Problem p03625 (C++)

Buggy submission (status: Runtime Error):
/* ---------- STL Libraries ---------- */

// IO library
#include <cstdio>
#include <fstream>
#include <iomanip>
#include <ios>
#include <iostream>

// algorithm library
#include <algorithm>
#include <cmath>
#include <numeric>
#include <random>

// container library
#include <array>
#include <bitset>
#include <deque>
#include <map>
#include <queue>
#include <set>
#include <string>
#include <tuple>
#include <vector>

/* ---------- Namespace ---------- */

using namespace std;

/* ---------- Type Abbreviation ---------- */

template <typename T> using PQ = priority_queue<T>;
template <typename T> using GPQ = priority_queue<T, vector<T>, greater<T>>;

using ll = long long;

#define fst first
#define snd second
#define mp make_pair
#define mt make_tuple

#define FOR(i, a, b) for (ll i = (a); i < (b); i++)
#define REP(i, n) FOR(i, 0, n)

/* ---------- conversion ---------- */

#define INT(c) static_cast<int>(c)
#define CHAR(n) static_cast<char>(n)
#define LL(n) static_cast<ll>(n)
#define DOUBLE(n) static_cast<double>(n)

/* ---------- container ---------- */

#define ALL(v) (v).begin(), (v).end()
#define SIZE(v) (LL((v).size()))
#define each(itr, v) for (auto(itr) : (v))

#define FIND(v, k) (v).find(k) != (v).end()
#define VFIND(v, k) find(ALL(v), k) != (v).end()

#define gsort(b, e) sort(b, e, greater<decltype(*b)>())

/* ----------- debug ---------- */

template <class T> ostream &operator<<(ostream &os, vector<T> v) {
  os << "[";
  for (auto vv : v)
    os << vv << ",";
  return os << "]";
}

template <class T> ostream &operator<<(ostream &os, set<T> v) {
  os << "[";
  for (auto vv : v)
    os << vv << ",";
  return os << "]";
}

template <class L, class R> ostream &operator<<(ostream &os, pair<L, R> p) {
  return os << "(" << p.fst << "," << p.snd << ")";
}

/* ---------- Constants ---------- */

// const ll MOD = 1e9 + 7;
// const int INF = 1 << 25;
// const ll INF = 1LL << 50;
// const double PI = acos(-1);
// const double EPS = 1e-10;
// mt19937 mert(LL(time(0)));

/* ---------- Short Functions ---------- */

template <typename T> T sq(T a) { return a * a; }

template <typename T> T gcd(T a, T b) {
  if (a > b)
    return gcd(b, a);
  return a == 0 ? b : gcd(b % a, a);
}

template <typename T, typename U> T mypow(T b, U n) {
  if (n == 0)
    return 1;
  if (n == 1)
    return b /* % MOD */;
  if (n % 2 == 0) {
    return mypow(b * b /* % MOD */, n / 2);
  } else {
    return mypow(b, n - 1) * b /* % MOD */;
  }
}

ll pcnt(ll b) { return __builtin_popcountll(b); }

// const ll dx[4] = {0, -1, 1, 0};
// const ll dy[4] = {-1, 0, 0, 1};
// const ll dx[8] = {-1, 0, 1, -1, 1, -1, 0, 1};
// const ll dy[8] = {-1, -1, -1, 0, 0, 1, 1, 1};

/* v-v-v-v-v-v-v-v-v Main Part v-v-v-v-v-v-v-v-v */

int main() {

  cin.tie(0);
  ios::sync_with_stdio(false);

  ll N;
  cin >> N;
  map<long long, int> mp;
  vector<long long> edge;
  REP(i, N) {
    ll a;
    cin >> a;
    mp[a]++;
  }
  each(itr, mp) {
    if (itr.second >= 4)
      edge.push_back(itr.first);
    if (itr.second >= 2)
      edge.push_back(itr.first);
  }
  sort(edge.begin(), edge.end());
  ll edge1, edge2;
  edge1 = edge.back();
  edge.pop_back();
  edge2 = edge.back();
  cout << edge1 * edge2 << endl;
  return 0;
}

Fixed submission:
/* ---------- STL Libraries ---------- */

// IO library
#include <cstdio>
#include <fstream>
#include <iomanip>
#include <ios>
#include <iostream>

// algorithm library
#include <algorithm>
#include <cmath>
#include <numeric>
#include <random>

// container library
#include <array>
#include <bitset>
#include <deque>
#include <map>
#include <queue>
#include <set>
#include <string>
#include <tuple>
#include <vector>

/* ---------- Namespace ---------- */

using namespace std;

/* ---------- Type Abbreviation ---------- */

template <typename T> using PQ = priority_queue<T>;
template <typename T> using GPQ = priority_queue<T, vector<T>, greater<T>>;

using ll = long long;

#define fst first
#define snd second
#define mp make_pair
#define mt make_tuple

#define FOR(i, a, b) for (ll i = (a); i < (b); i++)
#define REP(i, n) FOR(i, 0, n)

/* ---------- conversion ---------- */

#define INT(c) static_cast<int>(c)
#define CHAR(n) static_cast<char>(n)
#define LL(n) static_cast<ll>(n)
#define DOUBLE(n) static_cast<double>(n)

/* ---------- container ---------- */

#define ALL(v) (v).begin(), (v).end()
#define SIZE(v) (LL((v).size()))
#define each(itr, v) for (auto(itr) : (v))

#define FIND(v, k) (v).find(k) != (v).end()
#define VFIND(v, k) find(ALL(v), k) != (v).end()

#define gsort(b, e) sort(b, e, greater<decltype(*b)>())

/* ----------- debug ---------- */

template <class T> ostream &operator<<(ostream &os, vector<T> v) {
  os << "[";
  for (auto vv : v)
    os << vv << ",";
  return os << "]";
}

template <class T> ostream &operator<<(ostream &os, set<T> v) {
  os << "[";
  for (auto vv : v)
    os << vv << ",";
  return os << "]";
}

template <class L, class R> ostream &operator<<(ostream &os, pair<L, R> p) {
  return os << "(" << p.fst << "," << p.snd << ")";
}

/* ---------- Constants ---------- */

// const ll MOD = 1e9 + 7;
// const int INF = 1 << 25;
// const ll INF = 1LL << 50;
// const double PI = acos(-1);
// const double EPS = 1e-10;
// mt19937 mert(LL(time(0)));

/* ---------- Short Functions ---------- */

template <typename T> T sq(T a) { return a * a; }

template <typename T> T gcd(T a, T b) {
  if (a > b)
    return gcd(b, a);
  return a == 0 ? b : gcd(b % a, a);
}

template <typename T, typename U> T mypow(T b, U n) {
  if (n == 0)
    return 1;
  if (n == 1)
    return b /* % MOD */;
  if (n % 2 == 0) {
    return mypow(b * b /* % MOD */, n / 2);
  } else {
    return mypow(b, n - 1) * b /* % MOD */;
  }
}

ll pcnt(ll b) { return __builtin_popcountll(b); }

// const ll dx[4] = {0, -1, 1, 0};
// const ll dy[4] = {-1, 0, 0, 1};
// const ll dx[8] = {-1, 0, 1, -1, 1, -1, 0, 1};
// const ll dy[8] = {-1, -1, -1, 0, 0, 1, 1, 1};

/* v-v-v-v-v-v-v-v-v Main Part v-v-v-v-v-v-v-v-v */

int main() {

  cin.tie(0);
  ios::sync_with_stdio(false);

  ll N;
  cin >> N;
  map<long long, int> mp;
  vector<long long> edge;
  REP(i, N) {
    ll a;
    cin >> a;
    mp[a]++;
  }
  each(itr, mp) {
    if (itr.second >= 4)
      edge.push_back(itr.first);
    if (itr.second >= 2)
      edge.push_back(itr.first);
  }
  sort(edge.begin(), edge.end());
  if (edge.size() < 2) {
    cout << 0 << endl;
    return 0;
  }
  ll edge1, edge2;
  edge1 = edge.back();
  edge.pop_back();
  edge2 = edge.back();
  cout << edge1 * edge2 << endl;
  return 0;
}