#include <bits/stdc++.h>
using namespace std;
int n, uu, hea[200005], cnt, deg[200005], fa[200005];
bool vis[200005];
vector<int> shu;
vector<int> ans;
struct Edge {
  int too, nxt;
} edge[400005];
void add_edge(int fro, int too) {
  edge[++cnt].nxt = hea[fro];
  edge[cnt].too = too;
  hea[fro] = cnt;
}
void dfs(int x, int f) {
  shu.push_back(x);
  fa[x] = f;
  for (int i = hea[x]; i; i = edge[i].nxt) {
    int t = edge[i].too;
    if (t != f) dfs(t, x);
  }
}
void shanchu(int x) {
  vis[x] = true;
  ans.push_back(x);
  for (int i = hea[x]; i; i = edge[i].nxt) {
    int t = edge[i].too;
    deg[t]--;
    if (t != fa[x] && !vis[t]) {
      if (deg[t] % 2 == 0) shanchu(t);
    }
  }
}
int main() {
  cin >> n;
  for (int i = 1; i <= n; i++) {
    scanf("%d", &uu);
    if (!uu) continue;
    add_edge(uu, i);
    add_edge(i, uu);
    deg[i]++;
    deg[uu]++;
  }
  dfs(1, 0);
  for (int i = shu.size() - 1; i >= 0; i--) {
    int x = shu[i];
    if (deg[x] & 1) continue;
    shanchu(x);
  }
  if (ans.size() != n)
    printf("NO\n");
  else {
    printf("YES\n");
    for (auto i : ans) printf("%d\n", i);
  }
  return 0;
}
