解题思路：

class Solution {
    List<String> res = new ArrayList<>();

    public List<String> generateParenthesis(int n) {
        if (n <= 0) return res;
        getParenthesis("", n, n);
        return res;
    }

    private void getParenthesis(String str, int left, int right) {
        if (left == 0 && right == 0) {
            res.add(str);
            return;
        }
        if (left == right) {
            // 剩余左右括号数相等，下一个只能用左括号
            getParenthesis(str + "(", left - 1, right);
        } else if (left < right) {
            // 剩余左括号小于右括号，下一个可以用左括号也可以用右括号
            if (left > 0) getParenthesis(str + "(", left - 1, right);
            if (right > 0) getParenthesis(str + ")", left, right - 1);
        }
    }
}