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;
}