magento 1 extension converted in to magento 2 process

Combine products of 2 categories in to one category

I created 4 categories.

Category 1, 2 , 3, 4

all have 50 + 50 + 50 + 50 products,

I created category 5 and category 6 ,

I want to add products of "category 1 & 2" to Category 5

I want to add products of "category 3 & 4" to Category 6.

below is the solution for this in magento site.

$category1Id = 1;
$category2Id = 2;
$category5Id = 5;
//get the category instances
$category1 = Mage::getModel('catalog/category')->load($category1Id);
$category2 = Mage::getModel('catalog/category')->load($category2Id);
$category5 = Mage::getModel('catalog/category')->load($category5Id);

//get products from the first 2 categories
$category1Products = $category1->getProductsPosition();
$category2Products = $category2->getProductsPosition();
//merge the products into one big array
$merged = array_merge($category1Products, $category2Products);
//assigned the merged products to category 5.

$category5->setPostedProducts($merged);
//save the category 5;
$category5->save();