well formed string

well_formed_string

class StringForm
 def valid_string?(str)
   string_array = str.chars
   new_array = []

   string_array.length.times do
     if ["[", "{", "("].include?(string_array.first)
       new_array << string_array.shift
     elsif ["]", "}", ")"].index(string_array.first) == ["[", "{", "("].index(new_array.last)
       string_array.shift && new_array.pop
     else
      new_array << string_array.shift
     end
   end
   
   new_array.empty?
 end
end