matching parens

    PARENS = {
      "(" => ")",
      "{" => "}",
      "[" => "]"
    }
    OPENING_PARENS = PARENS.keys
    CLOSING_PARENS = PARENS.values
    def valid_parentheses(string)
      stack  = []
      string.each_char do |ch|
        if OPENING_PARENS.include?(ch)
          stack << ch
        elsif CLOSING_PARENS.include?(ch)
          ch == PARENS[stack.last] ? stack.pop : (return false)
        end
      end
      stack.empty?
    end
    p valid_parentheses("(){}[]") # true
    p valid_parentheses("[(])")   # false