#include <bits/stdc++.h>   using namespace std;   const int MOD = (int)1e9 + 7;   const int FFTMOD = 1007681537;   const int INF = (int)1e9;   const long long LINF = (long long)1e18;   const long double PI = acos((long double)-1);   const long double EPS = 1e-9;   inline long long gcd(long long a, long long b) {     long long r;     while (b) {       r = a % b;       a = b;       b = r;     }     return a;   }   inline long long lcm(long long a, long long b) { return a / gcd(a, b) * b; }   inline long long fpow(long long n, long long k, int p = MOD) {     long long r = 1;     for (; k; k >>= 1) {       if (k & 1) r = r * n % p;       n = n * n % p;     }     return r;   }   template <class T>   inline int chkmin(T& a, const T& val) {     return val < a ? a = val, 1 : 0;   }   template <class T>   inline int chkmax(T& a, const T& val) {     return a < val ? a = val, 1 : 0;   }   inline long long isqrt(long long k) {     long long r = sqrt(k) + 1;     while (r * r > k) r--;     return r;   }   inline long long icbrt(long long k) {     long long r = cbrt(k) + 1;     while (r * r * r > k) r--;     return r;   }   inline void addmod(int& a, int val, int p = MOD) {     if ((a = (a + val)) >= p) a -= p;   }   inline void submod(int& a, int val, int p = MOD) {     if ((a = (a - val)) < 0) a += p;   }   inline int mult(int a, int b, int p = MOD) { return (long long)a * b % p; }   inline int inv(int a, int p = MOD) { return fpow(a, p - 2, p); }   inline int sign(long double x) { return x < -EPS ? -1 : x > +EPS; }   inline int sign(long double x, long double y) { return sign(x - y); }   const int maxn = 2e5 + 5;   int n;   vector<int> adj[maxn];   int par[maxn];   int size[maxn];   int f[maxn];   void dfs(int u, int p) {     size[u] = 1;     par[u] = p;     for (int v : adj[u]) {       if (v != p) {         dfs(v, u);         size[u] += size[v];       }     }   }   void chemthan() {     cin >> n;     for (int i = (0); i < (n); ++i) {       int u;       cin >> u;       u--;       if (u != -1) {         adj[u].push_back(i);         adj[i].push_back(u);       }     }     if (!(n & 1)) {       cout <<  NO n ;       return;     }     dfs(0, -1);     for (int u = (0); u < (n); ++u) {       int tot = 0;       for (int v : adj[u]) {         if (v != par[u]) {           if (!(size[v] & 1)) {             f[u]++;           }           tot += size[v];         }       }       tot = n - tot - 1;       if (!(tot & 1) && tot) {         f[u]++;       }     }     static int deg[maxn];     static int rem[maxn];     for (int u = (0); u < (n); ++u) deg[u] = int((adj[u]).size());     set<int> st;     for (int u = (0); u < (n); ++u) {       if (!f[u] && !(deg[u] & 1)) {         st.insert(u);       }     }     vector<int> res;     while (int((st).size())) {       int u = *st.begin();       st.erase(u);       if (deg[u] & 1) {         continue;       }       res.push_back(u);       rem[u] = 1;       for (int v : adj[u]) {         deg[v]--;         f[v]--;         if (!f[v] && !rem[v] && !(deg[v] & 1)) {           st.insert(v);         }       }     }     if (int((res).size()) != n) {       cout <<  NO n ;       return;     }     cout <<  YES n ;     for (int u : res) {       cout << u + 1 <<   n ;     }   }   int main(int argc, char* argv[]) {     ios_base::sync_with_stdio(0), cin.tie(0);     if (argc > 1) {       assert(freopen(argv[1],  r , stdin));     }     if (argc > 2) {       assert(freopen(argv[2],  wb , stdout));     }     chemthan();     cerr <<   nTime elapsed:   << 1000 * clock() / CLOCKS_PER_SEC <<  ms n ;     return 0;   }