Tarjan无向图连通性

割点：去掉某点x，该无向图分割成两部分（及以上）

割边：去掉某条边x，该无向图分割成两部分（及以上）

时间戳：在搜索树上的遍历序号dfn

追溯值：subtree子树和非搜索树上可达的最小点

割边和割点判定

void tarjan(int x, int in_edge) {
    dfn[x] = low[x] = ++num;
    for (int i = head[x]; i; i = nex[i]) {
        int y = ver[i];
        if (!dfn[y]) {
            tarjan(y, i);
            low[x] = min(low[x], low[y]);
            if (low[y] > dfn[x])
                bridge[i] = bridge[i ^ 1] = true;
        } else if (i != (in_edge ^ 1))
            low[x] = min(low[x], dfn[y]);
    }
}

void tarjan(int x) {
    dfn[x] = low[x] = ++num;
    int flag = 0;
    for (int i = head[x]; i; i = nex[i]) {
        int y = ver[i];
        if (!dfn[y]) {
            tarjan(y);
            low[x] = min(low[x], low[y]);
            if (low[y] >= dfn[x]) {
                flag++;
                if (x != 1 || flag > 1) cut[x] = true;
            }
        } else low[x] = min(low[x], dfn[y]);
    }
}

边缩点：利用dfs进行标记编号

点缩点：%%%

https://github.com/lydrainbowcat/tedukuri/blob/master/%E9%85%8D%E5%A5%97%E5%85%89%E7%9B%98/%E6%AD%A3%E6%96%87%E5%8C%85%E5%90%AB%E7%9A%84%E7%A8%8B%E5%BA%8F%E7%89%87%E6%AE%B5/0x66%20tarjan_dcc_euler.cpp