sort_binary_tree_by_levels_solution.rb

# return the array containing the tree levels, from root to last level.
def tree_by_levels(node)
  return [] unless node
  unpacked_tree = {}
  breadth_first_search(node, unpacked_tree)
  unpacked_tree.values.flatten
end

def breadth_first_search(node, hash = {}, depth = 0)
  return unless node
  hash[depth] ||= []
  hash[depth] << node.value
  breadth_first_search(node.left, hash, depth + 1)
  breadth_first_search(node.right, hash, depth + 1)
end