只需检查是否存在某个 $i$ 使得 $f_{f_{f_i}} = i$，即 $f[f[f[i]]] == i$。

#include <bits/stdc++.h>
using namespace std;

int main() {
    int n;
    cin >> n;
    vector<int> f(n + 1);
    for (int i = 1; i <= n; ++i) {
        cin >> f[i];
    }
    for (int i = 1; i <= n; ++i) {
        if (f[f[f[i]]] == i) {
            cout << "YES" << endl;
            return 0;
        }
    }
    cout << "NO" << endl;
    return 0;
}