Problem p02774 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
using namespace std;

using ll = long long;
#define int ll

// [a -> b-1]
#define reps(i, a, b) for (int i = (int)(a); i < (int)(b); i++)
// [0 -> a-1]
#define rep(i, a) reps(i, 0, (a))
// [a-1 -> b]
#define rreps(i, a, b) for (int i = (int)((a)-1); i >= (int)(b); i--)
// [a-1 -> 0]
#define rrep(i, a) rreps(i, a, 0)
#define all(v) (v).begin(), (v).end()
// next_permutation(all(v))
#define PERM(v) next_permutation(all(v))
/*sort(all(v));
 * (v).erase(unique(all(v)), v.end())*/
#define UNIQUE(v)                                                              \
  sort(all(v));                                                                \
  (v).erase(unique(all(v)), v.end())
// b is [a, c)
#define RANGE(a, b, c) ((a) <= (b) && (b) < (c))
// c is [a, e) && d is [b, f)
#define RANGE2D(a, b, c, d, e, f) (RANGE((a), (c), (e)) && RANGE((b), (d), (f)))
#define chmin(a, b) a = min(a, (b))
#define chmax(a, b) a = max(a, (b))
#define fcout cout << fixed << setprecision(15)
#define YES(f) cout << ((f) ? YES_STR : NO_STR) << endl;

#ifdef LOCAL_DEBUG_OPTION
namespace debug_print_n {
struct DebugPrint {
  template <class T, class U = remove_const_t<remove_reference_t<T>>>
  static enable_if_t<is_integral<U>::value && !is_same<U, bool>::value &&
                     !is_same<U, char>::value>
  DUMP_SUB_FUNCTION(T x) {
    cerr << x;
  }
  template <class T, class U = remove_const_t<remove_reference_t<T>>>
  static enable_if_t<is_same<U, bool>::value> DUMP_SUB_FUNCTION(T x) {
    if (x)
      cerr << "True";
    else
      cerr << "False";
  }
  template <class T, class U = remove_const_t<remove_reference_t<T>>>
  static enable_if_t<is_same<U, char>::value> DUMP_SUB_FUNCTION(T x) {
    cerr << '\'' << x << '\'';
  }
  template <class T, class U = remove_const_t<remove_reference_t<T>>>
  static enable_if_t<is_same<U, string>::value> DUMP_SUB_FUNCTION(T x) {
    cerr << '\"' << x << '\"';
  }
  template <class T>
  static enable_if_t<is_same<T, const char *>::value> DUMP_SUB_FUNCTION(T x) {
    cerr << string(x);
  }
  template <class T> static enable_if_t<true> DUMP_SUB_FUNCTION(vector<T> x) {
    cerr << '{';
    for (size_t i = 0; i < x.size(); i++) {
      if (i)
        cerr << ", ";
      DUMP_SUB_FUNCTION(x[i]);
    }
    cerr << '}';
  }
  template <class T, size_t N>
  static enable_if_t<true> DUMP_SUB_FUNCTION(array<T, N> x) {
    cerr << '{';
    for (size_t i = 0; i < x.size(); i++) {
      if (i)
        cerr << ", ";
      DUMP_SUB_FUNCTION(x[i]);
    }
    cerr << '}';
  }
  template <class T> static enable_if_t<true> DUMP_SUB_FUNCTION(set<T> x) {
    cerr << '[';
    size_t i = 0;
    for (auto itr = x.begin(); itr != x.end(); itr++, i++) {
      if (i)
        cerr << ", ";
      DUMP_SUB_FUNCTION(*itr);
    }
    cerr << ']';
  }
  template <class T, class U>
  static enable_if_t<true> DUMP_SUB_FUNCTION(map<T, U> x) {
    cerr << '[';
    size_t i = 0;
    for (auto itr = x.begin(); itr != x.end(); itr++, i++) {
      if (i)
        cerr << ", ";
      DUMP_SUB_FUNCTION(itr->first);
      cerr << " : ";
      DUMP_SUB_FUNCTION(itr->second);
    }
    cerr << ']';
  }
  template <class T, class U = remove_const_t<remove_reference_t<T>>>
  static enable_if_t<is_floating_point<U>::value> DUMP_SUB_FUNCTION(T x) {
    cerr << fixed << setprecision(5) << x;
  }
  template <class T, class U>
  static enable_if_t<true> DUMP_SUB_FUNCTION(pair<T, U> p) {
    cerr << '(';
    DUMP_SUB_FUNCTION(p.first);
    cerr << ", ";
    DUMP_SUB_FUNCTION(p.second);
    cerr << ')';
  }
  template <int num = 0, class... Args>
  static enable_if_t<num == sizeof...(Args)>
  DUMP_SUB_FUNCTION(tuple<Args...> t) {
    if (num)
      cerr << ')';
  }
  template <int num = 0, class... Args>
      static enable_if_t <
      num<sizeof...(Args)> DUMP_SUB_FUNCTION(tuple<Args...> t) {
    if (num == 0)
      cerr << '(';
    DUMP_SUB_FUNCTION(get<num>(t));
    if (num + 1 < sizeof...(Args))
      cerr << ", ";
    DUMP_SUB_FUNCTION<num + 1>(t);
  }
  template <long unsigned N>
  static enable_if_t<true> DUMP_SUB_FUNCTION(bitset<N> x) {
    cerr << x;
  }
};
void dump() { cerr << endl; }
template <class Head, class... Tail> void dump(Head &&head, Tail &&...tail) {
  DebugPrint::DUMP_SUB_FUNCTION(head);
  if (sizeof...(Tail) != 0) {
    cerr << ' ';
  }
  dump(forward<Tail>(tail)...);
}
} // namespace debug_print_n
using debug_print_n::dump;
#else
template <class... Args> void dump(Args &&...args) {}
#endif

// meguru<T>(ok, ng, isok)で呼ぶ
template <class T> T meguru(T ok, T ng, function<bool(T)> isok) {
  while (abs(ok - ng) > 1) {
    T mid = (ok + ng) / 2;
    (isok(mid) ? ok : ng) = mid;
  }
  return ok;
}

signed main() {
  int N, K;
  cin >> N >> K;
  vector<int> A(N);
  rep(i, N) cin >> A[i];
  sort(all(A));
  auto cnt = [&](int mid) -> int { // mid 以下の数
    int res = 0;
    rep(i, N) {
      if (A[i] < 0) {
        res +=
            i - meguru<int>(i, -1, [&](int p) { return A[p] * A[i] <= mid; });
      } else if (A[i] == 0) {
        if (mid >= 0)
          res += i;
      } else {
        res += meguru<int>(i, -1, [&](int p) { return A[p] * A[i] > mid; });
      }
    }
    return res;
  };
  cout << meguru<int>(2e18, -2e18, [&](auto mid) { return cnt(mid) >= K; })
       << endl;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;

using ll = long long;
#define int ll

// [a -> b-1]
#define reps(i, a, b) for (int i = (int)(a); i < (int)(b); i++)
// [0 -> a-1]
#define rep(i, a) reps(i, 0, (a))
// [a-1 -> b]
#define rreps(i, a, b) for (int i = (int)((a)-1); i >= (int)(b); i--)
// [a-1 -> 0]
#define rrep(i, a) rreps(i, a, 0)
#define all(v) (v).begin(), (v).end()
// next_permutation(all(v))
#define PERM(v) next_permutation(all(v))
/*sort(all(v));
 * (v).erase(unique(all(v)), v.end())*/
#define UNIQUE(v)                                                              \
  sort(all(v));                                                                \
  (v).erase(unique(all(v)), v.end())
// b is [a, c)
#define RANGE(a, b, c) ((a) <= (b) && (b) < (c))
// c is [a, e) && d is [b, f)
#define RANGE2D(a, b, c, d, e, f) (RANGE((a), (c), (e)) && RANGE((b), (d), (f)))
#define chmin(a, b) a = min(a, (b))
#define chmax(a, b) a = max(a, (b))
#define fcout cout << fixed << setprecision(15)
#define YES(f) cout << ((f) ? YES_STR : NO_STR) << endl;

#ifdef LOCAL_DEBUG_OPTION
namespace debug_print_n {
struct DebugPrint {
  template <class T, class U = remove_const_t<remove_reference_t<T>>>
  static enable_if_t<is_integral<U>::value && !is_same<U, bool>::value &&
                     !is_same<U, char>::value>
  DUMP_SUB_FUNCTION(T x) {
    cerr << x;
  }
  template <class T, class U = remove_const_t<remove_reference_t<T>>>
  static enable_if_t<is_same<U, bool>::value> DUMP_SUB_FUNCTION(T x) {
    if (x)
      cerr << "True";
    else
      cerr << "False";
  }
  template <class T, class U = remove_const_t<remove_reference_t<T>>>
  static enable_if_t<is_same<U, char>::value> DUMP_SUB_FUNCTION(T x) {
    cerr << '\'' << x << '\'';
  }
  template <class T, class U = remove_const_t<remove_reference_t<T>>>
  static enable_if_t<is_same<U, string>::value> DUMP_SUB_FUNCTION(T x) {
    cerr << '\"' << x << '\"';
  }
  template <class T>
  static enable_if_t<is_same<T, const char *>::value> DUMP_SUB_FUNCTION(T x) {
    cerr << string(x);
  }
  template <class T> static enable_if_t<true> DUMP_SUB_FUNCTION(vector<T> x) {
    cerr << '{';
    for (size_t i = 0; i < x.size(); i++) {
      if (i)
        cerr << ", ";
      DUMP_SUB_FUNCTION(x[i]);
    }
    cerr << '}';
  }
  template <class T, size_t N>
  static enable_if_t<true> DUMP_SUB_FUNCTION(array<T, N> x) {
    cerr << '{';
    for (size_t i = 0; i < x.size(); i++) {
      if (i)
        cerr << ", ";
      DUMP_SUB_FUNCTION(x[i]);
    }
    cerr << '}';
  }
  template <class T> static enable_if_t<true> DUMP_SUB_FUNCTION(set<T> x) {
    cerr << '[';
    size_t i = 0;
    for (auto itr = x.begin(); itr != x.end(); itr++, i++) {
      if (i)
        cerr << ", ";
      DUMP_SUB_FUNCTION(*itr);
    }
    cerr << ']';
  }
  template <class T, class U>
  static enable_if_t<true> DUMP_SUB_FUNCTION(map<T, U> x) {
    cerr << '[';
    size_t i = 0;
    for (auto itr = x.begin(); itr != x.end(); itr++, i++) {
      if (i)
        cerr << ", ";
      DUMP_SUB_FUNCTION(itr->first);
      cerr << " : ";
      DUMP_SUB_FUNCTION(itr->second);
    }
    cerr << ']';
  }
  template <class T, class U = remove_const_t<remove_reference_t<T>>>
  static enable_if_t<is_floating_point<U>::value> DUMP_SUB_FUNCTION(T x) {
    cerr << fixed << setprecision(5) << x;
  }
  template <class T, class U>
  static enable_if_t<true> DUMP_SUB_FUNCTION(pair<T, U> p) {
    cerr << '(';
    DUMP_SUB_FUNCTION(p.first);
    cerr << ", ";
    DUMP_SUB_FUNCTION(p.second);
    cerr << ')';
  }
  template <int num = 0, class... Args>
  static enable_if_t<num == sizeof...(Args)>
  DUMP_SUB_FUNCTION(tuple<Args...> t) {
    if (num)
      cerr << ')';
  }
  template <int num = 0, class... Args>
      static enable_if_t <
      num<sizeof...(Args)> DUMP_SUB_FUNCTION(tuple<Args...> t) {
    if (num == 0)
      cerr << '(';
    DUMP_SUB_FUNCTION(get<num>(t));
    if (num + 1 < sizeof...(Args))
      cerr << ", ";
    DUMP_SUB_FUNCTION<num + 1>(t);
  }
  template <long unsigned N>
  static enable_if_t<true> DUMP_SUB_FUNCTION(bitset<N> x) {
    cerr << x;
  }
};
void dump() { cerr << endl; }
template <class Head, class... Tail> void dump(Head &&head, Tail &&...tail) {
  DebugPrint::DUMP_SUB_FUNCTION(head);
  if (sizeof...(Tail) != 0) {
    cerr << ' ';
  }
  dump(forward<Tail>(tail)...);
}
} // namespace debug_print_n
using debug_print_n::dump;
#else
template <class... Args> void dump(Args &&...args) {}
#endif

// meguru<T>(ok, ng, isok)で呼ぶ
template <class T> T meguru(T ok, T ng, function<bool(T)> isok) {
  while (abs(ok - ng) > 1) {
    T mid = (ok + ng) / 2;
    (isok(mid) ? ok : ng) = mid;
  }
  return ok;
}

signed main() {
  int N, K;
  cin >> N >> K;
  vector<int> A(N);
  rep(i, N) scanf("%lld", &A[i]);
  sort(all(A));
  auto cnt = [&](int mid) -> int { // mid 以下の数
    int res = 0;
    rep(i, N) {
      if (A[i] < 0) {
        res +=
            i - meguru<int>(i, -1, [&](int p) { return A[p] * A[i] <= mid; });
      } else if (A[i] == 0) {
        if (mid >= 0)
          res += i;
      } else {
        res += meguru<int>(i, -1, [&](int p) { return A[p] * A[i] > mid; });
      }
    }
    return res;
  };
  cout << meguru<int>(2e18, -2e18, [&](auto mid) { return cnt(mid) >= K; })
       << endl;
}