Problem p02845 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <bitset>
#include <cctype>
#include <cmath>
#include <complex>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <unordered_map>
#include <utility>
#include <vector>
#define _overload3(_1, _2, _3, name, ...) name
#define _rep(i, n) repi(i, 0, n)
#define repi(i, a, b) for (ll i = ll(a); i < ll(b); ++i)
#define rep(...) _overload3(__VA_ARGS__, repi, _rep, )(__VA_ARGS__)
#define all(x) (x).begin(), (x).end()
#define PRINT(V) cout << V << "\n"
#define SORT(V) sort((V).begin(), (V).end())
#define RSORT(V) sort((V).rbegin(), (V).rend())
using namespace std;
using ll = long long;
template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}
inline void Yes(bool condition) {
  if (condition)
    PRINT("Yes");
  else
    PRINT("No");
}
template <class itr> void cins(itr first, itr last) {
  for (auto i = first; i != last; i++) {
    cin >> (*i);
  }
}
template <class itr> void array_output(itr start, itr goal) {
  string ans = "", k = " ";
  for (auto i = start; i != goal; i++)
    ans += to_string(*i) + k;
  if (!ans.empty())
    ans.pop_back();
  PRINT(ans);
}
ll gcd(ll a, ll b) { return a ? gcd(b % a, a) : b; }
const ll INF = 1e15;
const ll MOD = 1000000007;
const ll MOD2 = 998244353;
typedef pair<ll, ll> P;
typedef pair<double, double> point;
const ll MAX = 200005;
constexpr ll nx[8] = {1, 0, -1, 0, 1, -1, 1, -1};
constexpr ll ny[8] = {0, 1, 0, -1, 1, 1, -1, -1};
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  ll n;
  cin >> n;
  vector<ll> a(n);
  cins(all(a));
  ll ans = 1;
  vector<ll> c(3, 0);
  rep(i, n) {
    ll cnt = 0, ind = -1;
    rep(j, 3) {
      if (a[i] == c[j]) {
        cnt++;
        ind = j;
      }
    }
    (ans *= cnt) %= MOD;
    c[ind]++;
  }
  PRINT(ans);
}


Fixed submission:
#include <algorithm>
#include <bitset>
#include <cctype>
#include <cmath>
#include <complex>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <unordered_map>
#include <utility>
#include <vector>
#define _overload3(_1, _2, _3, name, ...) name
#define _rep(i, n) repi(i, 0, n)
#define repi(i, a, b) for (ll i = ll(a); i < ll(b); ++i)
#define rep(...) _overload3(__VA_ARGS__, repi, _rep, )(__VA_ARGS__)
#define all(x) (x).begin(), (x).end()
#define PRINT(V) cout << V << "\n"
#define SORT(V) sort((V).begin(), (V).end())
#define RSORT(V) sort((V).rbegin(), (V).rend())
using namespace std;
using ll = long long;
template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}
inline void Yes(bool condition) {
  if (condition)
    PRINT("Yes");
  else
    PRINT("No");
}
template <class itr> void cins(itr first, itr last) {
  for (auto i = first; i != last; i++) {
    cin >> (*i);
  }
}
template <class itr> void array_output(itr start, itr goal) {
  string ans = "", k = " ";
  for (auto i = start; i != goal; i++)
    ans += to_string(*i) + k;
  if (!ans.empty())
    ans.pop_back();
  PRINT(ans);
}
ll gcd(ll a, ll b) { return a ? gcd(b % a, a) : b; }
const ll INF = 1e15;
const ll MOD = 1000000007;
const ll MOD2 = 998244353;
typedef pair<ll, ll> P;
typedef pair<double, double> point;
const ll MAX = 200005;
constexpr ll nx[8] = {1, 0, -1, 0, 1, -1, 1, -1};
constexpr ll ny[8] = {0, 1, 0, -1, 1, 1, -1, -1};
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  ll n;
  cin >> n;
  vector<ll> a(n);
  cins(all(a));
  ll ans = 1;
  vector<ll> c(3, 0);
  rep(i, n) {
    ll cnt = 0, ind = 0;
    rep(j, 3) {
      if (a[i] == c[j]) {
        cnt++;
        ind = j;
      }
    }
    (ans *= cnt) %= MOD;
    c[ind]++;
  }
  PRINT(ans);
}