How to use stl for permutation and combination?

#include <iostream>
#include <vector>
#include <algorithm>

//从 indexs 集合中选择 num 个元素进行组合并保证返回的组合中没有重复的元素
std::vector<std::vector<int>> combination(const int num,std::vector<int> &indexs)
{
    std::vector<int> elements;
    elements.resize(indexs.size());
    for (int i = 0; i < num; i++)
    {
        elements.at(i) = 1;
    }

    std::vector<std::vector<int>> set;
    do
    {
        //当前组合
        std::vector<int> currentCombination;
        for (size_t i = 0; i < elements.size(); ++i)
        {
            if (elements[i])
            {
                currentCombination.push_back(indexs[i]);
            }
        }
        //把当前组合存入向量容积中
        set.push_back(currentCombination);
    } while (prev_permutation(elements.begin(), elements.end()));

    //std::cout << set.size() << std::endl;
    return set;
}

std::ostream& operator<<(std::ostream& os, const std::vector<std::vector<int>>& data)
{
    for (int i = 0;i < data.size();i++)
    {
        os << i << ": ";
        for (int j = 0;j < data[i].size();j++)
        {
            os << data[i][j] << ",";
        }
        os << "\n";
    }
    return os;
}

inline unsigned int factorial(unsigned int value)
{
    unsigned int local_value = value;
    while (value-- > 1)
    {
        local_value *= value;
    }
    return local_value;
}

unsigned int combination(unsigned int k,unsigned int n)
{
    if (k > n)
    {
        return 0;
    }
    return factorial(n)/ (factorial(k)*factorial(n - k));
}

int main()
{
    std::vector<int> indexs = { 1,2,3,4,5,6,7,8,9 };
    const int num = 4;
    
    std::cout << combination(num, indexs) << std::endl;

    getchar();
        return 0;
}