trie.rb

class Trie < Hash
  def initialize
    # Ensure that this is not a special Hash by disallowing
    # initialization options.
    super
  end

  def add(string)
    string.chars.inject(self) do |trie, char|
      trie[char] ||= Trie.new
    end
  end

  def longest_common_prefix
    if keys.length == 1
      char = keys.first
      char + self[char].longest_common_prefix
    else
      ''
    end
  end
end