Language: C++

#include "bits/stdc++.h"

using namespace std;

#define DEBUG(x) cerr<<#x<<": "<<x<<endl;
#define DEBUG_VEC(v) cerr<<#v<<":";for(int i=0;i<v.size();i++) cerr<<" "<<v[i]; cerr<<endl

typedef long long ll;
#define vi vector<int>
#define vl vector<ll>
#define vii vector< vector<int> >
#define vll vector< vector<ll> >
#define vs vector<string>
#define pii pair<int,int>
#define pis pair<int,string>
#define psi pair<string,int>
#define pll pair<ll,ll>
template<class S, class T> pair<S, T> operator+(const pair<S, T> &s, const pair<S, T> &t) { return pair<S, T>(s.first + t.first, s.second + t.second); }
template<class S, class T> pair<S, T> operator-(const pair<S, T> &s, const pair<S, T> &t) { return pair<S, T>(s.first - t.first, s.second - t.second); }
template<class S, class T> ostream& operator<<(ostream& os, pair<S, T> p) { os << "(" << p.first << ", " << p.second << ")"; return os; }
#define X first
#define Y second
#define rep(i,n) for(ll i=0;i<(ll)(n);i++)
#define rep1(i,n) for(ll i=1;i<=(ll)(n);i++)
#define rrep(i,n) for(ll i=(ll)(n)-1;i>=0;i--)
#define rrep1(i,n) for(ll i=(ll)(n);i>0;i--)
#define REP(i,a,b) for(ll i=(ll)a;i<(ll)b;i++)
#define in(x, a, b) (a <= x && x < b)
#define all(c) c.begin(),c.end()
template<class T> bool chmax(T &a, const T &b) { if (a<b) { a = b; return 1; } return 0; }
template<class T> bool chmin(T &a, const T &b) { if (a>b) { a = b; return 1; } return 0; }
#define UNIQUE(v) v.erase(std::unique(v.begin(), v.end()), v.end());
const ll inf = 1000000001;
const ll INF = 1e18;
const ll MOD = 1000000007;
const double pi = 3.14159265358979323846;
#define Sp(p) cout<<setprecision(15)<< fixed<<p<<endl;
int dx[4] = { 1,0, -1,0 }, dy[4] = { 0,1,0,-1 };
int dx2[8] = { 1,1,0,-1,-1,-1,0,1 }, dy2[8] = { 0,1,1,1,0,-1,-1,-1 };
#define fio() cin.tie(0); ios::sync_with_stdio(false);
#define mp make_pair


class Bit {
public:
	int n;
	vl bit; // 0-index

	Bit(int _n) { n = _n; bit.resize(n); }

	// [0, i)の和
	ll sum(int i) {
		ll s = 0;
		while (i > 0) {
			s += bit[i - 1];
			i -= i & -i;
		}
		return s;
	}

	//iは0-index
	void add(int i, ll x) {
		i++;
		while (i <= n) {
			bit[i - 1] += x;
			i += i & -i;
		}
	}

	// bitに入ってる中でx番目に小さい数を求める
	// 1番小さい数はx = 1;
	// bitに3を入れる→add(3, 1), 3を出す→add(3, -1)
	// return == nの場合x個も入っていない
	ll min_xth(int x) {
		int left = -1, right = n;
		while (left + 1 < right) {
			int mid = (left + right) / 2;
			int temp = sum(mid + 1);
			if (temp < x) {
				left = mid;
			}
			else {
				right = mid;
			}
		}
		return right;
	}


	// pを順番通りにするための交換回数
	ll bubble(vl p) {
		int n = p.size();
		ll ans = 0;
		for (int j = 0; j < n; j++) {
			ans += (j - sum(p[j] + 1));
			add(p[j], 1);
		}
		return ans;
	}
};

ll merge(vl &a, int l, int r) {
  if (l + 1 >= r) {
    return 0;
  }
  ll res = 0;
  int mid = (l + r) / 2;
  res = merge(a, l, mid) + merge(a, mid, r);
  vl b(r - mid);
  for (int i = mid; i < r; i++) {
    b[i - mid] = a[i];
  }
  sort(all(b));
  REP (i, l, mid) {
    res += b.end() - lower_bound(all(b), a[i]);
  }
  return res;
}


int main() {
  fio();
  ll n;
  cin >> n;
  vl a(n);
  rep (i, n) {
    cin >> a[i];
  }
  if (n == 1) {
    cout << a[0] << endl;
    return 0;
  }
  ll ok = 1, ng = inf;
  while (ok + 1 < ng) {
    ll mid = (ok + ng) / 2;
    vl b(n), sum(n + 1);
    rep (i, n) {
      b[i] = (a[i] >= mid);
      sum[i + 1] = sum[i] + b[i];
    }
    //*
    vl sum2(n + 1);
    rep (i, n + 1) {
      sum2[i] = 2*sum[i] - i + 2 * n;
    }
    //*/
    //Bit bit(5 * n + 1);
    ll cnt = (n + 1) * n / 2 - bit.bubble(sum2);
    //ll cnt = merge(sum2, 0, n + 1);
    if (2 * cnt >= (n - 1) * n / 2) {
      ok = mid;
    }
    else {
      ng = mid;
    }
  }
  cout << ok << endl;
}


Compiler output:
a.cc: In function 'int main()':
a.cc:148:32: error: 'bit' was not declared in this scope; did you mean 'Bit'?
  148 |     ll cnt = (n + 1) * n / 2 - bit.bubble(sum2);
      |                                ^~~
      |                                Bit