Problem p02774 (C++)

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

// template {{{
#define pb push_back
#define eb emplace_back
#define mp make_pair
#define mt make_tuple
#define lb lower_bound
#define ub upper_bound
#define f first
#define s second
#define resz resize

#define sz(x) int((x).size())
#define all(x) (x).begin(), (x).end()

#define FOR(i, a, b) for (int i = (a); i < (b); i++)
#define F0R(i, a) for (int i = 0; i < (a); i++)
#define FORd(i, a, b) for (int i = (b)-1; i >= (a); i--)
#define F0Rd(i, a) for (int i = (a)-1; i >= 0; i--)
#define trav(a, x) for (auto &a : x)

#define sort_by(x, y)                                                          \
  sort(all(x), [&](const auto &a, const auto &b) { return y; })

using ll = long long;
using vi = vector<int>;
using vvi = vector<vi>;
using vll = vector<ll>;
using vvll = vector<vll>;
using vb = vector<bool>;
using vd = vector<double>;
using vs = vector<string>;

using pii = pair<int, int>;
using pll = pair<ll, ll>;
using pdd = pair<double, double>;

using vpii = vector<pii>;
using vvpii = vector<vpii>;
using vpll = vector<pll>;
using vvpll = vector<vpll>;
using vpdd = vector<pdd>;
using vvpdd = vector<vpdd>;

template <typename T> void ckmin(T &a, const T &b) { a = min(a, b); }
template <typename T> void ckmax(T &a, const T &b) { a = max(a, b); }

mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());

namespace __input {
template <class T1, class T2> void re(pair<T1, T2> &p);
template <class T> void re(vector<T> &a);
template <class T, size_t SZ> void re(array<T, SZ> &a);

template <class T> void re(T &x) { cin >> x; }
void re(double &x) {
  string t;
  re(t);
  x = stod(t);
}
template <class Arg, class... Args> void re(Arg &first, Args &...rest) {
  re(first);
  re(rest...);
}

template <class T1, class T2> void re(pair<T1, T2> &p) { re(p.f, p.s); }
template <class T> void re(vector<T> &a) { F0R(i, sz(a)) re(a[i]); }
template <class T, size_t SZ> void re(array<T, SZ> &a) { F0R(i, SZ) re(a[i]); }
} // namespace __input
using namespace __input;

namespace __output {
template <class T1, class T2> void pr(const pair<T1, T2> &x);
template <class T, size_t SZ> void pr(const array<T, SZ> &x);
template <class T> void pr(const vector<T> &x);
template <class T> void pr(const deque<T> &x);
template <class T> void pr(const set<T> &x);
template <class T1, class T2> void pr(const map<T1, T2> &x);

template <class T> void pr(const T &x) { cout << x; }
template <class Arg, class... Args>
void pr(const Arg &first, const Args &...rest) {
  pr(first);
  pr(rest...);
}

template <class T1, class T2> void pr(const pair<T1, T2> &x) {
  pr("{", x.f, ", ", x.s, "}");
}
template <class T, bool pretty = true> void prContain(const T &x) {
  if (pretty)
    pr("{");
  bool fst = 1;
  for (const auto &a : x)
    pr(!fst ? pretty ? ", " : " " : "", a), fst = 0;
  if (pretty)
    pr("}");
}
template <class T> void pc(const T &x) {
  prContain<T, false>(x);
  pr("\n");
}
template <class T, size_t SZ> void pr(const array<T, SZ> &x) { prContain(x); }
template <class T> void pr(const vector<T> &x) { prContain(x); }
template <class T> void pr(const deque<T> &x) { prContain(x); }
template <class T> void pr(const set<T> &x) { prContain(x); }
template <class T1, class T2> void pr(const map<T1, T2> &x) { prContain(x); }

void ps() { pr("\n"); }
template <class Arg> void ps(const Arg &first) {
  pr(first);
  ps();
}
template <class Arg, class... Args>
void ps(const Arg &first, const Args &...rest) {
  pr(first, " ");
  ps(rest...);
}
} // namespace __output
using namespace __output;

#define TRACE(x) x
#define __pn(x) pr(#x, " = ")
#define pd(...) __pn((__VA_ARGS__)), ps(__VA_ARGS__), cout << flush

namespace __algorithm {
template <typename T> void dedup(vector<T> &v) {
  sort(all(v));
  v.erase(unique(all(v)), v.end());
}
template <typename T>
typename vector<T>::iterator find(vector<T> &v, const T &x) {
  auto it = lower_bound(all(v), x);
  return it != v.end() && *it == x ? it : v.end();
}
template <typename T> size_t index(vector<T> &v, const T &x) {
  auto it = find(v, x);
  assert(it != v.end() && *it == x);
  return it - v.begin();
}
template <typename C, typename T, typename OP>
vector<T> prefixes(const C &v, T id, OP op) {
  vector<T> r(sz(v) + 1, id);
  F0R(i, sz(v)) r[i + 1] = op(r[i], v[i]);
  return r;
}
template <typename C, typename T, typename OP>
vector<T> suffixes(const C &v, T id, OP op) {
  vector<T> r(sz(v) + 1, id);
  F0Rd(i, sz(v)) r[i] = op(v[i], r[i + 1]);
  return r;
}
} // namespace __algorithm
using namespace __algorithm;

#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-parameter"
struct monostate {
  friend istream &operator>>(istream &is, const monostate &ms) { return is; }
  friend ostream &operator<<(ostream &os, const monostate &ms) { return os; }
  friend monostate operator+(const monostate &a, const monostate &b) {
    return a;
  }
} ms;
#pragma GCC diagnostic pop

namespace __io {
void setIn(string s) { freopen(s.c_str(), "r", stdin); }
void setOut(string s) { freopen(s.c_str(), "w", stdout); }
void setIO(string s = "") {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout << fixed << setprecision(15);
  if (sz(s)) {
    setIn(s + ".in"), setOut(s + ".out");
  }
}
} // namespace __io
using namespace __io;
// }}}

// integer division in C++ rounds towards 0

ll floor(ll top, ll bot) {
  assert(bot > 0);
  if (top < 0) {
    return (top - bot + 1) / bot;
  } else {
    return top / bot;
  }
}

ll ceil(ll top, ll bot) {
  assert(bot > 0);
  if (top < 0) {
    return top / bot;
  } else {
    return (top + bot - 1) / bot;
  }
}

int main() {
  setIO();

  int N;
  ll K;
  re(N, K);
  vi a(N);
  re(a);

  sort(all(a));

  auto count_geq = [&](ll v) {
    ll ans = 0;
    trav(_fv, a) {
      ll fv = _fv;

      if (fv == 0) {
        if (v <= 0)
          ans += N - 1;
        continue;
      }

      if (fv < 0) { // < 0
        ll minv = floor(-v, -fv);
        ans += ub(all(a), '0',
                  [&](char _, int pos) {
                    // return fv * pos < v;
                    // want pos > v/fv
                    return pos > minv;
                  }) -
               a.begin();
      } else { // >= 0
        ll minv = ceil(v, fv);
        ans += a.end() - ub(all(a), '0', [&](char _, int pos) {
                 // return fv * pos >= v;
                 // want pos >= v / fv
                 return pos >= minv;
               });
      }

      if (fv * fv >= v)
        ans--;
    }
    return ans / 2;
  };
  auto count_lt = [&](ll v) { return ll(N) * (N - 1) / 2 - count_geq(v); };

  --K;

  ll lo = -1e18, hi = 1e18 + 1;
  while (hi - lo > 1) {
    ll mi = (lo + 1 + hi) / 2;
    if (count_lt(mi) <= K)
      lo = mi;
    else
      hi = mi;
  }

  ps(lo);

  // did you check N=1? did you mix up N,M?
  // check your "infinity" and "null" values against the bounds
  return 0;
}


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

// template {{{
#define pb push_back
#define eb emplace_back
#define mp make_pair
#define mt make_tuple
#define lb lower_bound
#define ub upper_bound
#define f first
#define s second
#define resz resize

#define sz(x) int((x).size())
#define all(x) (x).begin(), (x).end()

#define FOR(i, a, b) for (int i = (a); i < (b); i++)
#define F0R(i, a) for (int i = 0; i < (a); i++)
#define FORd(i, a, b) for (int i = (b)-1; i >= (a); i--)
#define F0Rd(i, a) for (int i = (a)-1; i >= 0; i--)
#define trav(a, x) for (auto &a : x)

#define sort_by(x, y)                                                          \
  sort(all(x), [&](const auto &a, const auto &b) { return y; })

using ll = long long;
using vi = vector<int>;
using vvi = vector<vi>;
using vll = vector<ll>;
using vvll = vector<vll>;
using vb = vector<bool>;
using vd = vector<double>;
using vs = vector<string>;

using pii = pair<int, int>;
using pll = pair<ll, ll>;
using pdd = pair<double, double>;

using vpii = vector<pii>;
using vvpii = vector<vpii>;
using vpll = vector<pll>;
using vvpll = vector<vpll>;
using vpdd = vector<pdd>;
using vvpdd = vector<vpdd>;

template <typename T> void ckmin(T &a, const T &b) { a = min(a, b); }
template <typename T> void ckmax(T &a, const T &b) { a = max(a, b); }

mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());

namespace __input {
template <class T1, class T2> void re(pair<T1, T2> &p);
template <class T> void re(vector<T> &a);
template <class T, size_t SZ> void re(array<T, SZ> &a);

template <class T> void re(T &x) { cin >> x; }
void re(double &x) {
  string t;
  re(t);
  x = stod(t);
}
template <class Arg, class... Args> void re(Arg &first, Args &...rest) {
  re(first);
  re(rest...);
}

template <class T1, class T2> void re(pair<T1, T2> &p) { re(p.f, p.s); }
template <class T> void re(vector<T> &a) { F0R(i, sz(a)) re(a[i]); }
template <class T, size_t SZ> void re(array<T, SZ> &a) { F0R(i, SZ) re(a[i]); }
} // namespace __input
using namespace __input;

namespace __output {
template <class T1, class T2> void pr(const pair<T1, T2> &x);
template <class T, size_t SZ> void pr(const array<T, SZ> &x);
template <class T> void pr(const vector<T> &x);
template <class T> void pr(const deque<T> &x);
template <class T> void pr(const set<T> &x);
template <class T1, class T2> void pr(const map<T1, T2> &x);

template <class T> void pr(const T &x) { cout << x; }
template <class Arg, class... Args>
void pr(const Arg &first, const Args &...rest) {
  pr(first);
  pr(rest...);
}

template <class T1, class T2> void pr(const pair<T1, T2> &x) {
  pr("{", x.f, ", ", x.s, "}");
}
template <class T, bool pretty = true> void prContain(const T &x) {
  if (pretty)
    pr("{");
  bool fst = 1;
  for (const auto &a : x)
    pr(!fst ? pretty ? ", " : " " : "", a), fst = 0;
  if (pretty)
    pr("}");
}
template <class T> void pc(const T &x) {
  prContain<T, false>(x);
  pr("\n");
}
template <class T, size_t SZ> void pr(const array<T, SZ> &x) { prContain(x); }
template <class T> void pr(const vector<T> &x) { prContain(x); }
template <class T> void pr(const deque<T> &x) { prContain(x); }
template <class T> void pr(const set<T> &x) { prContain(x); }
template <class T1, class T2> void pr(const map<T1, T2> &x) { prContain(x); }

void ps() { pr("\n"); }
template <class Arg> void ps(const Arg &first) {
  pr(first);
  ps();
}
template <class Arg, class... Args>
void ps(const Arg &first, const Args &...rest) {
  pr(first, " ");
  ps(rest...);
}
} // namespace __output
using namespace __output;

#define TRACE(x) x
#define __pn(x) pr(#x, " = ")
#define pd(...) __pn((__VA_ARGS__)), ps(__VA_ARGS__), cout << flush

namespace __algorithm {
template <typename T> void dedup(vector<T> &v) {
  sort(all(v));
  v.erase(unique(all(v)), v.end());
}
template <typename T>
typename vector<T>::iterator find(vector<T> &v, const T &x) {
  auto it = lower_bound(all(v), x);
  return it != v.end() && *it == x ? it : v.end();
}
template <typename T> size_t index(vector<T> &v, const T &x) {
  auto it = find(v, x);
  assert(it != v.end() && *it == x);
  return it - v.begin();
}
template <typename C, typename T, typename OP>
vector<T> prefixes(const C &v, T id, OP op) {
  vector<T> r(sz(v) + 1, id);
  F0R(i, sz(v)) r[i + 1] = op(r[i], v[i]);
  return r;
}
template <typename C, typename T, typename OP>
vector<T> suffixes(const C &v, T id, OP op) {
  vector<T> r(sz(v) + 1, id);
  F0Rd(i, sz(v)) r[i] = op(v[i], r[i + 1]);
  return r;
}
} // namespace __algorithm
using namespace __algorithm;

#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-parameter"
struct monostate {
  friend istream &operator>>(istream &is, const monostate &ms) { return is; }
  friend ostream &operator<<(ostream &os, const monostate &ms) { return os; }
  friend monostate operator+(const monostate &a, const monostate &b) {
    return a;
  }
} ms;
#pragma GCC diagnostic pop

namespace __io {
void setIn(string s) { freopen(s.c_str(), "r", stdin); }
void setOut(string s) { freopen(s.c_str(), "w", stdout); }
void setIO(string s = "") {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout << fixed << setprecision(15);
  if (sz(s)) {
    setIn(s + ".in"), setOut(s + ".out");
  }
}
} // namespace __io
using namespace __io;
// }}}

// integer division in C++ rounds towards 0

ll floor(ll top, ll bot) {
  assert(bot > 0);
  if (top < 0) {
    return (top - bot + 1) / bot;
  } else {
    return top / bot;
  }
}

ll ceil(ll top, ll bot) {
  assert(bot > 0);
  if (top < 0) {
    return top / bot;
  } else {
    return (top + bot - 1) / bot;
  }
}

int main() {
  setIO();

  int N;
  ll K;
  re(N, K);
  vi a(N);
  re(a);

  sort(all(a));

  auto count_geq = [&](ll v) {
    ll ans = 0;
    trav(_fv, a) {
      ll fv = _fv;

      if (fv == 0) {
        if (v <= 0)
          ans += N - 1;
        continue;
      }

      if (fv < 0) { // < 0
        ll minv = floor(-v, -fv);
        ans += ub(all(a), '0',
                  [&](char _, int pos) {
                    // return fv * pos < v;
                    // want pos > v/fv
                    return pos > minv;
                  }) -
               a.begin();
      } else { // >= 0
        ll minv = ceil(v, fv);
        ans += a.end() - ub(all(a), '0', [&](char _, int pos) {
                 // return fv * pos >= v;
                 // want pos >= v / fv
                 return pos >= minv;
               });
      }

      if (fv * fv >= v)
        ans--;
    }
    return ans / 2;
  };
  auto count_lt = [&](ll v) { return ll(N) * (N - 1) / 2 - count_geq(v); };

  --K;

  ll lo = -1e18, hi = 1e18 + 1;
  while (hi - lo > 1) {
    ll mi = lo + (hi - lo) / 2;
    if (count_lt(mi) <= K)
      lo = mi;
    else
      hi = mi;
  }

  ps(lo);

  // did you check N=1? did you mix up N,M?
  // check your "infinity" and "null" values against the bounds
  return 0;
}