我感到非常的surprise，这么一道简单的模板题，竟然没人写题解，甚至没人通过

for (re i = 1; i <= n; i++)
{
    cin >> op >> x;
    if (op == 1)
        dq.push_front(x);
    else if (op == 2)
        dq.push_back(x);
    else if (op == 3)
    {
        if (dq.empty())
            cout << -1 << endl;
        else
            cout << dq.front() << endl;
    }
    else if (op == 4)
    {
        if (dq.empty())
            cout << -1 << endl;
        else
            cout << dq.back() << endl;
    }
    else if (op == 5)
    {
        if (!dq.empty())
            dq.pop_front();
    }
    else if (op == 6)
    {
        if (!dq.empty())
            dq.pop_back();
    }
}