Problem p02274 (C++)

Buggy submission (status: Runtime Error):
#include "bits/stdc++.h"
using namespace std;
using LL = long long;
using ULL = unsigned long long;
const double PI = acos(-1);
template <class T> constexpr T INF() { return ::std::numeric_limits<T>::max(); }
template <class T> constexpr T HINF() { return INF<T>() / 2; }
template <typename T_char> T_char TL(T_char cX) { return tolower(cX); };
template <typename T_char> T_char TU(T_char cX) { return toupper(cX); };
typedef pair<LL, LL> pii;
const int vy[] = {-1, -1, -1, 0, 1, 1, 1, 0},
          vx[] = {-1, 0, 1, 1, 1, 0, -1, -1};
const int dx[4] = {0, 1, 0, -1}, dy[4] = {1, 0, -1, 0};
int popcnt(unsigned long long n) {
  int cnt = 0;
  for (int i = 0; i < 64; i++)
    if ((n >> i) & 1)
      cnt++;
  return cnt;
}
int d_sum(LL n) {
  int ret = 0;
  while (n > 0) {
    ret += n % 10;
    n /= 10;
  }
  return ret;
}
int d_cnt(LL n) {
  int ret = 0;
  while (n > 0) {
    ret++;
    n /= 10;
  }
  return ret;
}
LL gcd(LL a, LL b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
};
LL lcm(LL a, LL b) {
  LL g = gcd(a, b);
  return a / g * b;
};
#define ALL(qpqpq) (qpqpq).begin(), (qpqpq).end()
#define UNIQUE(wpwpw)                                                          \
  sort(ALL((wpwpw)));                                                          \
  (wpwpw).erase(unique(ALL((wpwpw))), (wpwpw).end())
#define LOWER(epepe) transform(ALL((epepe)), (epepe).begin(), TL<char>)
#define UPPER(rprpr) transform(ALL((rprpr)), (rprpr).begin(), TU<char>)
#define FOR(i, tptpt, ypypy) for (LL i = (tptpt); i < (ypypy); i++)
#define REP(i, upupu) FOR(i, 0, upupu)
#define INIT                                                                   \
  std::ios::sync_with_stdio(false);                                            \
  std::cin.tie(0)
#pragma warning(disable : 4996)

int a[101010], n;

struct BIT {
  int bit[202020];
  void init() { REP(i, 202020) bit[i] = 0; }
  int sum(int i) {
    int s = 0;
    while (i > 0) {
      s += bit[i];
      i -= i & -i;
    }
    return s;
  }
  void add(int i, int x) {
    while (i <= 202020) {
      bit[i] += x;
      i += i & -i;
    }
  }
};

vector<pair<LL, LL>> v;

int main() {
  BIT bit;
  bit.init();
  cin >> n;
  for (int i = 0; i < n; i++) {
    cin >> a[i];
    v.emplace_back(make_pair(a[i], i));
  }
  sort(ALL(v));
  REP(i, n) { a[v[i].second] = i + 1; }
  LL ans = 0;
  for (int j = 0; j < n; j++) {
    ans += j - bit.sum(a[j]);
    bit.add(a[j], 1);
  }
  cout << ans << endl;
}


Fixed submission:
#include "bits/stdc++.h"
using namespace std;
using LL = long long;
using ULL = unsigned long long;
const double PI = acos(-1);
template <class T> constexpr T INF() { return ::std::numeric_limits<T>::max(); }
template <class T> constexpr T HINF() { return INF<T>() / 2; }
template <typename T_char> T_char TL(T_char cX) { return tolower(cX); };
template <typename T_char> T_char TU(T_char cX) { return toupper(cX); };
typedef pair<LL, LL> pii;
const int vy[] = {-1, -1, -1, 0, 1, 1, 1, 0},
          vx[] = {-1, 0, 1, 1, 1, 0, -1, -1};
const int dx[4] = {0, 1, 0, -1}, dy[4] = {1, 0, -1, 0};
int popcnt(unsigned long long n) {
  int cnt = 0;
  for (int i = 0; i < 64; i++)
    if ((n >> i) & 1)
      cnt++;
  return cnt;
}
int d_sum(LL n) {
  int ret = 0;
  while (n > 0) {
    ret += n % 10;
    n /= 10;
  }
  return ret;
}
int d_cnt(LL n) {
  int ret = 0;
  while (n > 0) {
    ret++;
    n /= 10;
  }
  return ret;
}
LL gcd(LL a, LL b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
};
LL lcm(LL a, LL b) {
  LL g = gcd(a, b);
  return a / g * b;
};
#define ALL(qpqpq) (qpqpq).begin(), (qpqpq).end()
#define UNIQUE(wpwpw)                                                          \
  sort(ALL((wpwpw)));                                                          \
  (wpwpw).erase(unique(ALL((wpwpw))), (wpwpw).end())
#define LOWER(epepe) transform(ALL((epepe)), (epepe).begin(), TL<char>)
#define UPPER(rprpr) transform(ALL((rprpr)), (rprpr).begin(), TU<char>)
#define FOR(i, tptpt, ypypy) for (LL i = (tptpt); i < (ypypy); i++)
#define REP(i, upupu) FOR(i, 0, upupu)
#define INIT                                                                   \
  std::ios::sync_with_stdio(false);                                            \
  std::cin.tie(0)
#pragma warning(disable : 4996)

int a[202020], n;

struct BIT {
  int bit[202020];
  void init() { REP(i, 202020) bit[i] = 0; }
  int sum(int i) {
    int s = 0;
    while (i > 0) {
      s += bit[i];
      i -= i & -i;
    }
    return s;
  }
  void add(int i, int x) {
    while (i <= 202020) {
      bit[i] += x;
      i += i & -i;
    }
  }
};

vector<pair<LL, LL>> v;

int main() {
  BIT bit;
  bit.init();
  cin >> n;
  for (int i = 0; i < n; i++) {
    cin >> a[i];
    v.emplace_back(make_pair(a[i], i));
  }
  sort(ALL(v));
  REP(i, n) { a[v[i].second] = i + 1; }
  LL ans = 0;
  for (int j = 0; j < n; j++) {
    ans += j - bit.sum(a[j]);
    bit.add(a[j], 1);
  }
  cout << ans << endl;
}