爆搜，过了7/11个数据

#include <iostream>
#include <cstring>
#include <algorithm>
using namespace std;
typedef long long LL;
const int N = 1e5+10;
int a[N];
bool st[N];
int n;
int ans;

void dfs(int start,int u,int step)
{
    st[u] = true;
    if(a[u] == start)
    {
        ans = max(ans,step + 1);
        return ;
    }
    if(!st[a[u]])dfs(start,a[u],step+1);
}
int main()
{
    cin>>n;
    for (int i = 1; i <= n; i ++ )cin>>a[i];

    for (int i = 1; i <= n; i ++ )
    {
        dfs(i,i,0);
        memset(st, 0, sizeof st);
    }
    cout<<ans<<endl;
    return 0;
}

后续补剪枝优化