Problem p03096 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
#define int long long
#define PI 3.141592653589793
#define rep(i, n) for (int i = 0; i < (n); i++)
#define REP(i, a, n) for (int i = a; i < (n); i++)
#define rrep(i, n, k)                                                          \
  for (int i = (n); i >= (k); i--)                                             \
    ;
#define all(x) (x).begin(), (x).end()
#define vi vector<int>
template <class T> istream &operator>>(istream &is, vector<T> &v) {
  for (auto &elemnt : v)
    is >> elemnt;
  return is;
}
template <class T, class U> istream &operator>>(istream &is, pair<T, U> &p) {
  is >> p.first >> p.second;
  return is;
}
template <class T> vector<T> make_vector(size_t a) { return vector<T>(a); }
template <class T, class... Ts> auto make_vector(size_t a, Ts... ts) {
  return vector<decltype(make_vector<T>(ts...))>(a, make_vector<T>(ts...));
}
const int MOD = 1e9 + 7;
const int INF = 2e18;

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>;

signed main() {
  int N;
  cin >> N;
  vector<int> C(1);
  cin >> C;
  vector<set<int>> idx(100100);
  for (int i = 1; i < N; i++) {
    int temp;
    cin >> temp;
    if (*C.rbegin() != temp)
      C.push_back(temp);
    idx[temp].insert(C.size() - 1);
  }
  vector<modint> ans(C.size(), 1);
  for (int i = C.size() - 2; i >= 0; i--) {
    auto itr = idx[C[i]].upper_bound(i);
    if (itr == idx[C[i]].end())
      ans[i] = ans[i + 1];
    else
      ans[i] = ans[i + 1] + ans[*itr];
  }
  cout << ans[0] << endl;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;
#define int long long
#define PI 3.141592653589793
#define rep(i, n) for (int i = 0; i < (n); i++)
#define REP(i, a, n) for (int i = a; i < (n); i++)
#define rrep(i, n, k)                                                          \
  for (int i = (n); i >= (k); i--)                                             \
    ;
#define all(x) (x).begin(), (x).end()
#define vi vector<int>
template <class T> istream &operator>>(istream &is, vector<T> &v) {
  for (auto &elemnt : v)
    is >> elemnt;
  return is;
}
template <class T, class U> istream &operator>>(istream &is, pair<T, U> &p) {
  is >> p.first >> p.second;
  return is;
}
template <class T> vector<T> make_vector(size_t a) { return vector<T>(a); }
template <class T, class... Ts> auto make_vector(size_t a, Ts... ts) {
  return vector<decltype(make_vector<T>(ts...))>(a, make_vector<T>(ts...));
}
const int MOD = 1e9 + 7;
const int INF = 2e18;

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>;

signed main() {
  int N;
  cin >> N;
  vector<int> C(1);
  cin >> C;
  vector<set<int>> idx(200100);
  for (int i = 1; i < N; i++) {
    int temp;
    cin >> temp;
    if (*C.rbegin() != temp)
      C.push_back(temp);
    idx[temp].insert(C.size() - 1);
  }
  vector<modint> ans(C.size(), 1);
  for (int i = C.size() - 2; i >= 0; i--) {
    auto itr = idx[C[i]].upper_bound(i);
    if (itr == idx[C[i]].end())
      ans[i] = ans[i + 1];
    else
      ans[i] = ans[i + 1] + ans[*itr];
  }
  cout << ans[0] << endl;
}