Problem p03268 (C++)

Buggy submission (status: Runtime Error):
#pragma GCC optimize("Ofast")
#include "bits/stdc++.h"

// Begin Header {{{
using namespace std;

#ifndef DEBUG
#define dump(...)
#endif

#define all(x) begin(x), end(x)
#define rall(x) rbegin(x), rend(x)
#define rep(i, b, e) for (intmax_t i = (b), i##_limit = (e); i < i##_limit; ++i)
#define repc(i, b, e)                                                          \
  for (intmax_t i = (b), i##_limit = (e); i <= i##_limit; ++i)
#define repr(i, b, e)                                                          \
  for (intmax_t i = (b), i##_limit = (e); i >= i##_limit; --i)
#define var(Type, ...)                                                         \
  Type __VA_ARGS__;                                                            \
  input(__VA_ARGS__)

constexpr size_t operator""_zu(unsigned long long value) { return value; };
constexpr intmax_t operator""_jd(unsigned long long value) { return value; };
constexpr uintmax_t operator""_ju(unsigned long long value) { return value; };

constexpr int INF = 0x3f3f3f3f;
constexpr intmax_t LINF = 0x3f3f3f3f3f3f3f3f_jd;

template <class T, class Compare = less<>>
using MaxHeap = priority_queue<T, vector<T>, Compare>;
template <class T, class Compare = greater<>>
using MinHeap = priority_queue<T, vector<T>, Compare>;

inline void input() {}
template <class Head, class... Tail>
inline void input(Head &&head, Tail &&...tail) {
  cin >> head;
  input(forward<Tail>(tail)...);
}

template <class Container, class Value = typename Container::value_type,
          enable_if_t<!is_same<Container, string>::value, nullptr_t> = nullptr>
inline istream &operator>>(istream &is, Container &vs) {
  for (auto &v : vs)
    is >> v;
  return is;
}

inline void output() { cout << "\n"; }
template <class Head, class... Tail>
inline void output(Head &&head, Tail &&...tail) {
  cout << head;
  if (sizeof...(tail))
    cout << " ";
  output(forward<Tail>(tail)...);
}

template <class Container, class Value = typename Container::value_type,
          enable_if_t<!is_same<Container, string>::value, nullptr_t> = nullptr>
inline ostream &operator<<(ostream &os, const Container &vs) {
  static constexpr const char *delim[] = {" ", ""};
  for (auto it = begin(vs); it != end(vs); ++it) {
    os << delim[it == begin(vs)] << *it;
  }
  return os;
}

template <class Iterator>
inline void join(const Iterator &Begin, const Iterator &End,
                 const string &delim = "\n", const string &last = "\n") {
  for (auto it = Begin; it != End; ++it) {
    cout << ((it == Begin) ? "" : delim) << *it;
  }
  cout << last;
}

template <class T> inline vector<T> makeVector(const T &initValue, size_t sz) {
  return vector<T>(sz, initValue);
}

template <class T, class... Args>
inline auto makeVector(const T &initValue, size_t sz, Args... args) {
  return vector<decltype(makeVector<T>(initValue, args...))>(
      sz, makeVector<T>(initValue, args...));
}

template <class Func> class FixPoint : Func {
public:
  explicit constexpr FixPoint(Func &&f) noexcept : Func(forward<Func>(f)) {}

  template <class... Args>
  constexpr decltype(auto) operator()(Args &&...args) const {
    return Func::operator()(*this, std::forward<Args>(args)...);
  }
};

template <class Func>
static inline constexpr decltype(auto) makeFixPoint(Func &&f) noexcept {
  return FixPoint<Func>{forward<Func>(f)};
}

template <class Container> struct reverse_t {
  Container &c;
  reverse_t(Container &c) : c(c) {}
  auto begin() { return c.rbegin(); }
  auto end() { return c.rend(); }
};

template <class Container> auto reversed(Container &c) {
  return reverse_t<Container>(c);
}

template <class T> inline bool chmax(T &a, const T &b) noexcept {
  return b > a && (a = b, true);
}

template <class T> inline bool chmin(T &a, const T &b) noexcept {
  return b < a && (a = b, true);
}

template <class T> inline T diff(const T &a, const T &b) noexcept {
  return a < b ? b - a : a - b;
}

void ioinit() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  cout << fixed << setprecision(10);
  clog << fixed << setprecision(10);
}
// }}} End Header

signed main() {
  ioinit();

  var(intmax_t, N, K);

  // a + b = 0(mod K)
  // b + c = 0(mod K)
  // c + a = 0(mod K)

  // b = -a(mod K) -> b = K - a(mod K)
  // c = -a(mod K) -> b = K - a(mod K)

  vector<intmax_t> cnt(N + 1);
  repc(n, 1, N) cnt[n % K]++;

  intmax_t res = 0;
  rep(a, 0, K) {
    const intmax_t b = (K - a) % K;
    const intmax_t c = (K - a) % K;
    if ((b + c) % K != 0)
      continue;
    res += cnt[a] * cnt[b] * cnt[c];
  }

  output(res);
  return 0;
}


Fixed submission:
#pragma GCC optimize("Ofast")
#include "bits/stdc++.h"

// Begin Header {{{
using namespace std;

#ifndef DEBUG
#define dump(...)
#endif

#define all(x) begin(x), end(x)
#define rall(x) rbegin(x), rend(x)
#define rep(i, b, e) for (intmax_t i = (b), i##_limit = (e); i < i##_limit; ++i)
#define repc(i, b, e)                                                          \
  for (intmax_t i = (b), i##_limit = (e); i <= i##_limit; ++i)
#define repr(i, b, e)                                                          \
  for (intmax_t i = (b), i##_limit = (e); i >= i##_limit; --i)
#define var(Type, ...)                                                         \
  Type __VA_ARGS__;                                                            \
  input(__VA_ARGS__)

constexpr size_t operator""_zu(unsigned long long value) { return value; };
constexpr intmax_t operator""_jd(unsigned long long value) { return value; };
constexpr uintmax_t operator""_ju(unsigned long long value) { return value; };

constexpr int INF = 0x3f3f3f3f;
constexpr intmax_t LINF = 0x3f3f3f3f3f3f3f3f_jd;

template <class T, class Compare = less<>>
using MaxHeap = priority_queue<T, vector<T>, Compare>;
template <class T, class Compare = greater<>>
using MinHeap = priority_queue<T, vector<T>, Compare>;

inline void input() {}
template <class Head, class... Tail>
inline void input(Head &&head, Tail &&...tail) {
  cin >> head;
  input(forward<Tail>(tail)...);
}

template <class Container, class Value = typename Container::value_type,
          enable_if_t<!is_same<Container, string>::value, nullptr_t> = nullptr>
inline istream &operator>>(istream &is, Container &vs) {
  for (auto &v : vs)
    is >> v;
  return is;
}

inline void output() { cout << "\n"; }
template <class Head, class... Tail>
inline void output(Head &&head, Tail &&...tail) {
  cout << head;
  if (sizeof...(tail))
    cout << " ";
  output(forward<Tail>(tail)...);
}

template <class Container, class Value = typename Container::value_type,
          enable_if_t<!is_same<Container, string>::value, nullptr_t> = nullptr>
inline ostream &operator<<(ostream &os, const Container &vs) {
  static constexpr const char *delim[] = {" ", ""};
  for (auto it = begin(vs); it != end(vs); ++it) {
    os << delim[it == begin(vs)] << *it;
  }
  return os;
}

template <class Iterator>
inline void join(const Iterator &Begin, const Iterator &End,
                 const string &delim = "\n", const string &last = "\n") {
  for (auto it = Begin; it != End; ++it) {
    cout << ((it == Begin) ? "" : delim) << *it;
  }
  cout << last;
}

template <class T> inline vector<T> makeVector(const T &initValue, size_t sz) {
  return vector<T>(sz, initValue);
}

template <class T, class... Args>
inline auto makeVector(const T &initValue, size_t sz, Args... args) {
  return vector<decltype(makeVector<T>(initValue, args...))>(
      sz, makeVector<T>(initValue, args...));
}

template <class Func> class FixPoint : Func {
public:
  explicit constexpr FixPoint(Func &&f) noexcept : Func(forward<Func>(f)) {}

  template <class... Args>
  constexpr decltype(auto) operator()(Args &&...args) const {
    return Func::operator()(*this, std::forward<Args>(args)...);
  }
};

template <class Func>
static inline constexpr decltype(auto) makeFixPoint(Func &&f) noexcept {
  return FixPoint<Func>{forward<Func>(f)};
}

template <class Container> struct reverse_t {
  Container &c;
  reverse_t(Container &c) : c(c) {}
  auto begin() { return c.rbegin(); }
  auto end() { return c.rend(); }
};

template <class Container> auto reversed(Container &c) {
  return reverse_t<Container>(c);
}

template <class T> inline bool chmax(T &a, const T &b) noexcept {
  return b > a && (a = b, true);
}

template <class T> inline bool chmin(T &a, const T &b) noexcept {
  return b < a && (a = b, true);
}

template <class T> inline T diff(const T &a, const T &b) noexcept {
  return a < b ? b - a : a - b;
}

void ioinit() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  cout << fixed << setprecision(10);
  clog << fixed << setprecision(10);
}
// }}} End Header

signed main() {
  ioinit();

  var(intmax_t, N, K);

  // a + b = 0(mod K)
  // b + c = 0(mod K)
  // c + a = 0(mod K)

  // b = -a(mod K) -> b = K - a(mod K)
  // c = -a(mod K) -> b = K - a(mod K)

  vector<intmax_t> cnt(K);
  repc(n, 1, N) cnt[n % K]++;

  intmax_t res = 0;
  rep(a, 0, K) {
    const intmax_t b = (K - a) % K;
    const intmax_t c = (K - a) % K;
    if ((b + c) % K != 0)
      continue;
    res += cnt[a] * cnt[b] * cnt[c];
  }

  output(res);
  return 0;
}