#include <bits/stdc++.h>
using namespace std;
const double PI = acos(-1.0);
const int N = 2e5 + 7;
vector<int> E[N];
int deg[N], n, des[N], p[N];
vector<int> pos;
int dep[N];
void dfs(int u, int f, int d) {
  dep[u] = d;
  for (auto &v : E[u]) {
    if (v != f) dfs(v, u, d + 1);
  }
}
inline void add(int s, int t) { E[s].push_back(t); }
void bfs() {
  priority_queue<pair<int, int> > Q;
  int i;
  for (i = 1; i <= n; ++i) {
    if (deg[i] & 1) continue;
    Q.push({dep[i], i});
  }
  while (!Q.empty()) {
    int u = Q.top().second;
    Q.pop();
    if (des[u] || ((deg[u] > 0) && (deg[u] & 1))) continue;
    des[u] = 1;
    pos.push_back(u);
    for (auto &v : E[u]) {
      if (des[v] || (deg[v] <= 0)) continue;
      --deg[v];
      --deg[u];
    }
    for (auto &v : E[u]) {
      if (des[v]) continue;
      if (deg[v] % 2 == 0) Q.push({dep[v], v});
    }
  }
}
int main(void) {
  int i;
  scanf("%d", &n);
  for (i = 1; i <= n; ++i) {
    scanf("%d", &p[i]);
    if (p[i]) {
      add(p[i], i);
      add(i, p[i]);
      ++deg[i];
      ++deg[p[i]];
    }
  }
  for (int i = 1; i <= n; ++i) {
    if (!p[i]) dfs(i, 0, 1);
  }
  bfs();
  int ans = pos.size();
  if (ans != n)
    puts("NO");
  else {
    puts("YES");
    for (auto &x : pos) printf("%d\n", x);
  }
  return 0;
}
