longest_common_prefix.rb

class PrefixTree
  attr_reader :longest

  def initialize
    @tree = {}
    @longest = 0
  end

  def insert( str )
    node = @tree
    cur_chain = 0
    str.each_char do |char|
      node[char] ? cur_chain += 1 : node[char] = {}
      node = node[char]
    end
    @longest = cur_chain if cur_chain > @longest
  end
end

f = File.open("common_prefix.txt", "r")

f.gets.to_i.times do
  tree = PrefixTree.new
  f.gets.to_i.times do
    tree.insert( f.gets.chomp )
  end
  puts tree.longest
end