static boolean haakjescorrect(String zin) {
    counter = 0;
    for (int i = 0; i < zin.length; i++) {
      if (zin.charAt(i) == '(')
        counter++;
      if (zin.charAt(i) == ')')
        counter--;
      if (counter < 0)
        return false;
    }
    if (counter == 0)
      return true;
    else
      return false;