I found the answer myself. :-)

In \Magento\Store\Model\Store, there is a function getRootCategoryId()

namespace Vendor\Module\Helper;

public class Store
{
    /**
     * @var \Magento\Store\Model\StoreManagerInterface $storeManager
     */
    protected $storeManager;

    public function __construct(\Magento\Store\Model\StoreManagerInterface $storeManager)
    {
        $this->storeManager = $storeManager;
    }//__construct

    /**
     * Get an associative array of [store_id => root_category_id] values for all stores
     * @return array
     */
    public function getAllStoreRootCategories()
    {
        $storeroots = [];
        foreach ($this->storeManager->getStores() as $store) {
            $storeroots[$store->getId()] = $store->getRootCategoryId();
        }
        return $storeroots;
    }//getAllStoreRootCategories

    /**
     * Get the root category id of a store
     * @param int|string|\Magento\Store\Model\Store $store The store to get category from, either by store_id, store_code or the \Magento\Store\Model\Store instance itself
     * @return int root category of store
     * @throws \Exception if no such store was found
     */
    public function getStoreRootCategoryId($store)
    {
        # Get \Magento\Store\Model\Store instance by id
        if (is_int($store)) {
            $store = $this->storeManager->getStore($store);
        }

        # Get \Magento\Store\Model\Store instance by code
        if (is_string($store)) {
            foreach ($this->storeManager->getStores() as $storeModel) {
                if ($storeModel->getCode() == $store) {
                    $store = $storeModel;
                    break;
                }
            }
        }

        # Get root category id from \Magento\Store\Model\Store instance
        if ($store instanceof \Magento\Store\Model\Store) {
            return $store->getRootCategoryId();
        }

        # If no \Magento\Store\Model\Store instance was supplied or found by id/code
        throw new \Exception('No such store found: ' . var_export($store, true));
    }//getStoreRootCategoryId
}//class Store

<?php

class your class_name
{

    public function __construct(
       \Magento\Store\Model\StoreManagerInterface $_storeManager
    )
    {
         $this->_storeManager = $_storeManager;
    }



    public function getRootCategoryId()
    {
        $store = 1;
        $rootCatId = $this->_storeManager->getStore($store)->getRootCategoryId();
        //$rootCatId = $this->_storeManager->getStore()->getRootCategoryId();//without assign store id
    }

}

Thanks Aron! so just to make it super easy to understand, if you are going to use this inside a module you should use direct injection as follows:

<?php

namespace ...

use ...

class Classname
{
    /** @var \Magento\Store\Model\Store */
    protected $_store;


    public function __construct(
        \Magento\Store\Model\Store $store
    )
    {
        $this->_store = $store;
    }



    public function getRootCategoryId()
    {
        return $this->_store->getStoreRootCategoryId();
    }

}