def arrange_nodes(nodes)
      node_ids = Set.new(nodes.map(&:id))
      index = Hash.new { |h, k| h[k] = {} }

      nodes.each_with_object({}) do |node, arranged|
        children = index[node.id]
        index[node.parent_id][node] = children
        arranged[node] = children unless node_ids.include?(node.parent_id)
      end
    end
def arrange_nodes(nodes)
      node_ids = Set.new(nodes.map(&:id))
      index = Hash.new { |h, k| h[k] = {} }

      nodes.each_with_object({}) do |node, arranged|
        children = index[node.id]
        index[node.parent_id][node] = children
        arranged[node] = children unless node_ids.include?(node.parent_id)
      end
    end
Arrange array of nodes into a nested hash of the form
 {node => children}, where children = {} if the node has no children
 If a node's parent is not included, the node will be included as if it is a top level node