双连通分量模板

边双连通分量（一般配上缩点、并查集）

tot = 1;
void add(int x,int y)
{
    nextt[++tot] = head[x];
    head[x] = tot;
    to[tot] = y;
    from[tot] = x;
}

void tarjan(int u)
{
    pre[u] = low[u] = ++dfs_clock;
    for (int i = head[u];i;i = nextt[i])
    {
        if (i == (bian[u] ^ 1))
        continue;
        int v = to[i];
        if (!pre[v])
        {
            bian[v] = i;
            tarjan(v);
            if (low[v] < low[u])
            low[u] = low[v];
            if (low[v] > pre[u])
            flag[bian[v]] = flag[bian[v] ^ 1] = 1;
        }
        else
        if (pre[v] < low[u])
        low[u] = pre[v];
    }
}

int find(int x)
{
    if (x == fa[x])
    return x;
    return fa[x] = find(fa[x]);
}

for (int i = 2; i <= tot; i += 2)
    if (!flag[i])
    fa[find(to[i])] = find(from[i]);

参考例题：传送门

点双连通分量：

void dfs(int u,int fa)
{
    pre[u] = low[u] = ++dfs_clock;
    int child = 0;
    for (int i = head[u];i;i = nextt[i])
    {
        int v = to[i];
        if (!pre[v])
        {
            node temp;
            temp.u = u;
            temp.v = v;
            e[++top] = temp;
            child++;
            dfs(v,u);
            low[u] = min(low[u],low[v]);
            if (low[v] >= pre[u])
            {
                iscut[u] = 1;
                num++;
                while (1)
                {
                    int tu = e[top].u,tv = e[top--].v;
                    if (belong[tu] != num)
                    belong[tu] = num;
                    if (belong[tv] != num)
                    belong[tv] = num;
                    if (tu == u && tv == v)
                    break;
                }
            }
        }
        else
        if (pre[v] < pre[u] && v != fa)
        {
            node temp;
            temp.u = u;
            temp.v = v;
            e[++top] = temp;
            low[u] = min(low[u],pre[v]);
        }
    }
    if (child == 1 && fa == -1)
    iscut[u] = 0;
}

参考例题：传送门