void tarjan(int u,int father){
    low[u] = dfn[u] = ++tar;
    ins[u] = 1;sta[++top] = u;
    for (auto v : E[u]){
        //if (v == father) continue;
        //把上面这一句去掉
        if (!dfn[v]){
            tarjan(v,u);low[u] = min(low[u],low[v]);
        } else if (ins[v]) low[u] = min(low[u],dfn[v]);
    } if (dfn[u] == low[u]){
        ++cnt;
        do{
            color[sta[top]] = cnt;
            ins[sta[top]] = 0;
        }while (sta[top--] != u);
    }
}