Problem p02774 (C++)

Buggy submission (status: Time Limit Exceeded):
////////////////////////////////////////
///  tu3 pro-con template            ///
////////////////////////////////////////
#include "bits/stdc++.h"
using namespace std;

// -- typedefs -- //
#define EPS 1e-9
typedef long long llong;

// -- loop macros -- //
#define LOOP_TYPE(s, n)                                                        \
  std::remove_reference_t<std::remove_cv_t<decltype((n) - (s))>>
#define FOR(i, s, n)                                                           \
  for (auto i = LOOP_TYPE(s, n)(s); i < LOOP_TYPE(s, n)(n); i++)
#define FORR(i, s, n)                                                          \
  for (auto i = LOOP_TYPE(s, n)(n) - 1; i != LOOP_TYPE(s, n)(s) - 1; i--)
#define REP(i, n) FOR(i, 0, n)
#define RREP(i, n) FORR(i, 0, n)

#define FORE(exp) for (auto &&exp)
#define allof(c) c.begin(), c.end()
#define partof(c, s, e) c.begin() + (s), c.begin() + (e)

// -- functors -- //
#define PREDICATE(t, a, ...) [&](const t &a) -> bool { return __VA_ARGS__; }
#define PRED(a, ...) PREDICATE(auto, a, __VA_ARGS__)
#define COMPARISON(t, a, b, ...)                                               \
  [&](const t &a, const t &b) -> bool { return __VA_ARGS__; }
#define COMP(a, b, ...) COMPARISON(auto, a, b, __VA_ARGS__)
#define CONV1(a, ...)                                                          \
  [&](const auto &a) -> auto{ return __VA_ARGS__; }
#define CONV2(a, b, ...)                                                       \
  [&](const auto &a, const auto &b) -> auto{ return __VA_ARGS__; }
#define CONV3(a, b, c, ...)                                                    \
  [&](const auto &a, const auto &b, const auto &c) -> auto{                    \
    return __VA_ARGS__;                                                        \
  }

#define TIE(...)                                                               \
  auto tie() const { return std::tie(__VA_ARGS__); }
#define LESS(t)                                                                \
  bool operator<(const t &rhs) const { return tie() < rhs.tie(); }
#define GREATER(t)                                                             \
  bool operator>(const t &rhs) const { return tie() > rhs.tie(); }

// -- I/O Helper -- //
struct _Reader {
  istream &cin;
  template <class T> _Reader operator,(T &rhs) {
    cin >> rhs;
    return *this;
  }
};
struct _Writer {
  ostream &cout;
  const char *d{" "};
  bool f{};
  template <class T> _Writer operator,(const T &rhs) {
    cout << (f ? d : "") << rhs;
    f = 1;
    return *this;
  }
};
#define READ(t, ...)                                                           \
  t __VA_ARGS__;                                                               \
  (_Reader{cin}), __VA_ARGS__
#define WRITE(...)                                                             \
  do {                                                                         \
    (_Writer{cout}), __VA_ARGS__;                                              \
    cout << endl;                                                              \
  } while (false)
#define WRITELN(...)                                                           \
  do {                                                                         \
    (_Writer{cout, "\n"}), __VA_ARGS__;                                        \
    cout << endl;                                                              \
  } while (false)
#define WRITE2D(vevector) FORE(row : vevector) WRITE(row)
#ifdef _DEBUG
#define DEBUG(...) (_Writer{cerr}), "DEBUG -> ", __VA_ARGS__, "\n"
#else
#define DEBUG(...)
#endif

// -- vevector -- //
template <class T> struct vevector : vector<vector<T>> {
  vevector(size_t n = 0, size_t m = 0, const T &initial = T())
      : vector<vector<T>>(n, vector<T>(m, initial)) {}
};
template <class T> struct vevevector : vector<vevector<T>> {
  vevevector(size_t n = 0, size_t m = 0, size_t l = 0, const T &initial = T())
      : vector<vevector<T>>(n, vevector<T>(m, l, initial)) {}
};
template <class T> struct vevevevector : vector<vevevector<T>> {
  vevevevector(size_t n = 0, size_t m = 0, size_t l = 0, size_t k = 0,
               const T &initial = T())
      : vector<vevevector<T>>(n, vevevector<T>(m, l, k, initial)) {}
};

namespace std {
template <class T1, class T2>
istream &operator>>(istream &in, pair<T1, T2> &p) {
  in >> p.first >> p.second;
  return in;
}
template <class T1, class T2>
ostream &operator<<(ostream &out, const pair<T1, T2> &p) {
  out << p.first << " " << p.second;
  return out;
}
} // namespace std

template <class T> T read() {
  T t;
  cin >> t;
  return t;
}
template <class T> vector<T> read(int n) {
  vector<T> v;
  REP(i, n) { v.push_back(read<T>()); }
  return v;
}
template <class T> vevector<T> read(int n, int m) {
  vevector<T> v;
  REP(i, n) v.push_back(read<T>(m));
  return v;
}
template <class T> vector<T> readjag() { return read<T>(read<int>()); }
template <class T> vevector<T> readjag(int n) {
  vevector<T> v;
  REP(i, n) v.push_back(readjag<T>());
  return v;
}

template <class T> struct iter_range_t {
  T beg, end;
};
template <class T> iter_range_t<T> iter_range(T beg, T end) {
  return iter_range_t<T>{beg, end};
}
template <class T> ostream &operator<<(ostream &out, iter_range_t<T> v) {
  if (v.beg != v.end) {
    out << *v.beg++;
    while (v.beg != v.end) {
      out << " " << *v.beg++;
    }
  }
  return out;
}
template <class T> ostream &operator<<(ostream &out, const vector<T> &v) {
  return out << iter_range(allof(v));
}

// -- etc -- //
template <class T> T infinity_value();
template <> int infinity_value<int>() { return int(1) << 30; }
template <> llong infinity_value<llong>() { return llong(1) << 60; }
template <> double infinity_value<double>() { return 1e+300 * 1e+300; }
#define INF(T) infinity_value<T>()

inline int sign_of(double x) { return (abs(x) < EPS ? 0 : x > 0 ? 1 : -1); }
template <class TInt> bool in_range(TInt val, TInt min, TInt max) {
  return val >= min && val < max;
}
template <> bool in_range<double>(double val, double min, double max) {
  return val - min > -EPS && val - max < EPS;
}
template <class TInt> bool in_range2d(TInt x, TInt y, TInt w, TInt h) {
  return x >= 0 && x < w && y >= 0 && y < h;
}
vector<int> iotavn(int start, int count) {
  vector<int> r(count);
  iota(allof(r), start);
  return r;
}

//// start up ////
void solve();
int32_t main() {
  cin.tie(nullptr);
  ios_base::sync_with_stdio(false);
  cout << fixed;
  cout << setprecision(std::numeric_limits<double>::max_digits10);
  solve();

  return 0;
}

struct Fixed {
  template <class F> struct FixedPoint : F {
    template <class... T> auto operator()(T &&...p) {
      return F::operator()(*this, std::forward<T>(p)...);
    }
    FixedPoint(F &&f) : F(std::forward<F>(f)) {}
  };

  template <class F> static auto fixed(F &&f) {
    return FixedPoint<F>(std::forward<F>(f));
  }
};

// にぶたん for 整数。
// lower_bound を求めたければ ok = min, ng = max + 1 とし、
// upper_bound を求めたければ ok = max, ng = min - 1 とするとよいと思う
template <class TPred>
llong binary_search(TPred pred, llong ok = 0,
                    llong ng = 1000000000000000000 + 1) {
  while (abs(ok - ng) > 1) {
    llong m = (ok + ng) / 2;
    if (pred(m))
      ok = m;
    else
      ng = m;
  }
  return ok;
}

template <class TPred, class RandomAccessIt>
pair<RandomAccessIt, RandomAccessIt>
binary_count_if(TPred pred, RandomAccessIt first, RandomAccessIt last) {
  if (first == last) {
    return {first, last};
  }
  assert(is_sorted(first, last));
  int N = last - first;
  if (pred(*first)) {
    return {first,
            first +
                binary_search(
                    [&](llong index) { return pred(*(first + index)); }, 0, N) +
                1};
  } else if (pred(*(last - 1))) {
    return {first + binary_search(
                        [&](llong index) { return pred(*(first + index)); },
                        N - 1, -1),
            last};
  } else {
    return {last, last};
  }
}

////////////////////
//// template end
////////////////////

void solve() {
  READ(llong, N, K);
  auto A = read<llong>(N);

  std::sort(allof(A));
  llong neg = lower_bound(allof(A), 0) - A.begin();
  llong pos = N - neg;

  if (K <= neg * pos) {
    auto val = binary_search(
        [&](llong x) {
          llong count = 0;
          REP(i, neg) // negpos
          {
            llong a = A[i];
            auto range =
                binary_count_if(PRED(b, a * b <= x), partof(A, neg, N));
            llong posCount = range.second - range.first;
            count += posCount;
          }
          return count >= K;
        },
        0, -INF(llong));
    WRITE(val);
  } else {
    K -= neg * pos;
    auto val = binary_search(
        [&](llong x) {
          llong count = 0;
          REP(i, neg) // negneg
          {
            llong a = A[i];
            auto range =
                binary_count_if(PRED(b, a * b <= x), partof(A, i + 1, neg));
            llong posCount = range.second - range.first;
            count += posCount;
          }

          REP(i, pos) // pospos
          {
            llong a = A[neg + i];
            auto range =
                binary_count_if(PRED(b, a * b <= x), partof(A, neg + i + 1, N));
            llong posCount = range.second - range.first;
            count += posCount;
          }

          return count >= K;
        },
        INF(llong), -1);
    WRITE(val);
  }
}


Fixed submission:
////////////////////////////////////////
///  tu3 pro-con template            ///
////////////////////////////////////////
#include "bits/stdc++.h"
using namespace std;

// -- typedefs -- //
#define EPS 1e-9
typedef long long llong;

// -- loop macros -- //
#define LOOP_TYPE(s, n)                                                        \
  std::remove_reference_t<std::remove_cv_t<decltype((n) - (s))>>
#define FOR(i, s, n)                                                           \
  for (auto i = LOOP_TYPE(s, n)(s); i < LOOP_TYPE(s, n)(n); i++)
#define FORR(i, s, n)                                                          \
  for (auto i = LOOP_TYPE(s, n)(n) - 1; i != LOOP_TYPE(s, n)(s) - 1; i--)
#define REP(i, n) FOR(i, 0, n)
#define RREP(i, n) FORR(i, 0, n)

#define FORE(exp) for (auto &&exp)
#define allof(c) c.begin(), c.end()
#define partof(c, s, e) c.begin() + (s), c.begin() + (e)

// -- functors -- //
#define PREDICATE(t, a, ...) [&](const t &a) -> bool { return __VA_ARGS__; }
#define PRED(a, ...) PREDICATE(auto, a, __VA_ARGS__)
#define COMPARISON(t, a, b, ...)                                               \
  [&](const t &a, const t &b) -> bool { return __VA_ARGS__; }
#define COMP(a, b, ...) COMPARISON(auto, a, b, __VA_ARGS__)
#define CONV1(a, ...)                                                          \
  [&](const auto &a) -> auto{ return __VA_ARGS__; }
#define CONV2(a, b, ...)                                                       \
  [&](const auto &a, const auto &b) -> auto{ return __VA_ARGS__; }
#define CONV3(a, b, c, ...)                                                    \
  [&](const auto &a, const auto &b, const auto &c) -> auto{                    \
    return __VA_ARGS__;                                                        \
  }

#define TIE(...)                                                               \
  auto tie() const { return std::tie(__VA_ARGS__); }
#define LESS(t)                                                                \
  bool operator<(const t &rhs) const { return tie() < rhs.tie(); }
#define GREATER(t)                                                             \
  bool operator>(const t &rhs) const { return tie() > rhs.tie(); }

// -- I/O Helper -- //
struct _Reader {
  istream &cin;
  template <class T> _Reader operator,(T &rhs) {
    cin >> rhs;
    return *this;
  }
};
struct _Writer {
  ostream &cout;
  const char *d{" "};
  bool f{};
  template <class T> _Writer operator,(const T &rhs) {
    cout << (f ? d : "") << rhs;
    f = 1;
    return *this;
  }
};
#define READ(t, ...)                                                           \
  t __VA_ARGS__;                                                               \
  (_Reader{cin}), __VA_ARGS__
#define WRITE(...)                                                             \
  do {                                                                         \
    (_Writer{cout}), __VA_ARGS__;                                              \
    cout << endl;                                                              \
  } while (false)
#define WRITELN(...)                                                           \
  do {                                                                         \
    (_Writer{cout, "\n"}), __VA_ARGS__;                                        \
    cout << endl;                                                              \
  } while (false)
#define WRITE2D(vevector) FORE(row : vevector) WRITE(row)
#ifdef _DEBUG
#define DEBUG(...) (_Writer{cerr}), "DEBUG -> ", __VA_ARGS__, "\n"
#else
#define DEBUG(...)
#endif

// -- vevector -- //
template <class T> struct vevector : vector<vector<T>> {
  vevector(size_t n = 0, size_t m = 0, const T &initial = T())
      : vector<vector<T>>(n, vector<T>(m, initial)) {}
};
template <class T> struct vevevector : vector<vevector<T>> {
  vevevector(size_t n = 0, size_t m = 0, size_t l = 0, const T &initial = T())
      : vector<vevector<T>>(n, vevector<T>(m, l, initial)) {}
};
template <class T> struct vevevevector : vector<vevevector<T>> {
  vevevevector(size_t n = 0, size_t m = 0, size_t l = 0, size_t k = 0,
               const T &initial = T())
      : vector<vevevector<T>>(n, vevevector<T>(m, l, k, initial)) {}
};

namespace std {
template <class T1, class T2>
istream &operator>>(istream &in, pair<T1, T2> &p) {
  in >> p.first >> p.second;
  return in;
}
template <class T1, class T2>
ostream &operator<<(ostream &out, const pair<T1, T2> &p) {
  out << p.first << " " << p.second;
  return out;
}
} // namespace std

template <class T> T read() {
  T t;
  cin >> t;
  return t;
}
template <class T> vector<T> read(int n) {
  vector<T> v;
  REP(i, n) { v.push_back(read<T>()); }
  return v;
}
template <class T> vevector<T> read(int n, int m) {
  vevector<T> v;
  REP(i, n) v.push_back(read<T>(m));
  return v;
}
template <class T> vector<T> readjag() { return read<T>(read<int>()); }
template <class T> vevector<T> readjag(int n) {
  vevector<T> v;
  REP(i, n) v.push_back(readjag<T>());
  return v;
}

template <class T> struct iter_range_t {
  T beg, end;
};
template <class T> iter_range_t<T> iter_range(T beg, T end) {
  return iter_range_t<T>{beg, end};
}
template <class T> ostream &operator<<(ostream &out, iter_range_t<T> v) {
  if (v.beg != v.end) {
    out << *v.beg++;
    while (v.beg != v.end) {
      out << " " << *v.beg++;
    }
  }
  return out;
}
template <class T> ostream &operator<<(ostream &out, const vector<T> &v) {
  return out << iter_range(allof(v));
}

// -- etc -- //
template <class T> T infinity_value();
template <> int infinity_value<int>() { return int(1) << 30; }
template <> llong infinity_value<llong>() { return llong(1) << 60; }
template <> double infinity_value<double>() { return 1e+300 * 1e+300; }
#define INF(T) infinity_value<T>()

inline int sign_of(double x) { return (abs(x) < EPS ? 0 : x > 0 ? 1 : -1); }
template <class TInt> bool in_range(TInt val, TInt min, TInt max) {
  return val >= min && val < max;
}
template <> bool in_range<double>(double val, double min, double max) {
  return val - min > -EPS && val - max < EPS;
}
template <class TInt> bool in_range2d(TInt x, TInt y, TInt w, TInt h) {
  return x >= 0 && x < w && y >= 0 && y < h;
}
vector<int> iotavn(int start, int count) {
  vector<int> r(count);
  iota(allof(r), start);
  return r;
}

//// start up ////
void solve();
int32_t main() {
  cin.tie(nullptr);
  ios_base::sync_with_stdio(false);
  cout << fixed;
  cout << setprecision(std::numeric_limits<double>::max_digits10);
  solve();

  return 0;
}

struct Fixed {
  template <class F> struct FixedPoint : F {
    template <class... T> auto operator()(T &&...p) {
      return F::operator()(*this, std::forward<T>(p)...);
    }
    FixedPoint(F &&f) : F(std::forward<F>(f)) {}
  };

  template <class F> static auto fixed(F &&f) {
    return FixedPoint<F>(std::forward<F>(f));
  }
};

// にぶたん for 整数。
// lower_bound を求めたければ ok = min, ng = max + 1 とし、
// upper_bound を求めたければ ok = max, ng = min - 1 とするとよいと思う
template <class TPred>
llong binary_search(TPred pred, llong ok = 0,
                    llong ng = 1000000000000000000 + 1) {
  while (abs(ok - ng) > 1) {
    llong m = (ok + ng) / 2;
    if (pred(m))
      ok = m;
    else
      ng = m;
  }
  return ok;
}

template <class TPred, class RandomAccessIt>
pair<RandomAccessIt, RandomAccessIt>
binary_count_if(TPred pred, RandomAccessIt first, RandomAccessIt last) {
  if (first == last) {
    return {first, last};
  }

  int N = last - first;
  if (pred(*first)) {
    return {first,
            first +
                binary_search(
                    [&](llong index) { return pred(*(first + index)); }, 0, N) +
                1};
  } else if (pred(*(last - 1))) {
    return {first + binary_search(
                        [&](llong index) { return pred(*(first + index)); },
                        N - 1, -1),
            last};
  } else {
    return {last, last};
  }
}

////////////////////
//// template end
////////////////////

void solve() {
  READ(llong, N, K);
  auto A = read<llong>(N);

  std::sort(allof(A));
  llong neg = lower_bound(allof(A), 0) - A.begin();
  llong pos = N - neg;

  if (K <= neg * pos) {
    auto val = binary_search(
        [&](llong x) {
          llong count = 0;
          REP(i, neg) // negpos
          {
            llong a = A[i];
            auto range =
                binary_count_if(PRED(b, a * b <= x), partof(A, neg, N));
            llong posCount = range.second - range.first;
            count += posCount;
          }
          return count >= K;
        },
        0, -INF(llong));
    WRITE(val);
  } else {
    K -= neg * pos;
    auto val = binary_search(
        [&](llong x) {
          llong count = 0;
          REP(i, neg) // negneg
          {
            llong a = A[i];
            auto range =
                binary_count_if(PRED(b, a * b <= x), partof(A, i + 1, neg));
            llong posCount = range.second - range.first;
            count += posCount;
          }

          REP(i, pos) // pospos
          {
            llong a = A[neg + i];
            auto range =
                binary_count_if(PRED(b, a * b <= x), partof(A, neg + i + 1, N));
            llong posCount = range.second - range.first;
            count += posCount;
          }

          return count >= K;
        },
        INF(llong), -1);
    WRITE(val);
  }
}