#include <vector>

​

template<typename T>

std::vector<T> flatten(const std::vector<std::vector<T>> &orig)

{   

    std::vector<T> ret;

    for(const auto &v: orig)

        ret.insert(ret.end(), v.begin(), v.end());                                                                                         

    return ret;

}   

​

int main() 

{   

    std::vector<std::vector<int>> vv; 

    vv.push_back(std::vector<int>{1, 2, 3});

    vv.push_back(std::vector<int>{10, 20});

    flatten(vv);

}