Problem p02774 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <bitset>
#include <climits>
#include <complex>
#include <deque>
#include <iomanip>
#include <iostream>
#include <istream>
#include <iterator>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <vector>
using namespace std;

typedef long long ll;
typedef unsigned long long ull;
// typedef pair<ll, ll> P;
#define rep(i, n) for (ll i = 0; i < (n); i++)
#define revrep(i, n) for (ll i = (n)-1; i >= 0; i--)
#define pb push_back
#define f first
#define s second
#define chmin(x, y) x = min(x, y);
#define chmax(x, y) x = max(x, y);

// const ll INFL = LLONG_MAX;//10^18 = 2^60
const ll INFL = 1LL << 61;
// const int INF = INT_MAX;
const ll INF = 1 << 30; // 10^9
ll MOD = 1000000007;
// ll MOD = 998244353;

vector<ll> dy = {0, 1, 0, -1, 1, 1, -1, -1, 0};
vector<ll> dx = {1, 0, -1, 0, 1, -1, 1, -1, 0};

void pres(double A, ll x = 10) {
  cout << fixed << setprecision(x) << A << endl;
}
void BinarySay(ll x, ll y = 60) {
  rep(i, y) cout << (x >> (y - 1 - i) & 1);
  cout << endl;
}
ll cnt_bit(ll x) { return __builtin_popcountll(x); }

ll pow_long(ll x, ll k) {
  ll res = 1;
  while (k > 0) {
    if (k % 2)
      res *= x;
    x *= x;
    k /= 2;
  }
  return res;
}

ll pow_mod(ll x, ll k) {
  ll res = 1;
  while (k > 0) {
    if (k % 2) {
      res *= x;
      res %= MOD;
    }
    x *= x;
    x %= MOD;
    k /= 2;
  }
  return res;
}

ll inverse(ll x) { return pow_mod(x, MOD - 2); };

ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}

ll lcm(ll x, ll y) {
  ll res = x / gcd(x, y);
  res *= y;
  return res;
};

// コンビネーション
const int MAXcomb = 200010;
ll fac[MAXcomb], finv[MAXcomb], inv[MAXcomb];
// facはn!,finvは1/n!
// invは逆元
void COMinit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (int i = 2; i < MAXcomb; i++) {
    fac[i] = fac[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}
ll comb(int n, int k) {
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * finv[k] % MOD * finv[n - k] % MOD;
}

const int MAXkai = 200010;
ll kai_memo[MAXkai];
ll kai(ll N) {
  if (kai_memo[N] != 0)
    return kai_memo[N];
  if (N <= 1)
    return 1;
  return kai_memo[N] = N * kai(N - 1) % MOD;
}

ll N, K;
ll zero;
vector<ll> P, M, A;

bool can(ll x) {
  ll res = 0;
  if (x > 0) {
    // P, P
    rep(i, P.size()) {
      if (P[0] * P[i] > x)
        continue;
      ll ok = 0, ng = P.size();
      while (ng - ok > 1) {
        ll mid = (ok + ng) / 2;
        if (P[i] * P[mid] <= x)
          ok = mid;
        else
          ng = mid;
      }
      res += ok + 1;
    }
    // M, M
    rep(i, M.size()) {
      if (M[0] * M[i] > x)
        continue;
      ll ok = 0, ng = M.size();
      while (ng - ok > 1) {
        ll mid = (ok + ng) / 2;
        if (M[i] * M[mid] <= x)
          ok = mid;
        else
          ng = mid;
      }
      res += ok + 1;
    }
    rep(i, P.size()) {
      if (P[i] * P[i] <= x)
        res--;
    }
    rep(i, M.size()) {
      if (M[i] * M[i] <= x)
        res--;
    }
    res /= 2;

    res += P.size() * M.size();
    res += zero * (P.size() + M.size());
    res += zero * (zero - 1) / 2;
  } else if (x < 0) {
    // P, M
    rep(i, P.size()) {
      if (-P[i] * M.back() > x)
        continue;
      ll ok = M.size() - 1, ng = -1;
      while (ok - ng > 1) {
        ll mid = (ok + ng) / 2;
        if (-P[i] * M[mid] <= x)
          ok = mid;
        else
          ng = mid;
      }
      res += M.size() - ok;
    }
  } else {
    res += P.size() * M.size();
    res += zero * (P.size() + M.size());
    res += zero * (zero - 1) / 2;
  }
  return res >= K;
}

void solve() {
  cin >> N >> K;
  rep(i, N) {
    ll a;
    cin >> a;
    if (a > 0)
      P.pb(a);
    else if (a < 0)
      M.pb(-a);
    else
      zero++;
    A.pb(a);
  }
  sort(A.begin(), A.end());
  sort(P.begin(), P.end());
  sort(M.begin(), M.end());
  ll ok = INFL, ng = -INFL;
  while (ok - ng > 1) {
    ll mid = (ng + ok) / 2;
    if (can(mid))
      ok = mid;
    else
      ng = mid;
  }
  cout << ok << endl;
}

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  solve();
}


Fixed submission:
#include <algorithm>
#include <bitset>
#include <climits>
#include <complex>
#include <deque>
#include <iomanip>
#include <iostream>
#include <istream>
#include <iterator>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <vector>
using namespace std;

typedef long long ll;
typedef unsigned long long ull;
// typedef pair<ll, ll> P;
#define rep(i, n) for (ll i = 0; i < (n); i++)
#define revrep(i, n) for (ll i = (n)-1; i >= 0; i--)
#define pb push_back
#define f first
#define s second
#define chmin(x, y) x = min(x, y);
#define chmax(x, y) x = max(x, y);

// const ll INFL = LLONG_MAX;//10^18 = 2^60
const ll INFL = 1LL << 61;
// const int INF = INT_MAX;
const ll INF = 1 << 30; // 10^9
ll MOD = 1000000007;
// ll MOD = 998244353;

vector<ll> dy = {0, 1, 0, -1, 1, 1, -1, -1, 0};
vector<ll> dx = {1, 0, -1, 0, 1, -1, 1, -1, 0};

void pres(double A, ll x = 10) {
  cout << fixed << setprecision(x) << A << endl;
}
void BinarySay(ll x, ll y = 60) {
  rep(i, y) cout << (x >> (y - 1 - i) & 1);
  cout << endl;
}
ll cnt_bit(ll x) { return __builtin_popcountll(x); }

ll pow_long(ll x, ll k) {
  ll res = 1;
  while (k > 0) {
    if (k % 2)
      res *= x;
    x *= x;
    k /= 2;
  }
  return res;
}

ll pow_mod(ll x, ll k) {
  ll res = 1;
  while (k > 0) {
    if (k % 2) {
      res *= x;
      res %= MOD;
    }
    x *= x;
    x %= MOD;
    k /= 2;
  }
  return res;
}

ll inverse(ll x) { return pow_mod(x, MOD - 2); };

ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}

ll lcm(ll x, ll y) {
  ll res = x / gcd(x, y);
  res *= y;
  return res;
};

// コンビネーション
const int MAXcomb = 200010;
ll fac[MAXcomb], finv[MAXcomb], inv[MAXcomb];
// facはn!,finvは1/n!
// invは逆元
void COMinit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (int i = 2; i < MAXcomb; i++) {
    fac[i] = fac[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}
ll comb(int n, int k) {
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * finv[k] % MOD * finv[n - k] % MOD;
}

const int MAXkai = 200010;
ll kai_memo[MAXkai];
ll kai(ll N) {
  if (kai_memo[N] != 0)
    return kai_memo[N];
  if (N <= 1)
    return 1;
  return kai_memo[N] = N * kai(N - 1) % MOD;
}

ll N, K;
ll zero;
vector<ll> P, M, A;

bool can(ll x) {
  ll res = 0;
  if (x > 0) {
    // P, P
    rep(i, P.size()) {
      if (P[0] * P[i] > x)
        continue;
      ll ok = 0, ng = P.size();
      while (ng - ok > 1) {
        ll mid = (ok + ng) / 2;
        if (P[i] * P[mid] <= x)
          ok = mid;
        else
          ng = mid;
      }
      res += ok + 1;
    }
    // M, M
    rep(i, M.size()) {
      if (M[0] * M[i] > x)
        continue;
      ll ok = 0, ng = M.size();
      while (ng - ok > 1) {
        ll mid = (ok + ng) / 2;
        if (M[i] * M[mid] <= x)
          ok = mid;
        else
          ng = mid;
      }
      res += ok + 1;
    }
    rep(i, P.size()) {
      if (P[i] * P[i] <= x)
        res--;
    }
    rep(i, M.size()) {
      if (M[i] * M[i] <= x)
        res--;
    }
    res /= 2;

    res += P.size() * M.size();
    res += zero * (P.size() + M.size());
    res += zero * (zero - 1) / 2;
  } else if (x < 0) {
    // P, M
    rep(i, P.size()) {
      if (M.size() == 0)
        continue;
      if (-P[i] * M.back() > x)
        continue;
      ll ok = M.size() - 1, ng = -1;
      while (ok - ng > 1) {
        ll mid = (ok + ng) / 2;
        if (-P[i] * M[mid] <= x)
          ok = mid;
        else
          ng = mid;
      }
      res += M.size() - ok;
    }
  } else {
    res += P.size() * M.size();
    res += zero * (P.size() + M.size());
    res += zero * (zero - 1) / 2;
  }
  return res >= K;
}

void solve() {
  cin >> N >> K;
  rep(i, N) {
    ll a;
    cin >> a;
    if (a > 0)
      P.pb(a);
    else if (a < 0)
      M.pb(-a);
    else
      zero++;
    A.pb(a);
  }
  sort(A.begin(), A.end());
  sort(P.begin(), P.end());
  sort(M.begin(), M.end());
  ll ok = INFL, ng = -INFL;
  while (ok - ng > 1) {
    ll mid = (ng + ok) / 2;
    if (can(mid))
      ok = mid;
    else
      ng = mid;
  }
  cout << ok << endl;
}

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  solve();
}