Extracting all keys from a multi-dimensional hash

class Hash
        def extract_keys(keys = [], temp = [])
                temp = temp.dup
                temp2 = temp.dup
                self.each { |key, value|
                        if value.class != Hash
                               temp << key
                                keys << temp
                                #puts temp.inspect
                                temp = []
                      end
                }
                temp = temp2
                self.each { |key, value|
                        if value.class == Hash
                                temp << key
                                keys = value.extract_keys(keys, temp)
                        end
                }
 
                keys
        end
end