How to get magento categories by an array of category ids?

Question

I have a situation where i have an array of category ids (containing both parent and child categories). now i want to fetch all categories by ids in the array in parent child relation i.e. if the category is top level (parent) then print it else if the category is child first get and print its parent category and then print the child categories. Until now i can get all categories from the list but the condition i applied to check if the category has parent category remain always true as all main categories are child of the root (default) category

<?php if ($_categories->getParentId()>0) {
    $_categories= $_categories->getParentCategory();
} ?>

Answer 1

Based on the comments....
Instead of $_categories->getParentId()>0 try to check

$_categories->getParentId() != 1 && $_categories->getParentId() != Mage::app()->getStore()->getRootCategoryId(). 

This should exclude the root of all roots (id 1) and the default root category of the store.

Answer 2

    $cats = Mage::getModel('catalog/category')->getCollection()->addAttributeToFilter('level', array('gteq' => 2));
    $final = array();
    foreach($cats as $cat)
    {
        if($cat->getLevel() > 2 && !$cat->hasChildren())
        {
            $final[$cat->getEntityId()] = implode(',', c_getAllCategories($cat));
        }
    }
    var_dump($final);  


    function c_getAllCategories($category,$cats = null)
    {
        if(!$cats)
            $cats = array($category->getEntityId());
        $category = $category->getParentCategory();
        $cats[]=$category->getEntityId();
        if($category && $category->getLevel()>=2)
        {
            $cats= c_getAllCategories($category,$cats);
        }
        return $cats;
    }

it will print out every categories and their children