求強連通。例題：

【NOIP2015】資訊傳遞

#pragma GCC optimize(3)
#pragma GCC optimize(2)
#include <iostream>
#include <string>
#include <cstdio>
#include <algorithm>
#include <cstring>
#include <cmath>
#include <queue>
#include <functional>
#include <vector>
#include 
 
<map>
#include <set>
#include <stack>
#include <fstream>
#include <sstream>
#include <unordered_map>
#define FT(a, b) memset(a, b, sizeof(a))
#define FAT(a) memset(a, 0, sizeof(a))
using namespace std;
typedef long long ll;
const int N = 2e5 + 10;
const int M = 3e5 + 100
 
;
const int INF = 0x3f3f3f3f;
const int mod = 1e9 + 7;
int n, m, k, x, y, t;

vector<int> e[N];
int a[N], dfn[N], low[N], idx = 0;
stack<int> st;
bool vis[N];
int ans = INF;
void tarjan(int now)
{
    dfn[now] = low[now] = ++idx;
    st.push(now);
    vis[now] = 1;
    for (int it : e[now])
    {
        
 
if (!dfn[it])
        {
            tarjan(it);
            low[now] = min(low[now], low[it]);
        }
        else if (vis[now])
        {
            low[now] = min(low[now], dfn[it]);
        }
    }
    if (low[now] == dfn[now])
    {

        int cnt = 0;
        while (st.size())
        {
            int x = st.top();
            st.pop();
            vis[x] = 0;
            cnt++;
            if (x == now)
                break;
        }
        if (cnt > 1)
            ans = min(ans, cnt);
    }
}
void solve()
{
    scanf("%d", &n);
    FAT(dfn);
    FAT(vis);
    FAT(low);
    for (int i = 1; i <= n; ++i)
    {
        int u;
        scanf("%d", &u);
        e[i].push_back(u);
    }
    for (int i = 1; i <= n; ++i)
        if (!dfn[i])
            tarjan(i);
    printf("%d\n", ans);
}

int main()
{
#ifdef ONLINE_JUDGE
#else
    freopen("/home/wjy/code/in.txt", "r", stdin);
    // freopen("/home/wjy/code/ans1.txt","w", stdout);
#endif
    ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);
    int T = 1;
    // scanf("%d", &T);
    // cin >> T;
    // time_t begin, end;
    // begin = clock();
    while (T--)
    {
        solve();
    }
    // end = clock();
    // double ret = double(end - begin) / CLOCKS_PER_SEC;
    return 0;
}