Problem p02553 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
typedef long long int ll;
// Factors of an num
/*
vector<ll> fact(ll n){
        vector<ll>v;
        for(int i=1;i*i<=n;i++){
                if(n%i ==0){
                        v.push_back(i);

                if(i!=n/i)
                        v.push_back(n/i);
            }
        }

        return v;
}
*/

/*
Seive Of Eratos..
int N=1e5;
vector<bool> isprime(N+1,true);
void pri(int n){
        for(int i=2;i*i<=n;i++){
                if(isprime[i]){
                        for(int j=i*i;j<=n;j+=i){
                                isprime[j]=false;
                        }
                }
        }
        return;
}
*/

/*
//Prime Factors
vector<bool> isprime(1e6+1,true);
 vector<int> primes;
 void sieve()
 {

    ll  i,j;
    isprime[0] = false;
    isprime[1] = false;

    for(i=2; i<=1e6 ;i++)
  {

    if(isprime[i])
    {

      primes.push_back(i);

      for(j=i*i; j<=1e6; j+=i)
      {
        isprime[j]=false;
      }

    }

  }
}

vector<int> primefactors(int  N)

 {

    vector<int> factors;
    ll pf_idx = 0, pf=primes[pf_idx];

    while(pf*pf<=N)
    {
        while(N % pf == 0)
        {
            N/=pf;
            factors.push_back(pf);
        }
        pf = primes[++pf_idx];
    }

    if(N != 1) factors.push_back(N);
    return factors;
}
 */

int main() {

#ifndef ONLINE_JUDGE
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
#endif
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cin.tie(NULL);
  int n = 1e9 + 1;
  ll a, a2;
  vector<ll> v(n + 1);
  for (int i = 0; i < n; ++i) {
    /* code */
    cin >> v[i];
    ll ans = max(ans, v[i]);
    a = ans;
    v.erase(remove(v.begin(), v.end(), ans), v.end());
    a2 = max(a2, v[i]);
  }
  cout << a2 * a << endl;
  ;
}

Fixed submission:
#include <bits/stdc++.h>
using namespace std;
typedef long long int ll;
// Factors of an num
/*
vector<ll> fact(ll n){
        vector<ll>v;
        for(int i=1;i*i<=n;i++){
                if(n%i ==0){
                        v.push_back(i);

                if(i!=n/i)
                        v.push_back(n/i);
            }
        }

        return v;
}
*/

/*
Seive Of Eratos..
int N=1e5;
vector<bool> isprime(N+1,true);
void pri(int n){
        for(int i=2;i*i<=n;i++){
                if(isprime[i]){
                        for(int j=i*i;j<=n;j+=i){
                                isprime[j]=false;
                        }
                }
        }
        return;
}
*/

/*
//Prime Factors
vector<bool> isprime(1e6+1,true);
 vector<int> primes;
 void sieve()
 {

    ll  i,j;
    isprime[0] = false;
    isprime[1] = false;

    for(i=2; i<=1e6 ;i++)
  {

    if(isprime[i])
    {

      primes.push_back(i);

      for(j=i*i; j<=1e6; j+=i)
      {
        isprime[j]=false;
      }

    }

  }
}

vector<int> primefactors(int  N)

 {

    vector<int> factors;
    ll pf_idx = 0, pf=primes[pf_idx];

    while(pf*pf<=N)
    {
        while(N % pf == 0)
        {
            N/=pf;
            factors.push_back(pf);
        }
        pf = primes[++pf_idx];
    }

    if(N != 1) factors.push_back(N);
    return factors;
}
 */

int main() {

#ifndef ONLINE_JUDGE
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
#endif
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cin.tie(NULL);
  ll a, b, c, d;
  cin >> a >> b >> c >> d;
  ll ans = INT_MIN;
  ll a1 = max(a * c, b * d);
  ll b1 = max(a * d, b * c);
  ans = max(a1, b1);
  cout << ans << endl;
  return 0;
}