下面代码用反射法生成 n 位格雷编码，横线处应填写（ ）。
vector<string> gray(int n) { vector<string> ans = {"0", "1"}; for (int bit = 2; bit <= n; ++bit) { int oldSize = ans.size(); for (int i = oldSize - 1; i >= 0; --i) ans.push_back(__________________________); for (int i = 0; i < oldSize; ++i) ans[i] = "0" + ans[i]; } return ans;}