public void helper(String temp, int left, int right) {
		if (left == 0 && right == 0) {
			res.add(temp);
			return;
		}
		if (left == 0) {
			helper(temp + ")", left, right - 1);
			return;
		}
		if (right == 0 || left > right) {
			// more right parenthesis
			return;
		}
		helper(temp + "(", left - 1, right);
		helper(temp + ")", left, right - 1);
	}