combinations.cpp

template <typename T>
std::vector<std::vector<T>> combinations(const std::vector<T> &vector,
                                         const size_t &combinations_len)
{
    std::function<
        void(const std::vector<T>&,
             const size_t&,
             std::vector<std::vector<T>>&,
             std::vector<T>&,
             size_t)
    > __combinations;
    __combinations = [&__combinations]
    (const std::vector<T> &v,
     const size_t &len,
     std::vector<std::vector<T>> &ans,
     std::vector<T> &comb,
     size_t current_index)
    {
        if (comb.size() == len)
        {
            ans.push_back(comb);
            return;
        }

        for (size_t i = current_index + 1; i < v.size(); ++i)
        {
            comb.push_back(v[i]);
            __combinations(v, len, ans, comb, i);
            comb.pop_back();
        }
    };

    std::vector<std::vector<T>> ans;
    std::vector<T> comb;
    __combinations(vector, combinations_len, ans, comb, -1);
    return ans;
}