#include <bits/stdc++.h>
const long long MOD = 1e9 + 9;
const int MAX_N = 2 * 1e5 + 100;
const long long INF = 1e18;
const long long delta = 10957;
using namespace std;
set<int> g[MAX_N];
vector<int> moves;
set<int> todo[MAX_N];
int mark[MAX_N];
void dfs(int v, int p) {
  if (g[v].size() <= 1) return;
  set<int> nows;
  if (!mark[v]) {
    set<int> st = g[v];
    for (auto x : st) {
      if (x != p) dfs(x, v);
    }
    int d = g[v].size();
    if (d % 2) {
      mark[v]++;
      todo[v] = nows;
      return;
    }
  } else
    nows = todo[v];
  moves.push_back(v);
  for (auto x : g[v]) {
    g[x].erase(v);
    if (x != p and g[x].size() > 0) dfs(x, v);
  }
  g[v].clear();
}
int main() {
  ios::sync_with_stdio(false);
  cout.tie();
  cin.tie();
  int n;
  cin >> n;
  for (int i = 1; i <= n; ++i) {
    int p;
    cin >> p;
    if (p) {
      g[i].insert(p);
      g[p].insert(i);
    }
  }
  if (n == 1) return cout << "YES\n1", 0;
  if (n % 2 == 0) return cout << "NO", 0;
  int v;
  for (int i = 1; i <= n; ++i) {
    if (g[i].size() % 2 == 0) {
      v = i;
      break;
    }
  }
  dfs(v, 0);
  cout << "YES\n";
  set<int> st;
  for (int i = 1; i <= n; ++i) st.insert(i);
  for (auto x : moves) {
    st.erase(x);
    cout << x << endl;
  }
  for (auto x : st) cout << x << endl;
  return 0;
}
