Problem p02821 (C++)

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

typedef long long ll;
#define pa pair<int, int>
#define pal pair<long long, long long>
#define pali pair<long long, int>
#define pad pair<double, double>
#define pb push_back
#define mp make_pair
#define COUT(v)                                                                \
  for (int64_t i = 0; i < (v).size(); ++i) {                                   \
    cout << v.at(i) << endl;                                                   \
  }
#define REP(i, n) for (int64_t i = 0; i < n; ++i)
#define FOR(i, r, n) for (int64_t i = (r); i < n; ++i)
#define VIN(v)                                                                 \
  for (int64_t i = 0; i < (v).size(); ++i) {                                   \
    cin >> (v).at(i);                                                          \
  }

typedef vector<bool> bvec;
typedef vector<int> ivec;
typedef vector<long long> lvec;
typedef vector<double> dvec;
typedef vector<pa> pavec;
typedef vector<pali> palivec;
typedef vector<pal> palvec;

typedef vector<vector<bool>> bmat;
typedef vector<vector<int>> imat;
typedef vector<vector<long long>> lmat;

typedef vector<string> svec;
typedef vector<vector<string>> smat;
const ll infll = (1LL << 60) - 1;
const int inf = (1 << 30) - 1;
const ll MOD = 1000000007;

ll gcd(ll x, ll y) {
  ll r = x % y;
  if (r == 0)
    return y;
  else
    return gcd(y, r);
}

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

lvec mfactor(ll n) {
  bvec ip(n, true);
  lvec mf(n, -1);
  ip[0] = false;
  ip[1] = false;
  mf[0] = 0;
  mf[1] = 1;
  REP(i, n) {
    if (ip[i]) {
      mf[i] = i;
      for (ll j = i * i; j < n; j += i) {
        ip[j] = false;
        if (mf[j] == -1)
          mf[j] = i;
      }
    }
  }
  return mf;
}

palivec get_prime(ll n, const lvec &mf) {
  palivec plist;
  while (n != 1) {
    int cnt = 0;
    ll m = mf[n];
    while (mf[n] == m) {
      cnt++;
      n /= m;
    }
    plist.pb(pali(m, cnt));
  }
  return plist;
}

void COMinit(int m, lvec &fac, lvec &finv) {
  lvec inv(m);
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (int i = 2; i < m; 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 pow_mod(ll a, ll n) {
  ll x = 1;
  while (n > 0) {
    if (n & 1) {
      x = x * a % MOD;
    }
    a = a * a % MOD;
    n >>= 1;
  }
  return x;
}

ll COM(int n, int k, const lvec &fac, const lvec &finv) {
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return (fac[n] * (finv[k] * finv[n - k] % MOD)) % MOD;
}

ll modinv(ll a, ll m) {
  ll b = m, u = 1, v = 0;
  while (b) {
    ll t = a / b;
    a -= t * b;
    swap(a, b);
    u -= t * v;
    swap(u, v);
  }
  u %= m;
  if (u < 0)
    u += m;
  return u;
}

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  ll n, m;
  cin >> n >> m;
  lvec a(n);
  lvec s(n);
  for (int i = 0; i < n; i++) {
    cin >> a[i];
  }

  sort(a.rbegin(), a.rend());
  ll ok = 2 * a[0] + 1, ng = 1;
  while (ok - ng > 1) {
    ll t = (ng + ok) / 2;
    ll cnt = 0;
    ll i = 0, j = n - 1;
    while (i < n && j >= 0) {
      if (a[i] + a[j] > t)
        cnt += j + 1;
      else {
        if (j > 0) {
          j--;
          continue;
        } else
          j = n - 1;
      }
      i++;
    }
    if (cnt < m)
      ok = t;
    else
      ng = t;
  }
  s[0] = a[0];
  for (int i = 1; i < n; i++) {
    s[i] = a[i] + s[i - 1];
  }

  ll ans = 0;
  ll cnt = 0;
  for (int i = 0; i < n; i++) {
    ll ok2 = 0, ng2 = n;
    while (ng2 - ok2 > 1) {
      ll t = (ng2 + ok2) / 2;
      if (a[i] + a[t] >= ok)
        ok2 = t;
      else
        ng2 = t;
    }
    if (ok2 > 0) {
      ans += a[i] * (ok2 + 1) + s[ok2];
      cnt += ok2 + 1;
    } else if (a[i] + a[ok2] >= ok) {
      ans += (a[i] + a[ok2]);
      cnt++;
    }
  }

  ans -= ok * (cnt - m);
  cout << ans << endl;
}


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

typedef long long ll;
#define pa pair<int, int>
#define pal pair<long long, long long>
#define pali pair<long long, int>
#define pad pair<double, double>
#define pb push_back
#define mp make_pair
#define COUT(v)                                                                \
  for (int64_t i = 0; i < (v).size(); ++i) {                                   \
    cout << v.at(i) << endl;                                                   \
  }
#define REP(i, n) for (int64_t i = 0; i < n; ++i)
#define FOR(i, r, n) for (int64_t i = (r); i < n; ++i)
#define VIN(v)                                                                 \
  for (int64_t i = 0; i < (v).size(); ++i) {                                   \
    cin >> (v).at(i);                                                          \
  }

typedef vector<bool> bvec;
typedef vector<int> ivec;
typedef vector<long long> lvec;
typedef vector<double> dvec;
typedef vector<pa> pavec;
typedef vector<pali> palivec;
typedef vector<pal> palvec;

typedef vector<vector<bool>> bmat;
typedef vector<vector<int>> imat;
typedef vector<vector<long long>> lmat;

typedef vector<string> svec;
typedef vector<vector<string>> smat;
const ll infll = (1LL << 60) - 1;
const int inf = (1 << 30) - 1;
const ll MOD = 1000000007;

ll gcd(ll x, ll y) {
  ll r = x % y;
  if (r == 0)
    return y;
  else
    return gcd(y, r);
}

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

lvec mfactor(ll n) {
  bvec ip(n, true);
  lvec mf(n, -1);
  ip[0] = false;
  ip[1] = false;
  mf[0] = 0;
  mf[1] = 1;
  REP(i, n) {
    if (ip[i]) {
      mf[i] = i;
      for (ll j = i * i; j < n; j += i) {
        ip[j] = false;
        if (mf[j] == -1)
          mf[j] = i;
      }
    }
  }
  return mf;
}

palivec get_prime(ll n, const lvec &mf) {
  palivec plist;
  while (n != 1) {
    int cnt = 0;
    ll m = mf[n];
    while (mf[n] == m) {
      cnt++;
      n /= m;
    }
    plist.pb(pali(m, cnt));
  }
  return plist;
}

void COMinit(int m, lvec &fac, lvec &finv) {
  lvec inv(m);
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (int i = 2; i < m; 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 pow_mod(ll a, ll n) {
  ll x = 1;
  while (n > 0) {
    if (n & 1) {
      x = x * a % MOD;
    }
    a = a * a % MOD;
    n >>= 1;
  }
  return x;
}

ll COM(int n, int k, const lvec &fac, const lvec &finv) {
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return (fac[n] * (finv[k] * finv[n - k] % MOD)) % MOD;
}

ll modinv(ll a, ll m) {
  ll b = m, u = 1, v = 0;
  while (b) {
    ll t = a / b;
    a -= t * b;
    swap(a, b);
    u -= t * v;
    swap(u, v);
  }
  u %= m;
  if (u < 0)
    u += m;
  return u;
}

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  ll n, m;
  cin >> n >> m;
  lvec a(n);
  lvec s(n);
  for (int i = 0; i < n; i++) {
    cin >> a[i];
  }

  sort(a.rbegin(), a.rend());
  ll ok = 2 * a[0] + 1, ng = 1;
  while (ok - ng > 1) {
    ll t = (ng + ok) / 2;
    ll cnt = 0;
    ll i = 0, j = n - 1;
    while (i < n && j >= 0) {
      if (a[i] + a[j] > t)
        cnt += j + 1;
      else {
        if (j > 0) {
          j--;
          continue;
        }
      }
      i++;
    }
    if (cnt < m)
      ok = t;
    else
      ng = t;
  }
  s[0] = a[0];
  for (int i = 1; i < n; i++) {
    s[i] = a[i] + s[i - 1];
  }

  ll ans = 0;
  ll cnt = 0;
  for (int i = 0; i < n; i++) {
    ll ok2 = 0, ng2 = n;
    while (ng2 - ok2 > 1) {
      ll t = (ng2 + ok2) / 2;
      if (a[i] + a[t] >= ok)
        ok2 = t;
      else
        ng2 = t;
    }
    if (ok2 > 0) {
      ans += a[i] * (ok2 + 1) + s[ok2];
      cnt += ok2 + 1;
    } else if (a[i] + a[ok2] >= ok) {
      ans += (a[i] + a[ok2]);
      cnt++;
    }
  }

  ans -= ok * (cnt - m);
  cout << ans << endl;
}