#include <bits/stdc++.h>
using namespace std;
const int N = 234567;
int n;
vector<int> e[N];
int f[N];
void dfs(int u, int p) {
  int go_before_parent = 0;
  int go_after_parent = 0;
  for (int i = 0; i < e[u].size(); ++i) {
    int v = e[u][i];
    if (v == p) {
      continue;
    }
    dfs(v, u);
    if (f[v] == 0) {
      ++go_before_parent;
    } else {
      ++go_after_parent;
    }
  }
  int son_degree = go_after_parent;
  if (son_degree & 1) {
    f[u] = 0;
  } else {
    f[u] = 1;
  }
}
void print_solution(int u, int p) {
  for (int i = 0; i < e[u].size(); ++i) {
    int v = e[u][i];
    if (v == p) {
      continue;
    }
    if (f[v] == 0) {
      print_solution(v, u);
    }
  }
  cout << u << endl;
  for (int i = 0; i < e[u].size(); ++i) {
    int v = e[u][i];
    if (v == p) {
      continue;
    }
    if (f[v] == 1) {
      print_solution(v, u);
    }
  }
}
int main() {
  cin >> n;
  for (int i = 1, j; i <= n; ++i) {
    cin >> j;
    if (j != 0) {
      e[i].push_back(j);
      e[j].push_back(i);
    }
  }
  dfs(1, 0);
  if (f[1] == 0) {
    cout << "NO" << endl;
    return 0;
  }
  cout << "YES" << endl;
  print_solution(1, 0);
  return 0;
}
