Creating an Array of Linked Lists from a BST
Your return types and the types left of the assignments should be as general as possible, so you only need to change the instantiation if you want to change the implementation to an other type of list.

List&lt;Node&gt; 
List&lt;List&lt;Node&gt;&gt;


In general it is a good idea to replace ifs that covers the whole function by a guard condition. So you get rid of a level of nesting.

{
    if (n == null) return arr;
    if (arr.get(level) == null) arr.add(level, new LinkedList&lt;Node&gt;());

    arr.get(level).add(n);
    addLists(arr, n.leftChild, level + 1);
    addLists(arr, n.rightChild, level + 1);

    return arr;
}


I only checked your code layout. If you want to see if it works, write a test as tb     suggested.