Problem p03748 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
#define FOR(i, n, m) for (ll i = n; i < (int)m; i++)
#define REP(i, n) FOR(i, 0, n)
#define ALL(v) v.begin(), v.end()
#define pb push_back
using namespace std;
using ll = std::int_fast64_t;
using P = pair<ll, ll>;
constexpr ll inf = 1000000000;
constexpr ll mod = 1000000007;
constexpr long double eps = 1e-15;
template <typename T1, typename T2>
ostream &operator<<(ostream &os, pair<T1, T2> p) {
  os << to_string(p.first) << " " << to_string(p.second);
  return os;
}
template <typename T> ostream &operator<<(ostream &os, vector<T> &v) {
  REP(i, v.size()) {
    if (i)
      os << " ";
    os << to_string(v[i]);
  }
  return os;
}

struct modint {
  ll n;

public:
  modint(const ll n = 0) : n((n % mod + mod) % mod) {}
  static modint pow(modint a, int m) {
    modint r = 1;
    while (m > 0) {
      if (m & 1) {
        r *= a;
      }
      a = (a * a);
      m /= 2;
    }
    return r;
  }
  modint &operator++() {
    *this += 1;
    return *this;
  }
  modint &operator--() {
    *this -= 1;
    return *this;
  }
  modint operator++(int) {
    modint ret = *this;
    *this += 1;
    return ret;
  }
  modint operator--(int) {
    modint ret = *this;
    *this -= 1;
    return ret;
  }
  modint operator~() const { return (this->pow(n, mod - 2)); } // inverse
  friend bool operator==(const modint &lhs, const modint &rhs) {
    return lhs.n == rhs.n;
  }
  friend bool operator<(const modint &lhs, const modint &rhs) {
    return lhs.n < rhs.n;
  }
  friend bool operator>(const modint &lhs, const modint &rhs) {
    return lhs.n > rhs.n;
  }
  friend modint &operator+=(modint &lhs, const modint &rhs) {
    lhs.n += rhs.n;
    if (lhs.n >= mod)
      lhs.n -= mod;
    return lhs;
  }
  friend modint &operator-=(modint &lhs, const modint &rhs) {
    lhs.n -= rhs.n;
    if (lhs.n < 0)
      lhs.n += mod;
    return lhs;
  }
  friend modint &operator*=(modint &lhs, const modint &rhs) {
    lhs.n = (lhs.n * rhs.n) % mod;
    return lhs;
  }
  friend modint &operator/=(modint &lhs, const modint &rhs) {
    lhs.n = (lhs.n * (~rhs).n) % mod;
    return lhs;
  }
  friend modint operator+(const modint &lhs, const modint &rhs) {
    return modint(lhs.n + rhs.n);
  }
  friend modint operator-(const modint &lhs, const modint &rhs) {
    return modint(lhs.n - rhs.n);
  }
  friend modint operator*(const modint &lhs, const modint &rhs) {
    return modint(lhs.n * rhs.n);
  }
  friend modint operator/(const modint &lhs, const modint &rhs) {
    return modint(lhs.n * (~rhs).n);
  }
};
istream &operator>>(istream &is, modint m) {
  is >> m.n;
  return is;
}
ostream &operator<<(ostream &os, modint m) {
  os << m.n;
  return os;
}

#define MAX_N 3030303
long long extgcd(long long a, long long b, long long &x, long long &y) {
  long long d = a;
  if (b != 0) {
    d = extgcd(b, a % b, y, x);
    y -= (a / b) * x;
  } else {
    x = 1;
    y = 0;
  }
  return d;
}
long long mod_inverse(long long a, long long m) {
  long long x, y;
  if (extgcd(a, m, x, y) == 1)
    return (m + x % m) % m;
  else
    return -1;
}
vector<long long> fact(MAX_N + 1, inf);
long long mod_fact(long long n, long long &e) {
  if (fact[0] == inf) {
    fact[0] = 1;
    if (MAX_N != 0)
      fact[1] = 1;
    for (ll i = 2; i <= MAX_N; ++i) {
      fact[i] = (fact[i - 1] * i) % mod;
    }
  }
  e = 0;
  if (n == 0)
    return 1;
  long long res = mod_fact(n / mod, e);
  e += n / mod;
  if ((n / mod) % 2 != 0)
    return (res * (mod - fact[n % mod])) % mod;
  return (res * fact[n % mod]) % mod;
}
// return nCk
long long mod_comb(long long n, long long k) {
  if (n < 0 || k < 0 || n < k)
    return 0;
  long long e1, e2, e3;
  long long a1 = mod_fact(n, e1), a2 = mod_fact(k, e2),
            a3 = mod_fact(n - k, e3);
  if (e1 > e2 + e3)
    return 0;
  return (a1 * mod_inverse((a2 * a3) % mod, mod)) % mod;
}

using mi = modint;

mi dp1[2][3030][2][2];
mi dp2[2][3030][2];

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n, m;
  cin >> n >> m;
  if (n == 1) {
    assert(false);
    mi ans = 2;
    REP(i, m) ans *= 2;
    cout << ans << endl;
    return 0;
  }
  for (int i = 0; i <= m; i++)
    for (int j = 0; j <= n; j++) {
      REP(k, 2) REP(l, 2) {
        dp1[(i % 2)][j][k][l] = 0;
        if (i == 0) {
          if (k && l)
            dp1[(i % 2)][j][k][l] = 1;
          else
            dp1[(i % 2)][j][k][l] = 0;
        } else {
          if (j) {
            if (j - 1 == 0)
              dp1[(i % 2)][j][k][l] +=
                  dp1[!(i % 2)][j - 1][1][l] + dp1[!(i % 2)][j][1][l];
            else
              dp1[(i % 2)][j][k][l] +=
                  dp1[!(i % 2)][j - 1][k][l] + dp1[!(i % 2)][j][k][l];
          }
          if (j < n) {
            if (j + 1 == n)
              dp1[(i % 2)][j][k][l] +=
                  dp1[!(i % 2)][j + 1][k][1] + dp1[!(i % 2)][j][k][1];
            else
              dp1[(i % 2)][j][k][l] +=
                  dp1[!(i % 2)][j + 1][k][l] + dp1[!(i % 2)][j][k][l];
          }
        }
      }
    }
  for (int i = 0; i <= m; i++)
    for (int j = 0; j <= n - 1; j++) {
      REP(k, 2) {
        dp2[i % 2][j][k] = 0;
        if (i == 0) {
          if (k)
            dp2[(i % 2)][j][k] = 1;
          else
            dp2[(i % 2)][j][k] = 0;
        } else {
          if (j) {
            if (j - 1 == 0)
              dp2[(i % 2)][j][k] +=
                  dp2[!(i % 2)][j - 1][1] + dp2[!(i % 2)][j][1];
            else
              dp2[(i % 2)][j][k] +=
                  dp2[!(i % 2)][j - 1][k] + dp2[!(i % 2)][j][k];
          }
          if (j < n - 1) {
            dp2[(i % 2)][j][k] += dp2[!(i % 2)][j + 1][k] + dp2[!(i % 2)][j][k];
          }
        }
      }
    }
  mi ans = 0;
  REP(i, n + 1) {
    if (i == 0)
      ans += dp1[m % 2][i][1][0];
    else if (i == n)
      ans += dp1[m % 2][i][0][1];
    else
      ans += dp1[m % 2][i][0][0];
  }
  REP(i, n + 1) {
    if (i == 0)
      ans += dp2[m % 2][i][1];
    else
      ans += dp2[m % 2][i][0];
  }
  cout << ans << endl;
  return 0;
}


Fixed submission:
#include <bits/stdc++.h>
#define FOR(i, n, m) for (ll i = n; i < (int)m; i++)
#define REP(i, n) FOR(i, 0, n)
#define ALL(v) v.begin(), v.end()
#define pb push_back
using namespace std;
using ll = std::int_fast64_t;
using P = pair<ll, ll>;
constexpr ll inf = 1000000000;
constexpr ll mod = 1000000007;
constexpr long double eps = 1e-15;
template <typename T1, typename T2>
ostream &operator<<(ostream &os, pair<T1, T2> p) {
  os << to_string(p.first) << " " << to_string(p.second);
  return os;
}
template <typename T> ostream &operator<<(ostream &os, vector<T> &v) {
  REP(i, v.size()) {
    if (i)
      os << " ";
    os << to_string(v[i]);
  }
  return os;
}

struct modint {
  ll n;

public:
  modint(const ll n = 0) : n((n % mod + mod) % mod) {}
  static modint pow(modint a, int m) {
    modint r = 1;
    while (m > 0) {
      if (m & 1) {
        r *= a;
      }
      a = (a * a);
      m /= 2;
    }
    return r;
  }
  modint &operator++() {
    *this += 1;
    return *this;
  }
  modint &operator--() {
    *this -= 1;
    return *this;
  }
  modint operator++(int) {
    modint ret = *this;
    *this += 1;
    return ret;
  }
  modint operator--(int) {
    modint ret = *this;
    *this -= 1;
    return ret;
  }
  modint operator~() const { return (this->pow(n, mod - 2)); } // inverse
  friend bool operator==(const modint &lhs, const modint &rhs) {
    return lhs.n == rhs.n;
  }
  friend bool operator<(const modint &lhs, const modint &rhs) {
    return lhs.n < rhs.n;
  }
  friend bool operator>(const modint &lhs, const modint &rhs) {
    return lhs.n > rhs.n;
  }
  friend modint &operator+=(modint &lhs, const modint &rhs) {
    lhs.n += rhs.n;
    if (lhs.n >= mod)
      lhs.n -= mod;
    return lhs;
  }
  friend modint &operator-=(modint &lhs, const modint &rhs) {
    lhs.n -= rhs.n;
    if (lhs.n < 0)
      lhs.n += mod;
    return lhs;
  }
  friend modint &operator*=(modint &lhs, const modint &rhs) {
    lhs.n = (lhs.n * rhs.n) % mod;
    return lhs;
  }
  friend modint &operator/=(modint &lhs, const modint &rhs) {
    lhs.n = (lhs.n * (~rhs).n) % mod;
    return lhs;
  }
  friend modint operator+(const modint &lhs, const modint &rhs) {
    return modint(lhs.n + rhs.n);
  }
  friend modint operator-(const modint &lhs, const modint &rhs) {
    return modint(lhs.n - rhs.n);
  }
  friend modint operator*(const modint &lhs, const modint &rhs) {
    return modint(lhs.n * rhs.n);
  }
  friend modint operator/(const modint &lhs, const modint &rhs) {
    return modint(lhs.n * (~rhs).n);
  }
};
istream &operator>>(istream &is, modint m) {
  is >> m.n;
  return is;
}
ostream &operator<<(ostream &os, modint m) {
  os << m.n;
  return os;
}

#define MAX_N 3030303
long long extgcd(long long a, long long b, long long &x, long long &y) {
  long long d = a;
  if (b != 0) {
    d = extgcd(b, a % b, y, x);
    y -= (a / b) * x;
  } else {
    x = 1;
    y = 0;
  }
  return d;
}
long long mod_inverse(long long a, long long m) {
  long long x, y;
  if (extgcd(a, m, x, y) == 1)
    return (m + x % m) % m;
  else
    return -1;
}
vector<long long> fact(MAX_N + 1, inf);
long long mod_fact(long long n, long long &e) {
  if (fact[0] == inf) {
    fact[0] = 1;
    if (MAX_N != 0)
      fact[1] = 1;
    for (ll i = 2; i <= MAX_N; ++i) {
      fact[i] = (fact[i - 1] * i) % mod;
    }
  }
  e = 0;
  if (n == 0)
    return 1;
  long long res = mod_fact(n / mod, e);
  e += n / mod;
  if ((n / mod) % 2 != 0)
    return (res * (mod - fact[n % mod])) % mod;
  return (res * fact[n % mod]) % mod;
}
// return nCk
long long mod_comb(long long n, long long k) {
  if (n < 0 || k < 0 || n < k)
    return 0;
  long long e1, e2, e3;
  long long a1 = mod_fact(n, e1), a2 = mod_fact(k, e2),
            a3 = mod_fact(n - k, e3);
  if (e1 > e2 + e3)
    return 0;
  return (a1 * mod_inverse((a2 * a3) % mod, mod)) % mod;
}

using mi = modint;

mi dp1[2][3030][2][2];
mi dp2[2][3030][2];

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n, m;
  cin >> n >> m;
  if (n == 1) {
    mi ans = 2;
    REP(i, m) ans *= 2;
    cout << ans << endl;
    return 0;
  }
  for (int i = 0; i <= m; i++)
    for (int j = 0; j <= n; j++) {
      REP(k, 2) REP(l, 2) {
        dp1[(i % 2)][j][k][l] = 0;
        if (i == 0) {
          if (k && l)
            dp1[(i % 2)][j][k][l] = 1;
          else
            dp1[(i % 2)][j][k][l] = 0;
        } else {
          if (j) {
            if (j - 1 == 0)
              dp1[(i % 2)][j][k][l] +=
                  dp1[!(i % 2)][j - 1][1][l] + dp1[!(i % 2)][j][1][l];
            else
              dp1[(i % 2)][j][k][l] +=
                  dp1[!(i % 2)][j - 1][k][l] + dp1[!(i % 2)][j][k][l];
          }
          if (j < n) {
            if (j + 1 == n)
              dp1[(i % 2)][j][k][l] +=
                  dp1[!(i % 2)][j + 1][k][1] + dp1[!(i % 2)][j][k][1];
            else
              dp1[(i % 2)][j][k][l] +=
                  dp1[!(i % 2)][j + 1][k][l] + dp1[!(i % 2)][j][k][l];
          }
        }
      }
    }
  for (int i = 0; i <= m; i++)
    for (int j = 0; j <= n - 1; j++) {
      REP(k, 2) {
        dp2[i % 2][j][k] = 0;
        if (i == 0) {
          if (k)
            dp2[(i % 2)][j][k] = 1;
          else
            dp2[(i % 2)][j][k] = 0;
        } else {
          if (j) {
            if (j - 1 == 0)
              dp2[(i % 2)][j][k] +=
                  dp2[!(i % 2)][j - 1][1] + dp2[!(i % 2)][j][1];
            else
              dp2[(i % 2)][j][k] +=
                  dp2[!(i % 2)][j - 1][k] + dp2[!(i % 2)][j][k];
          }
          if (j < n - 1) {
            dp2[(i % 2)][j][k] += dp2[!(i % 2)][j + 1][k] + dp2[!(i % 2)][j][k];
          }
        }
      }
    }
  mi ans = 0;
  REP(i, n + 1) {
    if (i == 0)
      ans += dp1[m % 2][i][1][0];
    else if (i == n)
      ans += dp1[m % 2][i][0][1];
    else
      ans += dp1[m % 2][i][0][0];
  }
  REP(i, n + 1) {
    if (i == 0)
      ans += dp2[m % 2][i][1];
    else
      ans += dp2[m % 2][i][0];
  }
  cout << ans << endl;
  return 0;
}