一：功能

        部分排序，即选出前n个最大值或者最小值，剩余部分是无序的。

二：用法

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

int main() {
    std::vector<int> data{9, 8, 7, 6, 5, 4, 3, 2, 1};
    std::partial_sort(data.begin(), data.begin()+3, data.end());
    for (auto v : data) {
        std::cout << v << " ";
    }
    std::cout << "\n";

    std::ranges::partial_sort(data, data.begin()+3, std::greater<>());
    for (auto v : data) {
        std::cout << v << " ";
    }
    std::cout << "\n";
}

三：实现

#include <algorithm>
#include <array>
#include <functional>
#include <iostream>

namespace impl
{
    template<typename RandomIt, typename Compare>
    constexpr //< since C++20
    void sift_down(RandomIt first, RandomIt last, const Compare& comp)
    {
        // sift down element at “first”
        const auto length = static_cast<std::size_t>(last - first);
        std::size_t current = 0;
        std::size_t next = 2;
        while (next < length)
        {
            if (comp(*(first + next), *(first + (next - 1))))
                --next;
            if (!comp(*(first + current), *(first + next)))
                return;
            std::iter_swap(first + current, first + next);
            current = next;
            next = 2 * current + 2;
        }
        --next;
        if (next < length && comp(*(first + current), *(first + next)))
            std::iter_swap(first + current, first + next);
    }
 
    template<typename RandomIt, typename Compare>
    constexpr //< since C++20
    void heap_select(RandomIt first, RandomIt middle, RandomIt last, const Compare& comp)
    {
        std::make_heap(first, middle, comp);
        for (auto i = middle; i != last; ++i)
        {
            if (comp(*i, *first))
            {
                std::iter_swap(first, i);
                sift_down(first, middle, comp);
            }
        }
    }
} // namespace impl
 
template<typename RandomIt, typename Compare>
constexpr //< since C++20
void my_partial_sort(RandomIt first, RandomIt middle, RandomIt last, Compare comp)
{
    impl::heap_select(first, middle, last, comp);
    std::sort_heap(first, middle, comp);
}

void print(const auto& s, int middle)
{
    for (int a : s)
        std::cout << a << ' ';
    std::cout << '\n';
    if (middle > 0)
    {
        while (middle-- > 0)
            std::cout << "--";
        std::cout << '^';
    }
    else if (middle < 0)
    {
        for (auto i = s.size() + middle; --i; std::cout << "  ")
        {}
 
        for (std::cout << '^'; middle++ < 0; std::cout << "--")
        {}
    }
    std::cout << '\n';
};
 
int main()
{
    std::array<int, 10> s{5, 7, 4, 2, 8, 6, 1, 9, 0, 3};
    print(s, 0);
    my_partial_sort(s.begin(), s.begin() + 3, s.end(), std::less{});
    print(s, 3);
    my_partial_sort(s.rbegin(), s.rbegin() + 4, s.rend(), std::less{});
    print(s, -4);
    my_partial_sort(s.rbegin(), s.rbegin() + 5, s.rend(), std::greater{});
    print(s, -5);
}