Tree sorting in ruby

sorting_tree.rb

# Question: https://www.dropbox.com/s/4dlhs6x69cmj8uv/Screenshot%202017-04-29%2012.21.05.png?dl=0
# Time limit: 30min
# Finished in 28min
# It should be possible to load the tree in one pass. I just didn't have time to rewrite that part.

require "rubygems"
require "json"
locations = JSON.parse(STDIN.read)

class Node
  # Confusing use of node.node... :)
  attr_accessor :node, :children
  def initialize(node)
    @node = node
    @children = []
  end
  
  def to_s(level=nil)
    str = level == nil ? "" : "#{"-" * level}#{@node["name"]}\n"
    level = level == nil ? 0 : level + 1
    return str + children.sort{|a,b| a.node["name"] <=> b.node["name"] }.map {|c| c.to_s(level) }.join("")
  end
  
  def append(value)
    children << Node.new(value)
  end

  def find(id)
    return self if @node["id"] == id
    for child in children
      node = child.find(id)
      return node if node != nil
    end
    nil
  end
end

def sort(list, into)
  unsorted = []
  list.each do |location|
    node = into.find(location["parent_id"])
    if node != nil
      node.append(location)
    else
      unsorted << location
    end
  end
  unsorted
end


STDERR.puts locations.inspect

root = Node.new({id: nil}) # makes the top-level nodes match on nil

unsorted = locations

while unsorted.count > 0
  unsorted = sort(unsorted, root)
end

puts root.to_s