#include <bits/stdc++.h>   using namespace std;   const int MOD = 1e9 + 7;   int n;   vector<int> v, lm, rm;   class SegmentTreeMax {    public:     int n;     vector<long long> vect;     SegmentTreeMax(int _n) : n(_n) {       vect.resize(2 * n);       fill(vect.begin(), vect.end(), 0);     }     void modify(int p, long long value) {       for (vect[p += n] = value; p > 1; p >>= 1) {         int val = rm[vect[p]] > rm[vect[p ^ 1]] ? vect[p] : vect[p ^ 1];         vect[p >> 1] = val;       }     }     long long query(int l, int r) {       long long res = 0;       for (l += n, r += n; l < r; l >>= 1, r >>= 1) {         if (l & 1) {           if (rm[res] < rm[vect[l]]) res = vect[l];           l++;         }         if (r & 1) {           if (rm[res] < rm[vect[r - 1]]) res = vect[r - 1];           r--;         }       }       return res;     }   };   int main() {     ios_base::sync_with_stdio(0);     cin.tie(0);     cin >> n;     v.resize(n + 1);     lm.resize(n + 1);     rm.resize(n + 1);     vector<int> v2;     for (int i = 1; i <= n; i++) {       cin >> v[i];       v2.push_back(v[i]);     }     v2.push_back(0);     sort(v2.begin(), v2.end());     auto it = unique(v2.begin(), v2.end());     v2.resize(it - v2.begin());     int l = 1, r = 1;     vector<int> ans;     SegmentTreeMax stm(n + 1);     vector<int> visited, ct(n + 1);     vector<vector<int>> updates(n + 1);     while (r <= n) {       int id = lower_bound(v2.begin(), v2.end(), v[r]) - v2.begin();       if (!lm[id]) lm[id] = r;       int val = stm.query(l, lm[id]);       updates[id].push_back(r);       rm[id] = r;       ct[id]++;       visited.push_back(id);       if (rm[val] > lm[id] || ct[id] == 4) {         if (ct[id] == 4) {           for (int i = 0; i < 4; i++) ans.push_back(v[r]);         } else {           ans.push_back(v2[val]);           ans.push_back(v[r]);           ans.push_back(v2[val]);           ans.push_back(v[r]);         }         for (int i : visited) {           lm[i] = 0;           rm[i] = 0;           ct[i] = 0;           updates[i].clear();         }         visited.clear();         l = r + 1;         r = l;       } else {         for (int i : updates[id]) {           stm.modify(i, id);         }         r++;       }     }     cout << ans.size() <<   n ;     for (int i : ans) {       cout << i <<    ;     }     return 0;   }   