RT。WA on #6。讨论区解法只会变成 70pts。

#include <bits/stdc++.h>

using namespace std;

namespace liuzimingc {
const int N = 1e5 + 5;

int n, m, dfn[N], low[N], cnt, bcc_cnt, tot, ans;
int f[N], g[N];
vector<int> e[N];
vector<int> bcc_e[N];
stack<int> s;
int a[N], fa[N], q[N];
bool vis[N];

void dfs(int u, int fa) {
	dfn[u] = low[u] = ++cnt;
	s.push(u);
	for (const int &v : e[u]) {
		if (v == fa) continue;
		if (!dfn[v]) {
			dfs(v, u);
			low[u] = min(low[u], low[v]);
			if (low[v] >= dfn[u]) {
				if (s.top() == v) {
					s.pop();
					bcc_e[u].push_back(v);
					bcc_e[v].push_back(u);
					continue;
				}
				bcc_cnt++;
				int t;
				do {
					t = s.top(); s.pop();
					bcc_e[bcc_cnt].push_back(t);
				} while (t != v);
				bcc_e[u].push_back(bcc_cnt);
			}
		}
		else low[u] = min(low[u], dfn[v]);
	}
}

void dfs2(int u, int fa) {
	if (u <= n) {
		for (const int &v : bcc_e[u]) {
		    if (v == fa) continue;
		    dfs2(v, u);
		}
		for (const int &v : bcc_e[u]) {
		    if (v == fa) continue;
			if (v <= n) {
				ans = max(ans, f[u] + f[v] + 1);
				f[u] = max(f[u], f[v] + 1);
			}
			else {
				ans = max(ans, f[u] + f[v]);
				f[u] = max(f[u], f[v]);
			}
		}
	}
	else {
		for (const int &v : bcc_e[u]) dfs2(v, u);
		int tot = 0;
		for (const int &v : bcc_e[u]) g[++tot] = f[v];
		g[++tot] = f[u];
		for (int i = 1; i <= tot; i++) g[i + tot] = g[i];
		int head = 1, tail = 0;
		q[++tail] = 0;
		for (int i = 1; i <= tot * 2; i++) {
			while (head < tail && i - q[head] > tot / 2) head++;
			ans = max(ans, i + g[i] + g[q[head]] - q[head]);
			while (head < tail && g[i] - i >= g[q[tail]] - q[tail]) tail--;
			q[++tail] = i;
		}
		for (int i = 1; i <= tot; i++) f[u] = max(f[u], g[i] + min(i, tot - i));
	}
}

int main() {
	cin >> n >> m;
	while (m--) {
		int k;
		cin >> k;
		for (int i = 1; i <= k; i++) {
			cin >> a[i];
			if (i > 1) e[a[i]].push_back(a[i - 1]), e[a[i - 1]].push_back(a[i]);
		}
	}
	bcc_cnt = n;
	dfs(1, 0);
	dfs2(1, 0);
	cout << ans << endl;
	return 0;
}
#undef int
}

int main() {
	liuzimingc::main();
	return 0;
}