How to get the category ID of the current post?

How to get the category ID of the current post? Just copy the codes below and paste it into your .php file.

$category = get_the_category( get_the_ID() );
$category_ID = $category[0]->term_id;

Another problem is, what if the category ID is a sub category and you need the parent category ID? For Example, the current category ID is 2 and this is a sub category of 1 ( which is 1 is the category you are looking ), then you need codes below.

$category = get_the_category( get_the_ID() );
$category_ID = $category[0]->term_id;
$parent_id = get_ancestors( $category_ID, 'category' ); 
if( in_array( 1, $parent_id ) ){
// codes if true
}