Frame-Stewart algorithm C++11 implementation

hanoi.cpp

#include <iostream>
#include <tuple>
#include <stack>
#include <vector>

std::stack<int> to132(std::stack<int> s) {
    int first = s.top();
    s.pop();
    int second = s.top();
    s.pop();
    int third = s.top();
    s.pop();

    s.push(second);
    s.push(third);
    s.push(first);

    return s;
}

std::stack<int> to12(std::stack<int> s) {
    int first = s.top();
    s.pop();
    int second = s.top();
    s.pop();

    s.pop();

    s.push(second);
    s.push(first);

    return s;
}

std::stack<int> to321(std::stack<int> s) {
    int first = s.top();
    s.pop();
    int second = s.top();
    s.pop();
    int third = s.top();
    s.pop();

    s.push(first);
    s.push(second);
    s.push(third);

    return s;
}

constexpr int kparam(int t, int size) {
    return (size <= 3) ? t - 1 : t / 2;
}

template<int T, int SIZE>
class Hanoi {
private:
    std::stack<int> pegs;

public:
    Hanoi(std::stack<int> v) : pegs(v) {}

    std::vector<std::tuple<int, int>> toString() {
        const int k = kparam(T, SIZE);

        Hanoi<k, SIZE> l{to132(pegs)};
        auto left = l.toString();
        Hanoi<T - k, SIZE - 1> c{to12(pegs)};
        auto center = c.toString();
        Hanoi<k, SIZE> r{to321(pegs)};
        auto right = r.toString();

        left.insert(left.end(), center.begin(), center.end());
        left.insert(left.end(), right.begin(), right.end());

        return left;
    }
};

template<int SIZE>
class Hanoi<0, SIZE> {
private:
    std::stack<int> pegs;

public:
    Hanoi(std::stack<int> v) : pegs(v) {}

    std::vector<std::tuple<int, int>> toString() {
        std::vector<std::tuple<int, int>> res{};

        return res;
    }
};

template <int SIZE>
class Hanoi<1, SIZE> {
private:
    std::stack<int> pegs;

public:
    Hanoi(std::stack<int> v) : pegs(v) {}

    std::vector<std::tuple<int, int>> toString() {
        std::vector<std::tuple<int, int>> res{};

        auto first = pegs.top();
        pegs.pop();
        auto second = pegs.top();
        pegs.pop();

        res.push_back(std::make_tuple(first, second));

        return res;
    }
};

int main() {
    std::stack<int> p;
    p.push(5);
    p.push(4);
    p.push(3);
    p.push(2);
    p.push(1);

    Hanoi<6,5> hanoi{p};

    auto res = hanoi.toString();

    for (auto r : res) {
        std::cout << std::get<0>(r) << ", " << std::get<1>(r) << std::endl;
    }
}