human
Optimize the performance and speed of this C++ code: std::vector<std::vector<int>>
convert_v1_to_v2(
    std::vector<int> v1) {
    
    std::vector<std::vector<int>> v2;
    for (auto it = v1.begin(); it != v1.end(); it++) {
        std::vector<int> sub_v1;
        auto it2 = it;
        for (auto it3 = v1.begin(); it3 != v1.end(); it3++) {
            sub_v1.push_back(*it3);
            it3++;
        }
        v2.push_back(sub_v1);
    }
    return v2;
}

gpt
--- Reasoning ---
1. Is the question multiple-choice question?
No
2. Is it implied multiple-choice question?
No
3. If two experts were independently asked the question, will the final answer be exactly the same?
No

--- Answer ---
open