Find string permutations using a Trie

trie.rb

class Trie
  attr_accessor :value, :children

  def initialize value = ''
    @value = value
    @children = {}
  end

  def [] val
    @children[val]
  end

  def []= val, trie
    @children[val] = trie
  end

  def next_val char
    self.value + char
  end

  def to_s
    puts @value unless @value.empty?
    @children.each {|c| c.to_s }
    nil
  end

  def self.build_trie str, root = nil
    root ||= Trie.new
    str.each_char do |char|
      # To allow duplicates switch these lines
      rest = str.delete char
      #rest = str.sub char, ''

      root[char] ||= Trie.build_trie(rest, Trie.new(root.next_val(char)))
    end

    root
  end 

end

Trie.build_trie('aac').to_s