Question

ArrayList<LinkedList<TreeNode>> result = new ArrayList<LinkedList<TreeNode>>();

    for(LinkedList<TreeNode> entry : result) 

Why in for loop result is LinkedList<TreeNode> and not ArrayList<LinkedList<TreeNode>>?

Answer 1

You create ArrayList of LinkedLists, so each element of your ArrayList is a LinkedList. This is what you wrote in your for loop: you are iterating over elements of ArrayList, i.e. over LinkedLists.

And BTW, avoid writing concrete classes at the left side of assignment, i.e.

ArrayList<LinkedList<TreeNode>> result = new ArrayList<LinkedList<TreeNode>>();
should be written as

List<List<TreeNode>> result = new ArrayList<List<TreeNode>>();

now you can say:

for(List<TreeNode> entry : result)

It is more flexible because you can change your implementation without changing any other code.