板子集合

//缩点
void tarjan(int u){
    dfn[u]=low[u]=++t;
    s[++top]=u;vis[u]=1;
    for(int i=0;i<g[u].size();++i){
        int v=g[u][i];
        if(!dfn[v]){
            tarjan(v);low[u]=min(low[u],low[v]);
        }else  if(vis[v])  low[u]=min(low[u],low[v]);
    }if(dfn[u]==low[u]){
        vis[u]=0;
        c[++sum]++;fa[u]=sum;
        while(s[top]^u){
            vis[st[top]]=0;
            c[sum]++;fa[s[top--]]=sum;
        }--top;
    }
}
//割点
void tarjan(int u){
    dfn[u]=low[u]=++t;int f=0;
    for(int i=0;i<g[u].size();++i){
        int v=g[u][i];
        if(!dfn[v]){
            tarjan(v);
            low[u]=min(low[u],low[v]);
            if(dfn[u]<=low[v])  ++f;
        }else  low[u]=min(low[u],dfn[v]);
    }if((u==rt&&f>1)||(u!=rt&&f))  q.push(u);
}
//割边
void tarjan(int u){
    dfn[u]=low[u]=++t;int f=0;
    for(int i=0;i<g[u].size();++i){
        int v=g[u][i];
        if(!dfn[v]){
            tarjan(v);
            low[u]=min(low[u],low[v]);
            if(dfn[u]<low[v])  a[++cnt]={min(u,v),max(u,v)};
        }else  low[u]=min(low[u],dfn[v]);
    }
}
//点双
void tarjan(int u){
    dfn[u]=low[u]=++t;st[++top]=u;int f=0;
    for(int i=0;i<g[u].size();++i){
        int v=g[u][i];
        if(!dfn[v]){
            tarjan(v);
            low[u]=min(low[u],low[v]);++f;
            if(dfn[u]<=low[v]){
                a[++ans].push_back(u);
                while(st[top+1]!=v)  a[ans].push_back(st[top--]);
            }  
        }else  low[u]=min(low[u],dfn[v]);
    }if(u==rt&&!f)  a[++ans].push_back(u);
}
//边双
void tarjan(int u,int fa){
    dfn[u]=low[u]=++t;s[++top]=u;
    for(in ti=0;i<g[u].size();++i){
        int v=g[u][i];
        if(!dfn[v]){
            tarjan(v,u);
            if(low[v]>=dfn[u])  continue;
            low[u]=min(low[u],low[v]);
        }else  if(v^fa)  low[u]=min(low[u],low[v]);
    }if(dfn[u]==low[u]){
        vis[u]=0;fa[u]=++sum;
        while(s[top]^u){
            vis[s[top]]=0;f[s[top--]]=sum;
        }--top;
    }
}