Remove "category" word from URL

Hello
I am trying to create an SEO friendly URL structure for my website.
I an trying to have an URL witohut thr word Category and parent category name for example
I have the below category structure
Category 1
-category 2
-category 3
–category 4
By default my category structure is
for 1st level category
https://www.mysite.com/category/category-1
For second level category
https://www.mysite.com/category/category-3/category-4

What I am trying to achieve is a URL structure like below
for 1st level category
https://www.mysite.com/category-1.html
For second level category
https://www.mysite.com/category-4.html

Till now I have achieved is
For second level category
https://www.mysite.com/category-4
The code I am using is

add_filter( 'category_link', 'wpse7807_category_link', 10, 2 );
function wpse7807_category_link( $catlink, $category_id )
{
    global $wp_rewrite;
    $catlink = $wp_rewrite->get_category_permastruct();

    if ( empty( $catlink ) ) {
        $catlink = home_url('?cat=' . $category_id);
    } else {
        $category = &get_category( $category_id );
        $category_nicename = $category->slug;

        $catlink = str_replace( '%category%', $category_nicename, $catlink );
        $catlink = home_url( user_trailingslashit( $catlink, 'category' ) );
    }
    return $catlink;
}

Please help me out in creating the desired structure

Thank in advance