本题要求你自行实现一个 queue 类，使以下示例代码能够正常运行。

#include <cassert>
#include <iostream>

class queue;

int main()
{
    queue<int> q;

    q.push(0); // back pushes 0
    q.push(1); // q = 0 1
    q.push(2); // q = 0 1 2
    q.push(3); // q = 0 1 2 3

    assert(q.front() == 0);
    assert(q.back() == 3);
    assert(q.size() == 4);

    q.pop(); // removes the front element, 0
    assert(q.size() == 3);

    // Print and remove all elements. Note that std::queue does not
    // support begin()/end(), so a range-for-loop cannot be used.
    std::cout << "q: ";
    for (; !q.empty(); q.pop())
        std::cout << q.front() << ' ';
    std::cout << '\n';
    assert(q.size() == 0);
}

正确的输出应为：

q: 1 2 3