Stack<Character> stack = new Stack<Character>();
     for (char c : s.toCharArray()) {
      if (c == '(')
       stack.push(')');
      else if (c == '{')
       stack.push('}');
      else if (c == '[')
   stack.push(']');
      else if (stack.isEmpty() || stack.pop() != c)
       return false;
     }
     return stack.isEmpty();