add_filter( 'single_template', 'my_single_template' );
function my_single_template($single_template)
{
    if (in_category(1)) {
        $file = get_template_directory().'/single-cat-1.php';
        if ( file_exists($file) ) {
            return $file;
        }
    }
    return $single_template;
}

function wpse_category_single_template( $single_template ) {
    global $post;
    $all_cats = get_the_category();

    if ( $all_cats[0]->cat_ID == '1' ) {
        if ( file_exists(get_template_directory() . "/single-cat1.php") ) return get_template_directory() . "/single-cat1.php";
    } elseif ( $all_cats[0]->cat_ID == '2' ) {
        if ( file_exists(get_template_directory() . "/single-cat2.php") ) return get_template_directory() . "/single-cat2.php";
    }
    return $single_template;
}
add_filter( 'single_template', 'wpse_category_single_template' );

function show_template() {
    global $template;
    print_r($template);
}
add_action( 'wp_head', 'show_template' );

function wpse_category_single_template( $single_template ) {
    global $post;
    $all_cats = get_the_category();

    if ( in_category(6) ) {
        if ( file_exists(get_template_directory() . "/page-custom.php") ) {
          return get_template_directory() . "/page-custom.php";
        } else {
          return get_template_directory() . "/page.php";
        }
    }
    return $single_template;
}
add_filter( 'single_template', 'wpse_category_single_template' );

add_filter( 'single_template', function ( $template )
{
    // Get the current single post
    $post_id = $GLOBALS['wp_the_query']->get_queried_object_id();

    // Test to see if our post belongs to category 1
    if ( !in_category( 1, $post_id ) ) {
        return $template;
    }

    // Our post is attached to category 1, lets look for single-1.php
    $locate_template = locate_template( 'single-1.php' );

    // Test if our template exist, if so, include it, otherwise bail
    if ( !$locate_template ) {
        return $template;
    }

    return $locate_template;
});