#include<bits/stdc++.h>
using namespace std;
#pragma region Macros
#define ll long long
#define ld long double
#define FOR(i,l,r) for(ll i=(l);i<(r);++i)
#define REP(i,n) FOR(i,0,n)
#define REPS(i,n) FOR(i,1,n+1)
#define RFOR(i,l,r) for(ll i=(l);i>=(r);--i)
#define RREP(i,n) RFOR(i,n-1,0)
#define RREPS(i,n) RFOR(i,n,1)
#define pb push_back
#define eb emplace_back
#define SZ(x) ((ll)(x).size())
#define all(x) (x).begin(),(x).end()
#define rall(x) (x).rbegin(),(x).rend()
template<class T = ll> using V = vector<T>;
template<class T = ll> using VV = V<V<T>>;
using P = pair<ll, ll>;
#define ios\
    ios::sync_with_stdio(false);\
    cin.tie(nullptr);
#define VEC(type, name, size)\
    V<type> name(size);\
    IN(name)
#define VVEC(type, name, h, w)\
    VV<type> name(h, V<type>(w));\
    IN(name)
#define INT(...)\
    int __VA_ARGS__;\
    IN(__VA_ARGS__)
#define LL(...)\
    ll __VA_ARGS__;\
    IN(__VA_ARGS__)
#define STR(...)\
    string __VA_ARGS__;\
    IN(__VA_ARGS__)
#define CHAR(...)\
    char __VA_ARGS__;\
    IN(__VA_ARGS__)
#define DOUBLE(...)\
    DOUBLE __VA_ARGS__;\
    IN(__VA_ARGS__)
#define LD(...)\
    LD __VA_ARGS__;\
    IN(__VA_ARGS__)
template <class T> void scan(T a) { cin >> a; }
void scan(int &a) { cin >> a; }
void scan(long long &a) { cin >> a; }
void scan(char &a) { cin >> a; }
void scan(double &a) { cin >> a; }
void scan(long double &a) { cin >> a; }
void scan(char a[]) { scanf("%s", a); }
void scan(string &a) { cin >> a; }
template <class T> void scan(V<T> &);
template <class T, class L> void scan(pair<T, L> &);
template <class T> void scan(V<T> &a) { for(auto &i : a) scan(i); }
template <class T, class L> void scan(pair<T, L> &p){ scan(p.first); scan(p.second); }
template <class T> void scan(T &a) { cin >> a; }
void IN() {}
template <class Head, class... Tail> void IN(Head &head, Tail &... tail) { scan(head); IN(tail...); }
template <class T> inline void print(T x){ cout << x << '\n';}
template <class T> V<T> press(V<T> &x){ 
    V<T> res = x;
    sort(all(res));
    res.erase(unique(all(res)), res.end());
    REP(i, SZ(x)){
        x[i] = lower_bound(all(res), x[i]) - res.begin();
    }
    return res;
}
template<class T> inline bool chmin(T& a, T b) {if (a > b) {a = b; return true; }return false; }
template<class T> inline bool chmax(T& a, T b) {if (a < b) {a = b; return true; }return false; }
inline void Yes(bool b = true) {cout << (b ? "Yes" : "No") << '\n';}
inline void YES(bool b = true) {cout << (b ? "YES" : "NO") << '\n';}
inline void err(bool b = true) {if(b) {cout << -1 << '\n'; exit(0);}}
template<class T> inline void fin(bool b = true, T e = 0) {if(b) {cout << e << '\n'; exit(0);}}
template<class T> T Roundup_div(T x, T y) {return (x+(y-1))/y;}
template <typename T> T pow(T a, long long n, T e = 1) {T ret = e; while (n) {if (n & 1) ret *= a; a *= a; n >>= 1; } return ret; }
const ll INF = 1e18;
#pragma endregion

//Segment Tree
//reference materials: <https://www.creativ.xyz/segment-tree-abstraction-979/>, <https://algo-logic.info/segment-tree/>
template <class Monoid>
class SegTree {
    using F = function<Monoid(Monoid,Monoid)>;
    int n;// 葉の数
    vector<Monoid> data; // データを格納する配列
    Monoid def;          // 初期値かつ単位元
    F operation; // 区間クエリ関数
    F update;    // 点更新関数
    // 区間[a,b)の総和。ノードk=[l,r)に着目
    Monoid _query(int a, int b, int k, int l, int r) {
        if (r <= a || b <= l) return def; // 交差しない
        if (a <= l && r <= b)
            return data[k]; // a,l,r,bの順で完全に含まれる
        else {
            Monoid c1 = _query(a, b, 2 * k + 1, l, (l + r) / 2); // 左の子
            Monoid c2 = _query(a, b, 2 * k + 2, (l + r) / 2, r); // 右の子
            return operation(c1, c2);
        }
    }
 
  public:
    // _n:SegTreeのサイズ, _def:初期値かつ単位元, _operation:クエリ関数,
    // _update:点更新関数
    SegTree(size_t _n, Monoid _def, F _operation, F _update)
        : def(_def), operation(_operation), update(_update) {
        n = 1;
        while (n < _n) {
            n *= 2;
        }
        data = vector<Monoid>(2 * n - 1, def);
    }
 
    // 場所i(0-indexed)の値をxで更新
    void set(int i, Monoid x) {
        i += n - 1;
        data[i] = update(data[i], x);
        while (i > 0) {
            i = (i - 1) / 2;
            data[i] = operation(data[i * 2 + 1], data[i * 2 + 2]);
        }
    }
 
    // 半開区間[a, b)の区間クエリ
    Monoid query(int a, int b) {
        return _query(a, b, 0, 0, n);
    }
 
    // 添字アクセス
    Monoid operator[](int i) {
        return data[i + n - 1];
    }
};

int main(){
    ios;
    INT(n);
    SegTree<P> even(n, {INF, INF}, [](P a, P b){return min(a, b);}, [](P a, P b){return b;});
    SegTree<P> odd(n, {INF, INF}, [](P a, P b){return min(a, b);}, [](P a, P b){return b;});
    REP(i, n){
        INT(p);
        if(i % 2) odd.set(i, {p, i});
        else even.set(i, {p, i}); 
    }

    priority_queue<pair<P, P>, V<pair<P, P>>, greater<pair<P, P>>> que;
    que.push({even.query(0, n), {0, n}});

    auto judge = [&](P p){
        if(p.first % 2) return odd.query(p.first, p.second);
        else return even.query(p.first, p.second);
    };
    auto judge2 = [&](int s, P p){
        if(s % 2) return even.query(p.first, p.second);
        else return odd.query(p.first, p.second);
    };
    auto used = [&](int s){
        if(s % 2) odd.set(s, {INF, INF});
        else even.set(s, {INF, INF});
    };

    REP(_, n / 2){
        P p = que.top().first, sect = que.top().second; que.pop();
        used(p.second);
        P q = judge2(sect.first, {p.second + 1, sect.second});
        used(q.second);
        cout << p.first << " " << q.first << " ";
        V<> d = {sect.first, sect.second, p.second + 1, q.second + 1};
        sort(all(d));
        REP(i, 3) que.push({judge({d[i], d[i + 1]}), {d[i], d[i + 1]}});
    }
    cout << endl;
}

Measured memory_bytes: 28092