Adding free product to cart when a product is added to cart has category A or B or C

functions.php

<?php

/**
 * Automatically adding the product to the cart.
 */
function aaptc_add_product_to_cart( $item_key, $product_id ) {

	$product_category_id 	      = array( 16, 17, 18 ); // Category ids of A, B and C
  $product_cats_ids 	        = wc_get_product_term_ids( $product_id, 'product_cat' );  
  $product_category_id_check  = false;

	foreach ( $product_category_id as $key => $value ) {
		if ( in_array( $value, $product_cats_ids ) ) { // Checking if the specified category is being matched or not.
			$product_category_id_check = true;
		}
	}

  if ( ! is_admin() && $product_category_id_check ) { // if front end and product is added to cart having category A or B or C.
      $free_product_id = 12989;  // Product Id of the free product which will get added to cart
      $found 		= false;

      //check if product already in cart
      if ( sizeof( WC()->cart->get_cart() ) > 0 ) {
          foreach ( WC()->cart->get_cart() as $cart_item_key => $values ) {
              $_product = $values['data'];
              if ( $_product->get_id() == $free_product_id )
                  $found = true;
          }
          // if product not found, add it
          if ( ! $found )
              WC()->cart->add_to_cart( $free_product_id );
      } else {
          // if no products in cart, add it
          WC()->cart->add_to_cart( $free_product_id );
      }
  }
}
add_action( 'woocommerce_add_to_cart', 'aaptc_add_product_to_cart', 10, 2 );