#include <iostream>
#include <array>
#include <vector>

std::string s = "aaaab";

std::array<size_t, 255> count_l = {0};

std::vector<unsigned char> stack;

void stack_write_up() {
    for (auto i : stack) {
        std::cout << i;
    }
}

void stack_write_down() {
    for (int i = stack.size() - 1; i >= 0; i--) {
        std::cout << stack[i];
    }
}

void write_pol() {
    for (int i = 0; i < 255; i++) {
        if (count_l[i] >= 1) {
            stack_write_up();
            std::cout << (char)i;
            stack_write_down();
            std::cout << std::endl;
            if (count_l[i] == 1) {
                continue;
            }
            count_l[i] -= 2;
            stack.push_back((unsigned char) i);
            stack_write_up();
            stack_write_down();
            std::cout << std::endl;
            write_pol();
            stack.pop_back();
            count_l[i] += 2;
        }
    }
}

int main() {
    for (auto i : s) {
        count_l[i]++;
    }
    write_pol();
    return 0;
}