洛谷 P3119【分层图】【tarjan】

// 这回只花了45min就打完了。
// 真好。记得多手造几组。最好有暴力对拍。

#include <bits/stdc++.h>

using namespace std;

const int N = 4e5 + 10;
int dfn[N], low[N], c, dis[N], belong[N], cnt[N], tot, scc[N];
bool instk[N], inque[N];
stack <int> stk;
vector <pair <int, int> > arr;
vector <int> z[N], y[N];

void dfs(int now)
{
  dfn[now] = low[now] = ++ c;
  stk.push(now); instk[now] = true;
  for (auto &p : z[now])
  {
    if (!dfn[p])
    {
      dfs(p);
      low[now] = min(low[now], low[p]);
    }
    else if (instk[p])
      low[now] = min(low[now], dfn[p]);
  }
  if (dfn[now] == low[now])
  {
    tot ++;
    while (stk.top() != now)
    {
      int p = stk.top();
      scc[p] = tot;
      stk.pop();
      instk[p] = false;
      cnt[tot] ++;
    }
    stk.pop();
    scc[now] = tot;
    instk[now] = false;
    cnt[tot] ++;
  }
}

void spfa(int s) {
  queue <int> q;
  q.push(s);
  inque[s] = true;
  dis[s] = 0;
  while (q.size()) {
    int f = q.front();
    q.pop();
    inque[f] = false;
    for (auto &u : y[f])
      if (dis[u] < dis[f] + cnt[u]) {
        dis[u] = dis[f] + cnt[u];
        if (!inque[u]) {
          inque[u] = true;
          q.push(u);
        }
      }
  }
}

int main()
{
  int n, m;
  cin >> n >> m;
  while (m --)
  {
    int x, y;
    cin >> x >> y;
    z[x].push_back(y);
    z[x + n].push_back(y + n);
    z[y].push_back(x + n);
    arr.push_back(make_pair(x, y));
    arr.push_back(make_pair(x + n, y + n));
    arr.push_back(make_pair(y, x + n));
  }
  for (int i = 1; i <= 2 * n; i ++)
    if (!dfn[i])
      dfs(i);
  for (auto &u : arr)
  {
    int x1 = scc[u.first], y1 = scc[u.second];
    if (x1 != y1)
      y[x1].push_back(y1);
  }
  spfa(scc[1]);
  cout << max(dis[scc[1]], dis[scc[n + 1]]) << '\n';
}

// 足下千里，移步换景，寰宇纷呈万花筒