找循环节求lcm就够了，若答案是12345应该输出1，被坑了下。

#include<iostream>
#include<cstdio>
#include<cstring>
#include<string>
#include<algorithm>
#include<cmath>
using namespace std;
#define INF 0x3FFFFFF
#define MAXN 2222
#define eps 1e-6
int a[MAXN],p[MAXN],b[MAXN],vis[MAXN];
int gcd(int a,int b) {return a%b==0?b:gcd(b,a%b);}
int lcm(int a,int b) {return a/gcd(a,b)*b;}
int main()
{
    int n;
    while(~scanf("%d",&n))
    {
        for(int i=1;i<=n;i++)
        {
            a[i]=i;
            vis[i]=0;
            scanf("%d",&p[i]);
        }
        int ans=1;
        for(int i=1;i<=n;i++)
        {
            if(vis[i]) continue;
            int now=a[i];
            int cnt=0;
            while(1)
            {
                vis[now]=1;
                now=p[now];
                cnt++;
                if(now==a[i]) break;
            }
            ans=lcm(ans,cnt);
        }
        printf("%d\n",ans);
    }
    return 0;
}