单选题库

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;
}

struct TreeNode {
    int val;
    TreeNode* left;
    TreeNode* right;
    TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
};
 
TreeNode* createTree() {
    TreeNode* root = new TreeNode(1);
    root->left = new TreeNode(2);
    root->right = new TreeNode(3);
    root->left->left = new TreeNode(4);
    root->left->right = new TreeNode(5);
    return root;
}