#include <bits/stdc++.h>
using namespace std;

const int N = 100010;
vector<int> v[N];
map<int, vector<int>> mp;

void dfs(int x, int h)
{
    mp[h].push_back(x);
    for (auto c : v[x])
        dfs(c, h + 1);
}

int main()
{
    int n;
    cin >> n;
    for (int i = 1; i <= n; i++)
    {
        int x;
        cin >> x;
        if (x == -1)
            x = 0;
        v[x].push_back(i);
    }
    dfs(0, 1);
    int MAX_h = mp.rbegin()->first;
    cout << MAX_h - 1 << endl;
    for (int i = 0; i < mp[MAX_h].size(); i++)
    {
        if (i) cout << ' ';
        cout << mp[MAX_h][i];
    }

    return 0;
}

或

#include <bits/stdc++.h>
using namespace std;

const int N = 100010;
vector<int> v[N];
vector<int> num;
int MAX_h;

void dfs(int x, int h)
{
    if (h > MAX_h)
    {
        MAX_h = h;
        num.clear();
    }
    if (h == MAX_h) num.push_back(x);

    for (auto c : v[x]) dfs(c, h + 1);
}

int main()
{
    int n;
    cin >> n;
    for (int i = 1; i <= n; i++)
    {
        int x;
        cin >> x;
        if (x == -1) x = 0;
        v[x].push_back(i);
    }

    dfs(0, 1);

    cout << MAX_h - 1 << endl;
    for (int i = 0; i < num.size(); i++)
    {
        if (i) cout << ' ';
        cout << num[i];
    }

    return 0;
}