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

    int main()
    {
        int pivot = 8;
        int arr [9] = {2,1,5,8,9,4,10,15,20};

        // get partition point  
        int *pt = std::stable_partition(arr, std::end(arr), [&](int n) {return n < pivot;});

        // create two vectors consisting of left and right hand side
        // of partition
        std::vector<int> a1(arr, pt);
        std::vector<int> a2(pt, std::end(arr));

        // output results
        for (auto& i : a1)
            std::cout << i << " ";
        std::cout << '\n';
        for (auto& i : a2)
            std::cout << i << " ";
    }