#include <vector>
#include <list>
#include <set>
#include <iostream>
#include <stdint.h>

std::set<int32_t> GetNext(const std::vector<std::list<int32_t>> &source)
{
    if (source.empty()) return std::set<int32_t>();

    std::set<int32_t> result;
    for (uint32_t sourceIndex = 0; sourceIndex < source.size(); ++sourceIndex) {
        result.insert(source[sourceIndex].begin(), source[sourceIndex].end());
    }
    return result;
}
int main()
{
    std::list<int32_t> source1 = { 1, 5, 12, 28, 31 };
    std::list<int32_t> source2 = { 6, 9, 13 };
    std::list<int32_t> source3 = { 3, 7, 10, 15 };

    std::vector<std::list<int32_t>> sources;
    sources.push_back(source1);
    sources.push_back(source2);
    sources.push_back(source3);

    auto result = GetNext(sources);
    for (auto value : result) {
        std::cout << value << std::endl;
    }

    return 0;
}