拓扑排序

    // 更新入度
    void updataIndegree( )
    {
        for (auto node : vexs)
            for (auto x : node.medges)
                ++x.mit->mindegree;
    }

    // 拓扑排序
    void topsort()
    {
        int counter = 0;
        updataIndegree();
        std::queue< Iterator > q;
        for (auto it = vexs.begin(); it != vexs.end(); ++it)
            if (it->mindegree == 0)
                q.push(it);                       // 入队

        while (!q.empty())
        {
            Iterator v = q.front();
            q.pop();                               // 出队
            v->mtopNum = ++counter;
            for (auto& w : v->medges)
                if (--w.mit->mindegree == 0)
                    q.push(w.mit);                // 入队
        }

        if (counter != vexs.size())
            throw CycleFoundException{ };
    }