noip2015 信息传递

const maxn=200000;var fa,son,q:array[0..maxn] of longint;    vis:array[0..maxn] of boolean;    l,r,i,n,x,ans:longint;function min(a,b:longint):longint;begin    if a<b then exit(a) else exit(b);end;procedure dfs(n,k,st:longint);begin    if (n=k) and (st>1) then begin ans:=min(ans,st-1); exit; end;     vis[n]:=false;    dfs(fa[n],k,st+1);end;begin    fillchar(vis,sizeof(vis),true);    fillchar(son,sizeof(son),0);    readln(n);    for i:=1 to n do begin        read(fa[i]);        inc(son[fa[i]]);    end;    l:=1;r:=1;    for i:=1 to n do if son[i]=0 then begin        q[r]:=i;        inc(r);    end;    while l<=r do begin        x:=q[l];        vis[x]:=false;        dec(son[fa[x]]);        if son[fa[x]]=0 then begin q[r]:=fa[x]; inc(r); end;        inc(l);    end;    ans:=maxlongint;    for i:=1 to n do if vis[i] then dfs(i,i,1);    writeln(ans);end.