dfs_final_answer.rb

class TreeNode
  # ...
 
  def dfs(target_value = nil,  &blk)
    # Depth-first search.  Returns TreeNode instance
    # with target value, or nil if not in tree.  
    # Optionally accepts a block and returns the 
    # first node encounted for which the block 
    # evaluates to true. This allows us to dfs
    # search using arbitrary criteria.

    if block_given? && !target_value.nil?
      raise ArgumentError, "takes either a block or a target value"
    end

    return self if @value == target_value
    return self if !blk.nil? && blk.call(@value)
 
    found_node = nil 
    @children.each do |child|
      found_node = child.dfs(target_value, &blk)
      return found_node unless found_node.nil?
    end
    return found_node
  end
  
end