Make subcategories inherit parent category templates

hierarchical_page_template_redirect.php

<?php
/*
 * Make subcategories inherit parent category templates
 */

function hierarchical_page_template_redirect () {

	$cat = get_queried_object();

		if ( is_category() && $cat->category_parent == 100 ) { // Parent category ID
			
			if ( file_exists( locate_template( array('category-cat_name.php'), true ) ) ) { // Category template
			} 
			elseif ( ! $cat->category_parent ) {
				return;
			} else {
				$ancestors = get_category_ancestors($cat);
				foreach ($ancestors as $ancestor_cat_id) {
					if ( file_exists ( locate_template( array('category-cat_name.php'), true ) ) ) {
        				exit; 
					} 
				}
				foreach ($ancestors as $ancestor_cat_id) { 
					$ancestor_slug = get_post($ancestor_cat_id)->post_name;
					if ( file_exists ( locate_template( array('category-cat_name.php'), true ) ) ) {
        				exit; 
					}
				}
				$current_page_template = get_post_meta($post->ID,'_wp_page_template',true); 
				if ( file_exists ( locate_template ( array('/' .  $current_page_template ), true ) ) ) {
					exit; 
				} else { 
					foreach ($ancestors as $ancestor_cat_id) {
						$ancestor_page_template = get_post_meta($ancestor_cat_id,'_wp_page_template',true);
						if ( file_exists ( locate_template ( array('/' .  $ancestor_page_template ), true ) ) ) {
							exit; 
						} 
					} 
				} 
			}
			exit; 
		} 
}
add_action('template_redirect','hierarchical_page_template_redirect');