How to create a custom dynamic WordPress widget?

class ts91_recent_post_widget extends WP_Widget 
{
 
	// The construct part of dynamic widget 
	function __construct() 
	{
		// construct code will go here
	}
	  
	// Creating dynamic widget front-end
	public function widget( $args, $instance ) 
	{
		// dynamic widget front end will go here
	}
	          
	// Creating dynamic widget Backend 
	public function form( $instance ) 
	{
		//admin form part will go here
	}
	      
	// Updating dynamic widget replacing old instances with new
	public function update( $new_instance, $old_instance ) 
	{
		//this function will update the instance
	}
 
// Class ts91_widget ends here
}

//Registering the dynamic widget
function ts91_register_widget() 
{
    register_widget( 'ts91_recent_post_widget' );
}
add_action( 'widgets_init', 'ts91_register_widget' );

class ts91_recent_post_widget extends WP_Widget 
{
 
	// The construct part of dynamic widget 
	function __construct() 
	{
		parent::__construct(
  
		// Base ID of your widget
		'ts91_recent_post_widget', 
		  
		// Widget name will appear in UI
		__('91 TechSquare Recent Posts', 'ts91_recent_post_widget_domain'), 
		  
		// Widget description
		array( 'description' => __( 'custom dynamic widget based on 91 TechSquare Tutorial', 'ts91_recent_post_widget_domain' ), ) 
		);
	}
	  
	// Creating dynamic widget front-end
	public function widget( $args, $instance ) 
	{
		$title = apply_filters( 'widget_title', $instance['title'] );
		$no_of_post = apply_filters( 'widget_number_of_post', $instance['number_of_post'] );
  
		// before and after widget arguments are defined by themes
		echo $args['before_widget'];
		if(empty($title))
		{
			$title = "91 TechSquare Recent Posts";
		}
		if ( ! empty( $title ) )
		echo $args['before_title'] . $title . $args['after_title'];

		
		  
		// This is where you run the code and display the output
		$args = array(
	        'posts_per_page' => $no_of_post, /* how many post you need to display */
	        'offset' => 0,
	        'orderby' => 'post_date',
	        'order' => 'DESC',
	        'post_type' => 'post', /* your post type name */
	        'post_status' => 'publish'
	    );
	    $query = new WP_Query($args);
	    if ($query->have_posts()) :
	    	echo "<ul>";
	        while ($query->have_posts()) : $query->the_post();
	            ?>
	            <li>
	            <a href="<?php the_permalink(); ?>"><?php the_title(); ?></a>
	       		</li>
	            <?php
	        endwhile;
	        echo "</ul>";
	    endif;
		echo $args['after_widget'];
	}
	          
	// Creating dynamic widget Backend 
	public function form( $instance ) 
	{
		if ( isset( $instance[ 'title' ] ) ) {
		$title = $instance[ 'title' ];
		}
		else {
		$title = __( '91 TechSquare Recent Posts', 'ts91_recent_post_widget_domain' );
		}

		//getting number of post value to form
		if ( isset( $instance[ 'number_of_post' ] ) ) {
		$number_of_post = $instance[ 'number_of_post' ];
		}
		else {
		$number_of_post = 5;
		}
		// Widget admin form
		?>
		<p>
		<label for="<?php echo $this->get_field_id( 'title' ); ?>"><?php _e( 'Title:' ); ?></label> 
		<input class="widefat" id="<?php echo $this->get_field_id( 'title' ); ?>" name="<?php echo $this->get_field_name( 'title' ); ?>" type="text" value="<?php echo esc_attr( $title ); ?>" />

		<label>No. of post:
		<input class="widefat" name="<?php echo $this->get_field_name( 'number_of_post' ); ?>" type="number" min="0" value="<?php echo esc_attr( $number_of_post ); ?>">
		</p>
		<?php
	}
	      
	// Updating dynamic widget replacing old instances with new
	public function update( $new_instance, $old_instance ) 
	{
		$instance = array();
		$instance['title'] = ( ! empty( $new_instance['title'] ) ) ? strip_tags( $new_instance['title'] ) : '';
		$instance['number_of_post'] = ( ! empty( $new_instance['number_of_post'] ) ) ? strip_tags( $new_instance['number_of_post'] ) : '';
		return $instance;
	}
 
// Class ts91_widget ends here
}

//Registering the dynamic widget
function ts91_register_widget() 
{
    register_widget( 'ts91_recent_post_widget' );
}
add_action( 'widgets_init', 'ts91_register_widget' );