for (int i = head[u]; i ; i = G[i].next){
    int v = G[i].v;
    if (!dfn[v]){
        tarjan(v);  //说明v还未在栈中
        low[u] = min(low[u], low[v]);
    }
    else if(!scc[v]){
        low[u] = min(low[u], dfn[v]);
      //为什么不能 low[u] = min(low[u], low[v]);
    }
}
}