Language: C++

#include<bits/stdc++.h>
using namespace std;

#define endl '\n'
#define ALL(a)  (a).begin(),(a).end()
#define ALLR(a)  (a).rbegin(),(a).rend()
#define spa << " " <<
#define lfs <<fixed<<setprecision(10)<<
#define test cout<<"test"<<endl;
#define fi first
#define se second
#define MP make_pair
#define PB push_back
#define EB emplace_back
#define rep(i,n,m) for(ll i = n; i < (ll)(m); i++)
#define rrep(i,n,m) for(ll i = n - 1; i >= (ll)(m); i--)
using ll = int;
using ld = long double;
const ll MOD = 1e9+7;
//const ll MOD = 998244353;
const ll INF = 1e9;
using P = pair<ll, ll>;
template<typename T>
void chmin(T &a,T b){if(a>b)a=b;}
template<typename T>
void chmax(T &a,T b){if(a<b)a=b;}
void pmod(ll &a,ll b){a=(a+b)%MOD;}
void pmod(ll &a,ll b,ll c){a=(b+c)%MOD;}
void qmod(ll &a,ll b){a=(a*b)%MOD;}
void qmod(ll &a,ll b,ll c){a=(b*c)%MOD;}
ll median(ll a,ll b, ll c){return a+b+c-max({a,b,c})-min({a,b,c});}
void ans1(bool x){if(x) cout<<"Yes"<<endl;else cout<<"No"<<endl;}
void ans2(bool x){if(x) cout<<"YES"<<endl;else cout<<"NO"<<endl;}
void ans3(bool x){if(x) cout<<"Yay!"<<endl;else cout<<":("<<endl;}
template<typename T1,typename T2>
void ans(bool x,T1 y,T2 z){if(x)cout<<y<<endl;else cout<<z<<endl;}  
template<typename T>
void debug(vector<vector<T>>v,ll h,ll w){for(ll i=0;i<h;i++)
{cout<<v[i][0];for(ll j=1;j<w;j++)cout spa v[i][j];cout<<endl;}};
void debug(vector<string>v,ll h,ll w){for(ll i=0;i<h;i++)
{for(ll j=0;j<w;j++)cout<<v[i][j];cout<<endl;}};
template<typename T>
void debug(vector<T>v,ll n){if(n!=0)cout<<v[0];
for(ll i=1;i<n;i++)cout spa v[i];cout<<endl;};
template<typename T>
vector<vector<T>>vec(ll x, ll y, T w){
  vector<vector<T>>v(x,vector<T>(y,w));return v;}
ll gcd(ll x,ll y){ll r;while(y!=0&&(r=x%y)!=0){x=y;y=r;}return y==0?x:y;}
template<typename T>
vector<ll>dx={1,0,-1,0,1,1,-1,-1};
vector<ll>dy={0,1,0,-1,1,-1,1,-1};
template<typename T>
vector<T> make_v(size_t a,T b){return vector<T>(a,b);}
template<typename... Ts>
auto make_v(size_t a,Ts... ts){
  return vector<decltype(make_v(ts...))>(a,make_v(ts...));
}
template<typename T>
struct SegmentTree{
  using F = function<T(T,T)>;
  vector<T> data;
  ll n,lastlen = 1;
  F func = [](T a, T b){return a < b ? a : b;};
  T iden = MP(INT_MAX,-1); //identity element
  SegmentTree(vector<T> v){
    n = (ll)v.size();
    while(lastlen < n)lastlen *= 2;
    data.assign(lastlen*2-1,iden);
    for(ll i=0;i<n;i++)data[i+lastlen-1] = v[i];
    for(ll i=lastlen-2;i>=0;i--){
      data[i] = func(data[2*i+1], data[2*i+2]);
    }
  }
  void update(ll point, T x){
    point+=lastlen-1;
    data[point] = x;
    while(point!=0){
      point=(point-1)/2;
      data[point]=func(data[2*point+1],data[2*point+2]);
    }
  }
  T query(ll a,ll b,ll point=0,ll left=0,ll right=-1){
    if(right<0)right=lastlen;
    T ret = iden;
    if(b <= left || right <= a);
    else if(a <= left && right <= b ){
      ret = func(ret, data[point]);
    }
    else{
      T p,q;
      p = query(a,b,point*2+1,left, (left+right)/2);
      q = query(a,b,point*2+2,(left+right)/2, right);
      ret = func(ret,p);
      ret = func(ret,q);
    }      
    return ret;
  }
};
vector<ll>a(220000);
vector<vector<ll>>g(220000);
vector<P>ret;
vector<P>ppppp(220000,MP(INT_MAX,-1));
struct SegmentTree<P>seg1(ppppp);
struct SegmentTree<P>seg2(ppppp);
ll solve(ll l,ll r){
  //cout<<l spa r<<endl;
  P p,q;
  if(l%2==0){
    p=seg1.query(l,r);
    q=seg2.query(p.se,r);
  }
  else{
    p=seg2.query(l,r);
    q=seg1.query(p.se,r);
  }
  ll idx=ret.size();
  ret.EB(p.fi,q.fi); 
  if(l<p.se){
    g[idx].PB(solve(l,p.se));
  }
  if(q.se+1<r)
    g[idx].PB(solve(q.se+1,r));
  }
  if(p.se+1<q.se){
    g[idx].PB(solve(p.se+1,q.se));
  } 
  return idx;
}
int main(){
  cin.tie(NULL);
  ios_base::sync_with_stdio(false);
  ll res=0,res1=INF,res2=-INF,buf=0;
  bool judge = true;
  ll n;cin>>n;
  rep(i,0,n){
    cin>>a[i];
    if(i%2==0)seg1.update(i,MP(a[i],i));
    else seg2.update(i,MP(a[i],i));
  }
  using PP=pair<P,ll>;
  priority_queue<PP,vector<PP>,greater<PP>> que;
  buf=solve(0,n);
  que.emplace(ret[buf],buf);
  vector<ll>v;
  while(!que.empty()){
    auto x=que.top();
    que.pop();
    rep(i,0,g[x.se].size()){
      que.emplace(ret[g[x.se][i]],g[x.se][i]);
    }
    v.PB(x.fi.fi);v.PB(x.fi.se);
  }
  debug(v,n);
  return 0;
}

Compiler output:
a.cc: In function 'll solve(ll, ll)':
a.cc:123:3: warning: no return statement in function returning non-void [-Wreturn-type]
  123 |   }
      |   ^
a.cc: At global scope:
a.cc:124:3: error: expected unqualified-id before 'if'
  124 |   if(p.se+1<q.se){
      |   ^~
a.cc:127:3: error: expected unqualified-id before 'return'
  127 |   return idx;
      |   ^~~~~~
a.cc:128:1: error: expected declaration before '}' token
  128 | }
      | ^