Incomplete implementation of a trie in ruby

trie.rb

class Trie 
  def initialize()
    @trie = Hash.new()
  end

  def build(str)
    chars = str.chars
    chars.inject(@trie) do |prev, char|       
      unless prev[char]
        prev[char] = Hash.new()     
        prev[char][:end] = true if chars.entries.last == char
      end
      prev[char]
    end
  end  

  def find(str)
    str.chars.inject(@trie) do |prev, char|
      return false unless prev[char]
      prev[char]  
    end
    true
  end

  def starts_with(str)
    get_entries([], str.chars.inject(@trie) { |prev, char| prev[char] } , str)
  end

  private
  def get_entries(entries, remaining, str)
    remaining.each do |key, value|
      entries << str if (key == :end) 
      if(value.is_a? Hash)
        get_entries(entries, value, str + key)
      end 
    end  
    entries.sort { |x, y| x.size <=> y.size }
  end
end

f = Trie.new
f.build "mark"
f.build "markus"
f.build "malicious"
f.starts_with "mark"