balanced_parentheses.rb

class String
  def balanced?
    pairs = { '{' => '}', '[' => ']', '(' => ')' }

    brackets = chars.each_with_object([]) do |char, stack|
      if bracket = pairs[char]
        stack << bracket
      elsif pairs.values.include?(char)
        next if stack.pop == char

        stack << char
        break stack
      end
    end

    brackets.empty?
  end
end

LIFO (Last In First Out) example, commonly known as a stack.