Problem p03013 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>

#define whole(f, x, ...)                                                       \
  ([&](decltype((x)) whole) {                                                  \
    return (f)(begin(whole), end(whole), ##__VA_ARGS__);                       \
  })(x)

using namespace std;
using ll = long long;

const int MOD = 1e9 + 7;

template <int mod> struct ModInt {
  int x;

  ModInt() : x(0) {}

  ModInt(int64_t y) : x(y >= 0 ? y % mod : (mod - (-y) % mod) % mod) {}

  ModInt &operator+=(const ModInt &p) {
    if ((x += p.x) >= mod)
      x -= mod;
    return *this;
  }

  ModInt &operator-=(const ModInt &p) {
    if ((x += mod - p.x) >= mod)
      x -= mod;
    return *this;
  }

  ModInt &operator*=(const ModInt &p) {
    x = (int)(1LL * x * p.x % mod);
    return *this;
  }

  ModInt &operator/=(const ModInt &p) {
    *this *= p.inverse();
    return *this;
  }

  ModInt operator-() const { return ModInt(-x); }

  ModInt operator+(const ModInt &p) const { return ModInt(*this) += p; }

  ModInt operator-(const ModInt &p) const { return ModInt(*this) -= p; }

  ModInt operator*(const ModInt &p) const { return ModInt(*this) *= p; }

  ModInt operator/(const ModInt &p) const { return ModInt(*this) /= p; }

  bool operator==(const ModInt &p) const { return x == p.x; }

  bool operator!=(const ModInt &p) const { return x != p.x; }

  ModInt inverse() const {
    int a = x, b = mod, u = 1, v = 0, t;
    while (b > 0) {
      t = a / b;
      swap(a -= t * b, b);
      swap(u -= t * v, v);
    }
    return ModInt(u);
  }

  ModInt pow(int64_t n) const {
    ModInt ret(1), mul(x);
    while (n > 0) {
      if (n & 1)
        ret *= mul;
      mul *= mul;
      n >>= 1;
    }
    return ret;
  }

  friend ostream &operator<<(ostream &os, const ModInt &p) { return os << p.x; }

  friend istream &operator>>(istream &is, ModInt &a) {
    int64_t t;
    is >> t;
    a = ModInt<mod>(t);
    return (is);
  }

  static int get_mod() { return mod; }
};

using modint = ModInt<MOD>;

void solve() {
  int n, m;
  cin >> n >> m;
  vector<int> a(m);
  for (int i = 0; i < m; i++) {
    cin >> a[i];
  }
  vector<modint> fibo(n + 1);
  fibo[1] = 1;
  fibo[2] = 1;
  for (int i = 3; i <= n; i++) {
    fibo[i] = fibo[i - 1] + fibo[i - 2];
  }

  modint sum(fibo[a[0]]);
  for (int i = 1; i < m; i++) {
    const int t = a[i] - a[i - 1] - 1;
    if (t == 0) {
      cout << 0 << endl;
      return;
    }
    sum *= fibo[t];
  }
  sum *= fibo[n - a[m - 1]];

  cout << sum << endl;
}

int main() {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);
  std::cout << std::fixed << std::setprecision(15);

  solve();
  return 0;
}


Fixed submission:
#include <bits/stdc++.h>

#define whole(f, x, ...)                                                       \
  ([&](decltype((x)) whole) {                                                  \
    return (f)(begin(whole), end(whole), ##__VA_ARGS__);                       \
  })(x)

using namespace std;
using ll = long long;

const int MOD = 1e9 + 7;

template <int mod> struct ModInt {
  int x;

  ModInt() : x(0) {}

  ModInt(int64_t y) : x(y >= 0 ? y % mod : (mod - (-y) % mod) % mod) {}

  ModInt &operator+=(const ModInt &p) {
    if ((x += p.x) >= mod)
      x -= mod;
    return *this;
  }

  ModInt &operator-=(const ModInt &p) {
    if ((x += mod - p.x) >= mod)
      x -= mod;
    return *this;
  }

  ModInt &operator*=(const ModInt &p) {
    x = (int)(1LL * x * p.x % mod);
    return *this;
  }

  ModInt &operator/=(const ModInt &p) {
    *this *= p.inverse();
    return *this;
  }

  ModInt operator-() const { return ModInt(-x); }

  ModInt operator+(const ModInt &p) const { return ModInt(*this) += p; }

  ModInt operator-(const ModInt &p) const { return ModInt(*this) -= p; }

  ModInt operator*(const ModInt &p) const { return ModInt(*this) *= p; }

  ModInt operator/(const ModInt &p) const { return ModInt(*this) /= p; }

  bool operator==(const ModInt &p) const { return x == p.x; }

  bool operator!=(const ModInt &p) const { return x != p.x; }

  ModInt inverse() const {
    int a = x, b = mod, u = 1, v = 0, t;
    while (b > 0) {
      t = a / b;
      swap(a -= t * b, b);
      swap(u -= t * v, v);
    }
    return ModInt(u);
  }

  ModInt pow(int64_t n) const {
    ModInt ret(1), mul(x);
    while (n > 0) {
      if (n & 1)
        ret *= mul;
      mul *= mul;
      n >>= 1;
    }
    return ret;
  }

  friend ostream &operator<<(ostream &os, const ModInt &p) { return os << p.x; }

  friend istream &operator>>(istream &is, ModInt &a) {
    int64_t t;
    is >> t;
    a = ModInt<mod>(t);
    return (is);
  }

  static int get_mod() { return mod; }
};

using modint = ModInt<MOD>;

void solve() {
  int n, m;
  cin >> n >> m;
  vector<int> a(m);
  for (int i = 0; i < m; i++) {
    cin >> a[i];
  }
  vector<modint> fibo(n + 1);
  fibo[1] = 1;
  fibo[2] = 1;
  for (int i = 3; i <= n; i++) {
    fibo[i] = fibo[i - 1] + fibo[i - 2];
  }

  if (m == 0) {
    cout << fibo[n] + fibo[n - 1] << endl;
    return;
  }

  modint sum(fibo[a[0]]);
  for (int i = 1; i < m; i++) {
    const int t = a[i] - a[i - 1] - 1;
    if (t == 0) {
      cout << 0 << endl;
      return;
    }
    sum *= fibo[t];
  }
  sum *= fibo[n - a[m - 1]];

  cout << sum << endl;
}

int main() {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);
  std::cout << std::fixed << std::setprecision(15);

  solve();
  return 0;
}