Случайное сочетание

#include <iostream>
#include <vector>
#include <unordered_set>
#include <algorithm>
#include <ctime>
 
const double SQRT_2 = ::std::sqrt(2);
 
::std::vector<size_t> random_sequence_shuffle(const size_t &k, const size_t &n)
{
    ::std::vector<size_t> ans(n);
    for (size_t i = 0; i < n; ++i)
        ans[i] = i;
    ::std::random_shuffle(ans.begin(), ans.end());
    ans.resize(k);
    return ans;
}
 
::std::vector<size_t> random_sequence_generate(const size_t &k, const size_t &n)
{
    ::std::unordered_set<size_t> r;
    for (size_t i = n - k; i < n; ++i)
    {
        size_t j = rand() % (i + 1);
        r.insert(r.find(j) == r.end() ? j : i);
    }
    return ::std::vector<size_t>(r.begin(), r.end());
}
 
::std::vector<size_t> random_sequence(const size_t &k, const size_t &n)
{
    return k > n * (SQRT_2 - 1) ? random_sequence_shuffle(k, n) : random_sequence_generate(k, n);
}
 
int main(int argc, char *argv[])
{
    int k = ::std::stoi(argv[1]),
        n = ::std::stoi(argv[2]);
    auto start = clock();
    ::random_sequence(k, n);
    auto end = clock();
    ::std::cout << double(end - start) / CLOCKS_PER_SEC << ::std::endl;
}