// solution with functional programming

function validParentheses(cad) {

  if (cad.length === 0) {
    return true;
  }

  if (cad[0] === ')' || cad[cad.length - 1] === '(') {
    return false;
  } else {
    if (cad[1] === ')') {
      return validParentheses(cad.slice(2));
    } else {
      var index = cad.indexOf(')');
      return validParentheses(cad.slice(0, index - 1) + cad.slice(index + 1));
    }
  }
}