被Hack的代码：

# include <bits/stdc++.h>

# define reg register

typedef long long ll;

using namespace std;

int n, a[200005], du[200005], f[200005], minx = 1e9, s[200005], x;

queue <int> q;

int find (int x) {

	return f[x] == x ? x : f[x] = find (f[x]);

}

int main () {

	ios::sync_with_stdio (0);

	cin.tie (0);

	cout.tie (0);

//	freopen ("dmes.in", "r", stdin);
//
//	freopen ("dmes.out", "w", stdout);

	cin >> n;

	for (reg int i = 1; i <= n; ++ i) {

		f[i] = i;

		cin >> a[i];

		++ du[a[i]];

	}

	for (reg int i = 1; i <= n; ++ i)
		if (! du[i])
			q.push (i);

	while (! q.empty ()) {

		x = q.front ();

		q.pop ();

		if (! -- du[a[x]])
			q.push (a[x]);

	}

	for (reg int i = 1; i <= n; ++ i)
		if (du[i])
			f[find (i)] = find (a[i]);

	for (reg int i = 1; i <= n; ++ i)
		if (du[i])
			++ s[find (i)];

	for (reg int i = 1; i <= n; ++ i)
		if (du[i])
			minx = min (minx, s[find (i)]);

	cout << minx;

	return 0;

}
/*
5
2 4 2 3 1
*/

Hack.in数据生成器:

#include<bits/stdc++.h>
using namespace std;
int main(){
	cout<<"200000\n";
	for(int i=1;i<200000;++i)
		cout<<i+1<<' ';
	cout<<1;
	return 0;
}

Hack.out

200000

我输出1，有没有题解可以撤暂时不知道，但是请求添加Hack