Problem p03767 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
#define int long long

#define _overload3(_1, _2, _3, name, ...) name
#define _rep(i, n) repi(i, 0, n)
#define repi(i, a, b) for (int i = (int)a, i##_len = (b); i < i##_len; i++)
#define rep(...) _overload3(__VA_ARGS__, repi, _rep, )(__VA_ARGS__)

#define all(box) box.begin(), box.end()

using namespace std;

using P = pair<int, int>;
using ll = long long;

const long long INF = LLONG_MAX / 3;

template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}

struct UnionFind {
  std::vector<int> par, siz;
  void init(int n) {
    par.resize(n);
    for (int i = 0; i < n; i++)
      par[i] = i;
    siz.resize(n, 1);
  }
  UnionFind(int n) { init(n); }
  int root(int n) {
    if (par[n] == n)
      return n;
    else
      return par[n] = root(par[n]);
  }
  bool issame(int x, int y) { return (root(x) == root(y)); }
  bool marge(int a, int b) {
    if (issame(a, b))
      return false;
    else {
      a = root(a), b = root(b);
      if (siz[a] < siz[b])
        swap(a, b);
      siz[b] += siz[a];
      par[a] = par[b];
      return true;
    }
  }
  int size(int n) { return siz[root(n)]; }
};

long long gcd(long long a, long long b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}
long long lcm(long long i, long long j) { return i * (j / gcd(i, j)); }

const int MOD = 998244353;
long long fac[510000], finv[510000], inv[510000];
long long MAX = 510000;
bool COMinited = false;
long long COM(int n, int k) {
  if (COMinited == false) {
    fac[0] = fac[1] = 1;
    finv[0] = finv[1] = 1;
    inv[1] = 1;
    for (int i = 2; i < MAX; 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;
    }
    COMinited = true;
  }
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}
long long modpow(long long a, long long n, long long mod) {
  long long res = 1;
  while (n > 0) {
    if (n & 1)
      res = res * a % mod;
    a = a * a % mod;
    n >>= 1;
  }
  return res;
}
long long modinv(long long a, long long mod) { return modpow(a, mod - 2, mod); }
template <typename T> struct BinaryIndexedTree {
  vector<T> data;

  BinaryIndexedTree(int sz) { data.assign(++sz, 0); }

  T sum(int k) {
    T ret = 0;
    for (++k; k > 0; k -= k & -k)
      ret += data[k];
    return (ret);
  }

  void add(int k, T x) {
    for (++k; k < data.size(); k += k & -k)
      data[k] += x;
  }
};
signed main() {
  int n;
  cin >> n;
  int a[100010];
  rep(i, 3 * n) cin >> a[i];
  sort(a, a + 3 * n);
  int re = 0;
  rep(i, n, 3 * n) {
    re += a[i];
    i++;
  }
  cout << re << endl;
}

Fixed submission:
#include <bits/stdc++.h>
#define int long long

#define _overload3(_1, _2, _3, name, ...) name
#define _rep(i, n) repi(i, 0, n)
#define repi(i, a, b) for (int i = (int)a, i##_len = (b); i < i##_len; i++)
#define rep(...) _overload3(__VA_ARGS__, repi, _rep, )(__VA_ARGS__)

#define all(box) box.begin(), box.end()

using namespace std;

using P = pair<int, int>;
using ll = long long;

const long long INF = LLONG_MAX / 3;

template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}

struct UnionFind {
  std::vector<int> par, siz;
  void init(int n) {
    par.resize(n);
    for (int i = 0; i < n; i++)
      par[i] = i;
    siz.resize(n, 1);
  }
  UnionFind(int n) { init(n); }
  int root(int n) {
    if (par[n] == n)
      return n;
    else
      return par[n] = root(par[n]);
  }
  bool issame(int x, int y) { return (root(x) == root(y)); }
  bool marge(int a, int b) {
    if (issame(a, b))
      return false;
    else {
      a = root(a), b = root(b);
      if (siz[a] < siz[b])
        swap(a, b);
      siz[b] += siz[a];
      par[a] = par[b];
      return true;
    }
  }
  int size(int n) { return siz[root(n)]; }
};

long long gcd(long long a, long long b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}
long long lcm(long long i, long long j) { return i * (j / gcd(i, j)); }

const int MOD = 998244353;
long long fac[510000], finv[510000], inv[510000];
long long MAX = 510000;
bool COMinited = false;
long long COM(int n, int k) {
  if (COMinited == false) {
    fac[0] = fac[1] = 1;
    finv[0] = finv[1] = 1;
    inv[1] = 1;
    for (int i = 2; i < MAX; 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;
    }
    COMinited = true;
  }
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}
long long modpow(long long a, long long n, long long mod) {
  long long res = 1;
  while (n > 0) {
    if (n & 1)
      res = res * a % mod;
    a = a * a % mod;
    n >>= 1;
  }
  return res;
}
long long modinv(long long a, long long mod) { return modpow(a, mod - 2, mod); }
template <typename T> struct BinaryIndexedTree {
  vector<T> data;

  BinaryIndexedTree(int sz) { data.assign(++sz, 0); }

  T sum(int k) {
    T ret = 0;
    for (++k; k > 0; k -= k & -k)
      ret += data[k];
    return (ret);
  }

  void add(int k, T x) {
    for (++k; k < data.size(); k += k & -k)
      data[k] += x;
  }
};
signed main() {
  int n;
  cin >> n;
  int a[300010];
  rep(i, 3 * n) cin >> a[i];
  sort(a, a + 3 * n);
  int re = 0;
  rep(i, n, 3 * n) {
    re += a[i];
    i++;
  }
  cout << re << endl;
}