Language: C++

#include <cassert>
#include <algorithm>
#include <array>
#include <bitset>
#include <cctype>
#include <cstdint>
#include <cmath>
#include <complex>
#include <chrono>
#include <deque>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <unordered_map>
#include <vector>
#include <random>
#include <memory>
#include <utility>
#include <limits>
#include "limits.h"
 
#define rep(i, a, b) for (long long (i) = (a); (i) < (b); (i)++)
#define all(i) i.begin(), i.end()
#define debug(...) std::cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)

void debug_out(){std::cerr<<std::endl;}

template<typename Head,typename... Tail>
void debug_out(Head h,Tail... t){
  std::cerr<<" "<<h;
  debug_out(t...);
}
 
template <typename T1, typename T2>
std::ostream& operator<<(std::ostream& os, std::pair<T1, T2> pa) {
  return os << pa.first << " " << pa.second;
}
 
template <typename T>
std::ostream& operator<<(std::ostream& os, std::vector<T> vec) {
  for (int i = 0; i < vec.size(); i++)os << vec[i] << (i + 1 == vec.size() ? "" : " ");
  return os;
}
 
template<typename T1,typename T2>
inline bool chmax(T1& a,T2 b){return a<b && (a=b,true);}
 
template<typename T1,typename T2>
inline bool chmin(T1& a,T2 b){return a>b && (a=b,true);}
 
long long pow_mod(long long a, long long b, long long mod=-1) {
  if(b==0)return 1;
  if ((a == 0)||(mod!=-1&&(a+mod)%mod==0)) {
    return 0;
  }
  long long x = 1;
  while (b > 0) {
    if (b & 1) {
      x = (mod!=-1)?(x * a) % mod:x*a;
    }
    a = (mod!=-1)?(a * a) % mod:a*a;
    b >>= 1;
  }
  return x;
}

// const long long MOD = 998244353;
const long long MOD = 1e9 + 7;

using ll = long long;
using P=std::pair<long long,long long>;

//非再帰抽象セグ木 0-indexed 単位元はiで指定
template <class T>
class SegmentTree {
 private:

  using F=std::function<T(T,T)>;

  long long n;
  T init;
  std::vector<T> dat;
  F fn;

 public:
  SegmentTree(long long i, T para, F fun)
      : init(para), fn(fun) {
    n = 1;
    while (n < i) {
      n *= 2;
    }
    dat = std::vector<T>(2 * n - 1, init);
  }

  SegmentTree(const std::vector<T>& seq,T para,F fun)
      : init(para),fn(fun){
    n=1;
    while(n<seq.size())n<<=1;
    dat=std::vector<T>(2*n-1,init);
    for(long long i=0;i<seq.size();i++)dat[i+n-1]=seq[i];
    for(long long i=n-2;i>=0;i--)dat[i]=fn(dat[i*2+1],dat[i*2+2]);
  }

  // k番目(0-indexed)を値aで更新,dest=trueのときは更新前を破壊して初期化する
  void update(long long k, T a, bool dest) {
    assert(0<=k&&k<n);
    k += n - 1;
    if (dest) dat[k] = a;
    else dat[k] = fn(dat[k], a);

    while (k > 0) {
      k = (k - 1) / 2;
      dat[k] = fn(dat[k * 2 + 1], dat[k * 2 + 2]);
    }
  }

  //[a,b)の値を返す
  T query(long long a, long long b){
    assert(0<=a&&b<=n);
    T rval=init,lval=init;

    long long l=a+n-1,r=b+n-1;
    for(;l<r;l=(l>>1),r=(r>>1)){
      if(!(r&1)){
        r--;
        rval=fn(dat[r],rval);
      }
      if(!(l&1)){
        lval=fn(lval,dat[l]);
        l++;
      }
    }

    return fn(lval,rval);
  }
};

int main() {
  std::cin.tie(nullptr);
  std::ios::sync_with_stdio(false);

  ll n;
  std::cin>>n;

  SegmentTree<ll> seg(n,0,[](ll a,ll b){return a+b;});

  ll ans=0;

  rep(i,0,n){
    ll p;
    std::cin>>p;

    seg.update(p,1,false);
    ans+=seg.query(0,p+1);
  }

  std::cout<<ans<<"\n"

  return 0;
}


Compiler output:
a.cc: In function 'int main()':
a.cc:163:23: error: expected ';' before 'return'
  163 |   std::cout<<ans<<"\n"
      |                       ^
      |                       ;
  164 | 
  165 |   return 0;
      |   ~~~~~~