Query showing subcategory of post's category

I want to show the subcategory of the category related to each post in my website,
I mean when the user visit any post in my website, I want him to see at the end of the post the following:

• SubCat1

• Post 1
• Post 2

• SubCat2

• Post 1
• Post 2

• SubCat3

• Post 1
• Post 2

I managed to find this code:

$categories =  get_categories('child_of=31');
foreach  ($categories as $category) {
    //Display the sub category information using $category values like $category->cat_name
    echo '<h2>'.$category->name.'</h2>';
    echo '<ul>';

    foreach (get_posts('cat='.$category->term_id) as $post) {
        setup_postdata( $post );
        echo '<li><a href="'.get_permalink($post->ID).'">'.get_the_title().'</a></li>';
    }
    echo '</ul>';
}

but this is for category with id= 31 , I want to make it variable.
any help please ?