Problem p03096 (C++)

Buggy submission (status: Runtime Error):

// #pragma GCC target("avx2")
#pragma GCC optimize("O3")
// #pragma GCC optimize("unroll-loops")

#include <bits/stdc++.h>

using namespace std;

#ifdef LOCAL
#include <prettyprint.hpp>
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]: ", d_err(__VA_ARGS__);
#else
#define debug(...) 83;
#endif

void d_err() { cerr << endl; }

template <typename H, typename... T> void d_err(H h, T... t) {
  cerr << h << " ";
  d_err(t...);
}

template <typename T> void print(T x) { cout << x << "\n"; }

#define ALL(x) (x).begin(), (x).end()
#define FOR(i, m, n) for (int i = (m); i < (n); ++i)
#define REVFOR(i, m, n) for (int i = (n - 1); i >= (m); --i)
#define REP(i, n) FOR(i, 0, n)
#define REVREP(i, n) REVFOR(i, 0, n)
#define fi first
#define se second
#define pb push_back
#define mp make_pair
#define eb emplace_back
#define bcnt __builtin_popcountll

typedef long long ll;
typedef unsigned long long ull;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef pair<ll, ll> Pll;
typedef pair<int, int> Pin;

ll INF = 1e16;
int inf = 1e9;

const ll MOD = 1000000007;
// const ll MOD = 998244353;
struct mint {
  ll x; // typedef long long ll;
  mint(ll x = 0) : x((x % MOD + MOD) % MOD) {}
  mint operator-() const { return mint(-x); }
  mint &operator+=(const mint a) {
    if ((x += a.x) >= MOD)
      x -= MOD;
    return *this;
  }
  mint &operator-=(const mint a) {
    if ((x += MOD - a.x) >= MOD)
      x -= MOD;
    return *this;
  }
  mint &operator*=(const mint a) {
    (x *= a.x) %= MOD;
    return *this;
  }
  mint operator+(const mint a) const {
    mint res(*this);
    return res += a;
  }
  mint operator-(const mint a) const {
    mint res(*this);
    return res -= a;
  }
  mint operator*(const mint a) const {
    mint res(*this);
    return res *= a;
  }
  mint pow(ll t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }
  // for prime MOD
  mint inv() const { return pow(MOD - 2); }
  mint &operator/=(const mint a) { return (*this) *= a.inv(); }
  mint operator/(const mint a) const { return mint(*this) /= a; }
};
istream &operator>>(istream &is, mint &a) { return is >> a.x; }
ostream &operator<<(ostream &os, const mint &a) { return os << a.x; }

int main() {
  cin.tie(0);
  ios_base::sync_with_stdio(false);
  cout << fixed << setprecision(20);

  int n;
  cin >> n;

  vector<int> a(n);
  REP(i, n) cin >> a[i];

  vector<int> ca;
  int prev = -1;
  REP(i, n) {
    if (prev == a[i])
      continue;
    ca.pb(a[i]);
    prev = a[i];
  }

  vector<int> last(n + 1, -1);
  vector<mint> dp(ca.size(), 0);
  dp[0] = 1;
  last[ca[0]] = 0;
  FOR(i, 1, ca.size()) {
    if (last[ca[i]] > -1)
      dp[i] += dp[last[ca[i]]];
    last[ca[i]] = i;
    dp[i] += dp[i - 1];
  }

  print(dp[ca.size() - 1]);
}


Fixed submission:

// #pragma GCC target("avx2")
#pragma GCC optimize("O3")
// #pragma GCC optimize("unroll-loops")

#include <bits/stdc++.h>

using namespace std;

#ifdef LOCAL
#include <prettyprint.hpp>
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]: ", d_err(__VA_ARGS__);
#else
#define debug(...) 83;
#endif

void d_err() { cerr << endl; }

template <typename H, typename... T> void d_err(H h, T... t) {
  cerr << h << " ";
  d_err(t...);
}

template <typename T> void print(T x) { cout << x << "\n"; }

#define ALL(x) (x).begin(), (x).end()
#define FOR(i, m, n) for (int i = (m); i < (n); ++i)
#define REVFOR(i, m, n) for (int i = (n - 1); i >= (m); --i)
#define REP(i, n) FOR(i, 0, n)
#define REVREP(i, n) REVFOR(i, 0, n)
#define fi first
#define se second
#define pb push_back
#define mp make_pair
#define eb emplace_back
#define bcnt __builtin_popcountll

typedef long long ll;
typedef unsigned long long ull;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef pair<ll, ll> Pll;
typedef pair<int, int> Pin;

ll INF = 1e16;
int inf = 1e9;

const ll MOD = 1000000007;
// const ll MOD = 998244353;
struct mint {
  ll x; // typedef long long ll;
  mint(ll x = 0) : x((x % MOD + MOD) % MOD) {}
  mint operator-() const { return mint(-x); }
  mint &operator+=(const mint a) {
    if ((x += a.x) >= MOD)
      x -= MOD;
    return *this;
  }
  mint &operator-=(const mint a) {
    if ((x += MOD - a.x) >= MOD)
      x -= MOD;
    return *this;
  }
  mint &operator*=(const mint a) {
    (x *= a.x) %= MOD;
    return *this;
  }
  mint operator+(const mint a) const {
    mint res(*this);
    return res += a;
  }
  mint operator-(const mint a) const {
    mint res(*this);
    return res -= a;
  }
  mint operator*(const mint a) const {
    mint res(*this);
    return res *= a;
  }
  mint pow(ll t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }
  // for prime MOD
  mint inv() const { return pow(MOD - 2); }
  mint &operator/=(const mint a) { return (*this) *= a.inv(); }
  mint operator/(const mint a) const { return mint(*this) /= a; }
};
istream &operator>>(istream &is, mint &a) { return is >> a.x; }
ostream &operator<<(ostream &os, const mint &a) { return os << a.x; }

int main() {
  cin.tie(0);
  ios_base::sync_with_stdio(false);
  cout << fixed << setprecision(20);

  int n;
  cin >> n;

  vector<int> a(n);
  REP(i, n) cin >> a[i];

  vector<int> ca;
  int prev = -1;
  REP(i, n) {
    if (prev == a[i])
      continue;
    ca.pb(a[i]);
    prev = a[i];
  }

  vector<int> last(2e5 + 5, -1);
  vector<mint> dp(ca.size(), 0);
  dp[0] = 1;
  last[ca[0]] = 0;
  FOR(i, 1, ca.size()) {
    if (last[ca[i]] > -1)
      dp[i] += dp[last[ca[i]]];
    last[ca[i]] = i;
    dp[i] += dp[i - 1];
  }

  print(dp[ca.size() - 1]);
}