Problem p02845 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <bitset>
#include <cassert>
#include <climits>
#include <cmath>
#include <complex>
#include <cstddef>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <tuple>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

using namespace std;
#define REP(i, n) for (int(i) = 0; (i) < (n); (i)++)
#define FOR(i, a, b) for (int(i) = (a); (i) < (b); (i)++)
#define RREP(i, a) for (int(i) = (a)-1; (i) >= 0; (i)--)
#define FORR(i, a, b) for (int(i) = (a)-1; (i) >= (b); (i)--)
#define DEBUG(C) cerr << #C << " = " << C << endl;
using LL = long long;
using VI = vector<int>;
using VVI = vector<VI>;
using VL = vector<LL>;
using VVL = vector<VL>;
using VD = vector<double>;
using VVD = vector<VD>;
using PII = pair<int, int>;
using PDD = pair<double, double>;
using PLL = pair<LL, LL>;
using VPII = vector<PII>;
template <typename T> using VT = vector<T>;
#define ALL(a) begin((a)), end((a))
#define RALL(a) rbegin((a)), rend((a))
#define SORT(a) sort(ALL((a)))
#define RSORT(a) sort(RALL((a)))
#define REVERSE(a) reverse(ALL((a)))
#define MP make_pair
#define FORE(a, b) for (auto &&a : (b))
#define EB emplace_back
#define GREATER(T) T, VT<T> greater<T>
template <typename T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}

const int INF = 1e9;
const int MOD = INF + 7;
const LL LLINF = 1e18;

const int MAX = 1e5 + 10;
int N;
int A[MAX];

int main(void) {
  cin >> N;
  REP(i, N) { cin >> A[i]; }

  LL ans = 1;
  int cnt[3] = {-1, -1, -1};
  REP(i, N) {
    VI cand;
    REP(j, 3) {
      if (cnt[j] + 1 == A[i]) {
        cand.EB(j);
      }
    }

    if (cnt[cand[0]] >= 0) {
      (ans *= (cand.size() + (cand.size() + 1)) / 2) %= MOD;
    }
    cnt[cand[0]]++;
  }

  int exists_cnt = 0;
  REP(i, 3) if (cnt[i] >= 0) { exists_cnt++; }

  (ans *= (exists_cnt == 1 ? 3 : 6)) %= MOD;

  cout << ans << endl;
}


Fixed submission:
#include <algorithm>
#include <bitset>
#include <cassert>
#include <climits>
#include <cmath>
#include <complex>
#include <cstddef>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <tuple>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

using namespace std;
#define REP(i, n) for (int(i) = 0; (i) < (n); (i)++)
#define FOR(i, a, b) for (int(i) = (a); (i) < (b); (i)++)
#define RREP(i, a) for (int(i) = (a)-1; (i) >= 0; (i)--)
#define FORR(i, a, b) for (int(i) = (a)-1; (i) >= (b); (i)--)
#define DEBUG(C) cerr << #C << " = " << C << endl;
using LL = long long;
using VI = vector<int>;
using VVI = vector<VI>;
using VL = vector<LL>;
using VVL = vector<VL>;
using VD = vector<double>;
using VVD = vector<VD>;
using PII = pair<int, int>;
using PDD = pair<double, double>;
using PLL = pair<LL, LL>;
using VPII = vector<PII>;
template <typename T> using VT = vector<T>;
#define ALL(a) begin((a)), end((a))
#define RALL(a) rbegin((a)), rend((a))
#define SORT(a) sort(ALL((a)))
#define RSORT(a) sort(RALL((a)))
#define REVERSE(a) reverse(ALL((a)))
#define MP make_pair
#define FORE(a, b) for (auto &&a : (b))
#define EB emplace_back
#define GREATER(T) T, VT<T> greater<T>
template <typename T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}

const int INF = 1e9;
const int MOD = INF + 7;
const LL LLINF = 1e18;

const int MAX = 1e5 + 10;
int N;
int A[MAX];

int main(void) {
  cin >> N;
  REP(i, N) { cin >> A[i]; }

  LL ans = 1;
  int cnt[3] = {-1, -1, -1};
  REP(i, N) {
    VI cand;
    REP(j, 3) {
      if (cnt[j] + 1 == A[i]) {
        cand.EB(j);
      }
    }

    if (cand.size() == 0) {
      cout << 0 << endl;
      return 0;
    }

    if (cnt[cand[0]] >= 0) {
      (ans *= (cand.size() + (cand.size() + 1)) / 2) %= MOD;
    }
    cnt[cand[0]]++;
  }

  int exists_cnt = 0;
  REP(i, 3) if (cnt[i] >= 0) { exists_cnt++; }

  (ans *= (exists_cnt == 1 ? 3 : 6)) %= MOD;

  cout << ans << endl;
}