Getting count of strings using Binary Search Tree in Ruby

I want to get the count of various strings stored in different files. Actually I need two types of counts. For a string str,

1.) Total no of occurrences of the string str throughout all the files. 2.) Number of files having the string str.

Below is my RUBY Code for it in which i have managed to get the total count. But i can't get the file count. I have used arrays instead of files for simplicity(analogous to files) and an instance variable 'flag'.

My concept is that if flag = 0, it means the string is encountered for the first time in the first array. So the filecount(or arraycount in this case) is incremented and flag is set to 1. So when the same string appears in the same array , flag is already set to 1 and nothing happens. When an array is over, all the flag values are set back to 0.(I think so)

But something's not working as expected. Thanks in Advance..!

    class Tree
  attr_accessor :left
  attr_accessor :right
  attr_accessor :data
  attr_accessor :count
  attr_accessor :flag
  attr_accessor :howmanyfiles

  def initialize(x=nil)
   @left = nil
   @right = nil
   @data = x
   @count = 1
   @flag = 0
   @howmanyfiles = 1
  end


  def search(x)
      if self.data == x
        self.count = self.count + 1
        if self.flag == 0
          self.howmanyfiles = self.howmanyfiles + 1
        end
       return "#{self.data} found" #self
      else
       ltree = left != nil ? left.search(x) : nil
       return ltree if ltree != nil
       rtree = right != nil ? right.search(x) : nil
       return rtree if rtree != nil
      end
    nil
  end


  def insert(x)
   list = []

   if @data == nil
    @data = x
    self.flag = 1
   elsif @left == nil
    @left = Tree.new(x)
    self.flag = 1
   elsif @right == nil
    @right = Tree.new(x)
    self.flag = 1
   else
    list << @left
    list << @right
    loop do
     node = list.shift
     if node.left == nil
      node.insert(x)
      break
     else
      list << node.left
     end
     if node.right == nil
      node.insert(x)
      break
     else
      list << node.right
     end
    end
   end



  end

  def traverse()
   list = []
   yield @data
   list << @left if @left != nil
   list << @right if @right != nil
   loop do
    break if list.empty?
    node = list.shift
    yield node.data
    list << node.left if node.left != nil
    list << node.right if node.right != nil
   end
  end






end


  items = ["Amal","Hai", "Bob", "Bob", "Cat", "Cat", "Amal", "Dog", "Rizu", "Zol","Amal"]

  tree = Tree.new

  items.each {|x|

    if tree.search(x) == nil
       tree.insert(x)
     end}


     ObjectSpace.each_object(Tree) do |obj|
      obj.flag = 0
     end

     items1 = ["Amal","wet", "jjj", "Cat"]
     items1.each {|x|

       if tree.search(x) == nil
          tree.insert(x)
        end}



        ObjectSpace.each_object(Tree) do |obj|
         obj.flag = 0
        end

        items2 = ["aa","Amal", "jjj"]
        items2.each {|x|

          if tree.search(x) == nil
             tree.insert(x)
           end}

     ObjectSpace.each_object(Tree) do |obj|
    puts obj.data.to_s + " " + obj.count.to_s + " " + obj.howmanyfiles.to_s
   end

  tree.traverse {|x|  print "#{x} "}
  print "\n"