#include <iostream>
#include <vector>

class Queue {

public:

    void push(const int value) {
        vec1.push_back(value);
    }

    void pop() {
        check_queue();
        if (vec2.empty()) {
            std::cout << "error" << std::endl;
        }
        else {
            vec2.pop_back();
        }
    }

    void peek() {
        check_queue();
        if (vec2.empty()) {
            std::cout << "error" << std::endl;
        }
        else {
            std::cout << vec2.back() << std::endl;
        }
    }

private:
    std::vector<int> vec1; // 6 7 8 9
    std::vector<int> vec2; // 5 4 3 2 1

    void check_queue() {
        if (vec2.empty()) {
            for (int i = vec1.size() - 1; i >= 0; i--) {
                vec2.push_back(vec1[i]);
            }
            vec1.clear();
        }
    }
};


int main() {
    setlocale(LC_ALL, "ru");

    Queue my_queue;

    for (int i = 0; i < 20; i++) {
        my_queue.push(i);
    }

    for (int i = 0; i < 20; i++) {
        my_queue.peek();
        my_queue.pop();
    }

}