关于 tarjan 建图常数优化

if(low[v] == dfn[u]) {
    g[f[++tot] = u].push_back(tot);
    while(q.top() != v) {
        g[f[q.top()] = tot].push_back(q.top());
        q.pop();
    }
    g[f[v] = tot].push_back(v);
    q.pop();
}