vector<string> grayCode(int n) {
    if (n == 0) return {"0"};
    if (n == 1) return {"0", "1"};

    vector<string> prev = grayCode(n-1);
    vector<string> result;
    for (string s : prev) {
        result.push_back("0" + s);
    }
    for (_______________) { // 在此处填写代码
        result.push_back("1" + prev[i]);
    }
    return result;
}