样例能过，但是提交不了

import os
import sys

from collections import deque
N=100010
n=int(input())
ans=[0]*N;st=[False]*N
ne=[0]+[int(x) for x in input().split()]
def dfs(x):
  global ne,idx,cnt
  if d[x]:
    if x!=path[1]:
      st[path[1]]=True
      return
    cnt=idx
    for i in range(idx,0,-1):
      st[path[i]]=True
    return cnt
  d[x]=1
  idx+=1
  path[idx]=x
  dfs(ne[x])
res=0
for i in range(1,n+1):
  if st[i]:
    continue
  path=[0]*N;d=[0]*N;idx=0;cnt=0
  dfs(i)
  res=max(res,cnt)
print(res)