出题人故意将DFS退出顺序调整为 1 2 3，导致以下代码可过样例：

#include <bits/stdc++.h>
using namespace std;
vector<int> G1[10005],G2[10005];
vector<int> t;
bool vis[10005];
int pos[10005], cnt[10005], out[10005];
int n, m;
void DFS1(int x)
{
    if (vis[x])
        return;
    vis[x] = 1;
    for (auto p : G1[x])
        DFS1(p);
    t.push_back(x);
}
void DFS2(int x, int k)
{
    if (pos[x])
        return;
    pos[x] = k;
    cnt[k]++;
    for (auto p : G2[x])
        DFS2(p, k);
}
int kosaraju()
{
    int k = 0;
    for (int i = 1; i <= n; i++)
        if (!vis[i])
            DFS1(i);
    reverse(t.begin(), t.end());
    for (int i = 0; i < t.size(); i++)
        if (!pos[i])//这里错了
            DFS2(i, ++k);
    return k;
}
signed main()
{
    cin >> n >> m;
    for (int i = 1; i <= m; i++)
    {
        int u, v;
        cin >> u >> v;
        G1[u].push_back(v);
        G2[v].push_back(u);
    }
    int k = kosaraju();
    for (int i = 1; i <= n; i++)
        for (auto j : G1[i])
            if (pos[i] != pos[j])
                out[pos[i]]++;
    int tt = -1;
    for (int i = 1; i <= k; i++)
    {
        if (out[i] == 0)
        {
            if (tt != -1)
            {
                cout << "0";
                return 0;
            }
            else
                tt = i;

        }
    }
    cout << cnt[tt];
}