// UUID: f7657d16-94b6-456f-b733-b3f0985d0bff
#include <bits/stdc++.h>
using namespace std;

int main() {
    cin.tie(0), ios::sync_with_stdio(0);

    int n;
    cin >> n;
    vector<vector<int>> g(n);
    for (int i = 0, j; i < n;) {
        cin >> j;
        if (!j) {
            i++;
            continue;
        }
        g[i].push_back(j - 1);
    }

    vector<int> id(n), result;
    while (1) {
        bool done = 1;
        for (int i = 0; i < n; i++) {
            int cnt = 0;
            for (int j : g[i]) cnt += id[i] == id[j];
            if (cnt >= 2) {
                id[i] ^= 1;
                done = 0;
            }
        }
        if (done) break;
    }

    for (int i = 0; i < n; i++) {
        if (id[i]) result.push_back(i);
    }
    cout << result.size() << "\n";
    for (int x : result) cout << x + 1 << " ";
    cout << "\n";
}