Input:

3
2 0
3 0
1 0

Output:

1
0

我的错误代码（但是AC了）：

#include <bits/stdc++.h>

using namespace std;

const int N = 2e5 + 10;
int belong[N], odeg[N], ideg[N];
vector <int> z[N];
int dfn[N], low[N], tot, cnt;
bool instk[N];
stack <int> stk;

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

signed main()
{
  int n;
  cin >> n;
  for (int i = 1; i <= n; i ++)
  {
    int p;
    while (cin >> p, p)
      z[i].push_back(p);
  }
  for (int i = 1; i <= n; i ++)
    if (!dfn[i])
      dfs(i);
  for (int i = 1; i <= n; i ++)
    for (auto &j : z[i])
      if (belong[i] != belong[j])
        odeg[belong[i]] ++, ideg[belong[j]] ++;
  int cnt = 0;
  for (int i = 1; i <= tot; i ++)
    if (!ideg[i])
      cnt ++;
  cout << cnt << '\n';
  int dnt = 0;
  for (int i = 1; i <= tot; i ++)
    if (!odeg[i])
      dnt ++;
  cout << max(cnt, dnt) << '\n';
  return 0;
}

叉了一篇题解：https://luogu.com.cn/blog/Eiffel-Ablog/p2812-xiao-yuan-wang-lao