#include <algorithm>
#include <iostream>
#include <iterator>
#include <numeric>
#include <set>
#include <sstream>
#include <vector>

struct Line
{
    std::string str;
};

std::istream& operator>>(std::istream& is, Line& line)
{
    std::getline(is, line.str, '\n');
    return is;
}

std::set<std::string> get_common_words(std::set<std::string> const& a,
                                       std::set<std::string> const& b)
{
    std::set<std::string> common { };
    std::set_intersection(std::begin(a), std::end(a), std::begin(b), std::end(b),
                          std::inserter(common, std::end(common)));
    return common;
}

int main()
{
    std::cout << "Enter your text: " << std::endl;
    std::vector<std::set<std::string>> lines { };
    std::transform(std::istream_iterator<Line> { std::cin },
                   std::istream_iterator<Line> { },
                   std::back_inserter(lines),
                   [](Line const& line)
                   {
                       std::istringstream iss { line.str };
                       std::set<std::string> words {
                           std::istream_iterator<std::string>{ iss },
                           std::istream_iterator<std::string>{ }
                       };
                       return words;
                   });

    lines.erase(std::remove_if(std::begin(lines), std::end(lines),
                               [](auto&& words) { return words.empty(); }),
                std::end(lines));

    if (lines.size() > 0)
    {
        std::set<std::string> common_words { 
            std::accumulate(std::next(std::begin(lines)), std::end(lines),
                            lines.front(), get_common_words)
        };
        if (common_words.size() > 0)
        {
            std::cout << "The words common with all lines are: " << std::endl;
            std::copy(std::begin(common_words), std::end(common_words),
                      std::ostream_iterator<std::string>{ std::cout, "\n" });
            return 0;
        }
    }
    std::cout << "No word appeared on every line." << std::endl;
}