Problem p02821 (C++)

Buggy submission (status: Runtime Error):
//{{{
#include <bits/stdc++.h>
using namespace std;

#define repX(a, b, c, x, ...) x
#define repN(a) repX a
#define rep(...) repN((__VA_ARGS__, rep3, rep2, loop))(__VA_ARGS__)
#define rrep(...) repN((__VA_ARGS__, rrep3, rrep2))(__VA_ARGS__)
#define loop(n) rep2(i_, n)
#define rep2(i, n) rep3(i, 0, n)
#define rep3(i, begin, end)                                                    \
  for (int i = (int)(begin), i##_end = (int)(end); i < i##_end; ++i)
#define rrep2(i, n) rrep3(i, 0, n)
#define rrep3(i, begin, end)                                                   \
  for (int i = (int)(end - 1), i##_end = (int)(begin); i >= i##_end; --i)
#define foreach(x, a) for (auto &x : a)
#define each(x, a) for (auto &x : a)
#define sz(x) ((int)(x).size())

struct IoSetup {
  IoSetup() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(10);
    cerr << fixed << setprecision(10);
  };
} ioSetup;

using ll = long long;

template <class T, class U>
ostream &operator<<(ostream &o, const pair<T, U> &j) {
  o << "{" << j.first << ", " << j.second << "}";
  return o;
}
template <class T, class U>
ostream &operator<<(ostream &o, const map<T, U> &j) {
  o << "{";
  for (auto t = j.begin(); t != j.end(); ++t)
    o << (t != j.begin() ? ", " : "") << *t;
  o << "}";
  return o;
}
template <class T> ostream &operator<<(ostream &o, const set<T> &j) {
  o << "{";
  for (auto t = j.begin(); t != j.end(); ++t)
    o << (t != j.begin() ? ", " : "") << *t;
  o << "}";
  return o;
}
template <class T> ostream &operator<<(ostream &o, const multiset<T> &j) {
  o << "{";
  for (auto t = j.begin(); t != j.end(); ++t)
    o << (t != j.begin() ? ", " : "") << *t;
  o << "}";
  return o;
}
template <class T> ostream &operator<<(ostream &o, const vector<T> &j) {
  o << "{";
  for (int i = 0; i < (int)j.size(); ++i)
    o << (i > 0 ? ", " : "") << j[i];
  o << "}";
  return o;
}
#ifdef LOCAL
inline void dump(void) { cerr << endl; }
template <class Head, class... Tail>
inline void dump(Head &&head, Tail &&...tail) {
  cerr << head << " ";
  dump(tail...);
}
#define debug(...)                                                             \
  do {                                                                         \
    cerr << "[L." << __LINE__ << "]\t" << #__VA_ARGS__ << "=";                 \
    dump(__VA_ARGS__);                                                         \
  } while (0)
#else
#define dump(...)
#define debug(...)
#endif

template <class T> inline void sort(T &a) { sort(a.begin(), a.end()); }
template <class T, class Compare> inline void sort(T &a, Compare comp) {
  sort(a.begin(), a.end(), comp);
}
template <class T> inline void rsort(T &a) { sort(a.rbegin(), a.rend()); }
template <class T> inline void reverse(T &a) { reverse(a.begin(), a.end()); }
template <class T> inline T Sum(vector<T> &a) {
  return accumulate(a.begin(), a.end(), (T)0);
}
template <class T> inline T max(vector<T> &a) {
  return *max_element(a.begin(), a.end());
}
template <class T> inline T min(vector<T> &a) {
  return *min_element(a.begin(), a.end());
}

template <class T, class U> inline bool chmax(T &a, const U &b) {
  return (b > a) ? (a = b, true) : false;
}
template <class T, class U> inline bool chmin(T &a, const U &b) {
  return (b < a) ? (a = b, true) : false;
}

ll gcd(const ll a, const ll b) { return b ? gcd(b, a % b) : a; }
ll lcm(const ll a, const ll b) { return a / gcd(a, b) * b; }

class in {
  int n;

public:
  in() { n = -1; }
  in(int n_) : n(n_){};
  template <class T> operator T() {
    T ret;
    cin >> ret;
    return ret;
  }
  template <class T> operator vector<T>() {
    assert(n >= 0);
    vector<T> ret(n);
    for (int i = 0; i < n; ++i)
      cin >> ret[i];
    return ret;
  }
};

template <class T> void print(const T &a) { cout << a; }
int out() {
  cout << '\n';
  return 0;
}
template <class T> int out(const T &t) {
  print(t);
  cout << '\n';
  return 0;
}
template <class Head, class... Tail>
int out(const Head &head, const Tail &...tail) {
  print(head);
  cout << " ";
  out(tail...);
  return 0;
}

template <std::uint_fast64_t Mod> class Modular {
  using u64 = std::uint_fast64_t;

public:
  u64 a;
  constexpr Modular(const u64 x = 0) noexcept : a(x % Mod) {}
  constexpr u64 val() const noexcept { return a; }
  constexpr Modular operator+(const Modular rhs) const noexcept {
    return Modular(*this) += rhs;
  }
  constexpr Modular operator-(const Modular rhs) const noexcept {
    return Modular(*this) -= rhs;
  }
  constexpr Modular operator*(const Modular rhs) const noexcept {
    return Modular(*this) *= rhs;
  }
  constexpr Modular operator/(const Modular rhs) const noexcept {
    return Modular(*this) /= rhs;
  }
  constexpr bool operator==(const Modular rhs) const noexcept {
    return Modular(*this).val() == rhs.val();
  }
  Modular &operator+=(const Modular rhs) noexcept {
    a += rhs.a;
    if (a >= Mod)
      a -= Mod;
    return *this;
  }
  Modular &operator-=(const Modular rhs) noexcept {
    if (a < rhs.a)
      a += Mod;
    a -= rhs.a;
    return *this;
  }
  Modular &operator++() noexcept { return *this += 1; }
  Modular &operator--() noexcept { return *this -= 1; }
  Modular &operator*=(const Modular rhs) noexcept {
    a = a * rhs.a % Mod;
    return *this;
  }
  Modular &operator/=(Modular rhs) noexcept {
    u64 exp = Mod - 2;
    while (exp) {
      if (exp % 2)
        *this *= rhs;
      rhs *= rhs;
      exp /= 2;
    }
    return *this;
  }
};
template <std::uint_fast64_t Mod>
ostream &operator<<(ostream &os, const Modular<Mod> &m) {
  return os << m.a;
}

const double pi = acos(-1);
const double eps = 1e-9;
const ll inf = 1001001001;
const ll mod = (ll)1e9 + 7;
using mint = Modular<mod>;

//}}}

int main() {
  ll N = in();
  ll M = in();
  vector<ll> A = in(N);
  sort(A);
  vector<ll> B(A);
  reverse(B);

  // 上位 i 番目までの人の値の和
  vector<ll> cum(N);
  cum[0] = 0;
  rep(i, 1, N + 1) { cum[i] = cum[i - 1] + B[i - 1]; }

  // 和がx以上の組がM組以上ならtrue
  auto f = [&](ll x) {
    ll cnt = 0;
    rep(i, N) {
      ll num = A.end() - lower_bound(A.begin(), A.end(), x - A[i]);
      cnt += num;
    }
    return cnt >= M;
  };

  // x以上の要素をM組まで足した値を返す。
  // ただし、上位M組目の和がちょうどxであることがわかっているものとする。
  auto g = [&](ll x) {
    ll res = 0;
    ll cnt = 0;
    rep(i, N) {
      ll num = A.end() - lower_bound(A.begin(), A.end(), x - A[i]);
      cnt += num;
      res += num * A[i] + cum[num];
    }
    res -= (cnt - M) * x;
    return res;
  };

  ll ok = 0;
  ll ng = A[N - 1] * 2 + 1;
  while (abs(ok - ng) > 1) {
    ll md = (ok + ng) / 2;
    (f(md) ? ok : ng) = md;
  }
  ll ans = g(ok);
  out(ans);
}


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

#define repX(a, b, c, x, ...) x
#define repN(a) repX a
#define rep(...) repN((__VA_ARGS__, rep3, rep2, loop))(__VA_ARGS__)
#define rrep(...) repN((__VA_ARGS__, rrep3, rrep2))(__VA_ARGS__)
#define loop(n) rep2(i_, n)
#define rep2(i, n) rep3(i, 0, n)
#define rep3(i, begin, end)                                                    \
  for (int i = (int)(begin), i##_end = (int)(end); i < i##_end; ++i)
#define rrep2(i, n) rrep3(i, 0, n)
#define rrep3(i, begin, end)                                                   \
  for (int i = (int)(end - 1), i##_end = (int)(begin); i >= i##_end; --i)
#define foreach(x, a) for (auto &x : a)
#define each(x, a) for (auto &x : a)
#define sz(x) ((int)(x).size())

struct IoSetup {
  IoSetup() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(10);
    cerr << fixed << setprecision(10);
  };
} ioSetup;

using ll = long long;

template <class T, class U>
ostream &operator<<(ostream &o, const pair<T, U> &j) {
  o << "{" << j.first << ", " << j.second << "}";
  return o;
}
template <class T, class U>
ostream &operator<<(ostream &o, const map<T, U> &j) {
  o << "{";
  for (auto t = j.begin(); t != j.end(); ++t)
    o << (t != j.begin() ? ", " : "") << *t;
  o << "}";
  return o;
}
template <class T> ostream &operator<<(ostream &o, const set<T> &j) {
  o << "{";
  for (auto t = j.begin(); t != j.end(); ++t)
    o << (t != j.begin() ? ", " : "") << *t;
  o << "}";
  return o;
}
template <class T> ostream &operator<<(ostream &o, const multiset<T> &j) {
  o << "{";
  for (auto t = j.begin(); t != j.end(); ++t)
    o << (t != j.begin() ? ", " : "") << *t;
  o << "}";
  return o;
}
template <class T> ostream &operator<<(ostream &o, const vector<T> &j) {
  o << "{";
  for (int i = 0; i < (int)j.size(); ++i)
    o << (i > 0 ? ", " : "") << j[i];
  o << "}";
  return o;
}
#ifdef LOCAL
inline void dump(void) { cerr << endl; }
template <class Head, class... Tail>
inline void dump(Head &&head, Tail &&...tail) {
  cerr << head << " ";
  dump(tail...);
}
#define debug(...)                                                             \
  do {                                                                         \
    cerr << "[L." << __LINE__ << "]\t" << #__VA_ARGS__ << "=";                 \
    dump(__VA_ARGS__);                                                         \
  } while (0)
#else
#define dump(...)
#define debug(...)
#endif

template <class T> inline void sort(T &a) { sort(a.begin(), a.end()); }
template <class T, class Compare> inline void sort(T &a, Compare comp) {
  sort(a.begin(), a.end(), comp);
}
template <class T> inline void rsort(T &a) { sort(a.rbegin(), a.rend()); }
template <class T> inline void reverse(T &a) { reverse(a.begin(), a.end()); }
template <class T> inline T Sum(vector<T> &a) {
  return accumulate(a.begin(), a.end(), (T)0);
}
template <class T> inline T max(vector<T> &a) {
  return *max_element(a.begin(), a.end());
}
template <class T> inline T min(vector<T> &a) {
  return *min_element(a.begin(), a.end());
}

template <class T, class U> inline bool chmax(T &a, const U &b) {
  return (b > a) ? (a = b, true) : false;
}
template <class T, class U> inline bool chmin(T &a, const U &b) {
  return (b < a) ? (a = b, true) : false;
}

ll gcd(const ll a, const ll b) { return b ? gcd(b, a % b) : a; }
ll lcm(const ll a, const ll b) { return a / gcd(a, b) * b; }

class in {
  int n;

public:
  in() { n = -1; }
  in(int n_) : n(n_){};
  template <class T> operator T() {
    T ret;
    cin >> ret;
    return ret;
  }
  template <class T> operator vector<T>() {
    assert(n >= 0);
    vector<T> ret(n);
    for (int i = 0; i < n; ++i)
      cin >> ret[i];
    return ret;
  }
};

template <class T> void print(const T &a) { cout << a; }
int out() {
  cout << '\n';
  return 0;
}
template <class T> int out(const T &t) {
  print(t);
  cout << '\n';
  return 0;
}
template <class Head, class... Tail>
int out(const Head &head, const Tail &...tail) {
  print(head);
  cout << " ";
  out(tail...);
  return 0;
}

template <std::uint_fast64_t Mod> class Modular {
  using u64 = std::uint_fast64_t;

public:
  u64 a;
  constexpr Modular(const u64 x = 0) noexcept : a(x % Mod) {}
  constexpr u64 val() const noexcept { return a; }
  constexpr Modular operator+(const Modular rhs) const noexcept {
    return Modular(*this) += rhs;
  }
  constexpr Modular operator-(const Modular rhs) const noexcept {
    return Modular(*this) -= rhs;
  }
  constexpr Modular operator*(const Modular rhs) const noexcept {
    return Modular(*this) *= rhs;
  }
  constexpr Modular operator/(const Modular rhs) const noexcept {
    return Modular(*this) /= rhs;
  }
  constexpr bool operator==(const Modular rhs) const noexcept {
    return Modular(*this).val() == rhs.val();
  }
  Modular &operator+=(const Modular rhs) noexcept {
    a += rhs.a;
    if (a >= Mod)
      a -= Mod;
    return *this;
  }
  Modular &operator-=(const Modular rhs) noexcept {
    if (a < rhs.a)
      a += Mod;
    a -= rhs.a;
    return *this;
  }
  Modular &operator++() noexcept { return *this += 1; }
  Modular &operator--() noexcept { return *this -= 1; }
  Modular &operator*=(const Modular rhs) noexcept {
    a = a * rhs.a % Mod;
    return *this;
  }
  Modular &operator/=(Modular rhs) noexcept {
    u64 exp = Mod - 2;
    while (exp) {
      if (exp % 2)
        *this *= rhs;
      rhs *= rhs;
      exp /= 2;
    }
    return *this;
  }
};
template <std::uint_fast64_t Mod>
ostream &operator<<(ostream &os, const Modular<Mod> &m) {
  return os << m.a;
}

const double pi = acos(-1);
const double eps = 1e-9;
const ll inf = 1001001001;
const ll mod = (ll)1e9 + 7;
using mint = Modular<mod>;

//}}}

int main() {
  ll N = in();
  ll M = in();
  vector<ll> A = in(N);
  sort(A);
  vector<ll> B(A);
  reverse(B);

  // 上位 i 番目までの人の値の和
  vector<ll> cum(N + 1);
  cum[0] = 0;
  rep(i, 1, N + 1) { cum[i] = cum[i - 1] + B[i - 1]; }

  // 和がx以上の組がM組以上ならtrue
  auto f = [&](ll x) {
    ll cnt = 0;
    rep(i, N) {
      ll num = A.end() - lower_bound(A.begin(), A.end(), x - A[i]);
      cnt += num;
    }
    return cnt >= M;
  };

  // x以上の要素をM組まで足した値を返す。
  // ただし、上位M組目の和がちょうどxであることがわかっているものとする。
  auto g = [&](ll x) {
    ll res = 0;
    ll cnt = 0;
    rep(i, N) {
      ll num = A.end() - lower_bound(A.begin(), A.end(), x - A[i]);
      cnt += num;
      res += num * A[i] + cum[num];
    }
    res -= (cnt - M) * x;
    return res;
  };

  ll ok = 0;
  ll ng = A[N - 1] * 2 + 1;
  while (abs(ok - ng) > 1) {
    ll md = (ok + ng) / 2;
    (f(md) ? ok : ng) = md;
  }
  ll ans = g(ok);
  out(ans);
}