins/list-sub-categories-lsc/