Problem p02716 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <array>
#include <bitset>
#include <cassert>
#include <complex>
#include <cstdio>
#include <cstring>
#include <functional>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <limits.h>
#include <map>
#include <math.h>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <time.h>
#include <unordered_map>
#include <unordered_set>
#include <vector>
using namespace std;
using ll = long long;
#define rep(i, a, b) for (int i = a; i < b; i++)
#define rrep(i, a, b) for (int i = b - 1; i >= a; i--)
#define ALL(a) a.begin(), a.end()
using pii = pair<int, int>;
using piii = pair<pii, int>;
using pll = pair<long long, long long>;
using plll = pair<pll, long long>;
// #pragma GCC optimize("Ofast")
#define pcnt __builtin_popcount
#define buli(x) __builtin_popcountll(x)
#define pb push_back
#define mp make_pair
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end());
#define isSquare(x) (sqrt(x) * sqrt(x) == x)
template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
};
template <class T> bool chmin(T &a, const T &b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
};
inline void in(void) { return; }
template <typename First, typename... Rest>
void in(First &first, Rest &...rest) {
  cin >> first;
  in(rest...);
  return;
}
inline void out(void) {
  cout << "\n";
  return;
}
template <typename First, typename... Rest>
void out(First first, Rest... rest) {
  cout << first << " ";
  out(rest...);
  return;
}
const double EPS = 1e-9;
const int mod = 1e9 + 7;
const int INF = 1e9;
const long long INFLL = 1e18;
void iosetup() {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(10);
  cerr << fixed << setprecision(10);
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) {
  os << p.first << " " << p.second;
  return os;
}
template <typename T1, typename T2>
istream &operator>>(istream &is, pair<T1, T2> &p) {
  is >> p.first >> p.second;
  return is;
}
template <typename T> ostream &operator<<(ostream &os, const vector<T> &v) {
  for (int i = 0; i < (int)v.size(); i++) {
    os << v[i] << (i + 1 != v.size() ? " " : "");
  }
  return os;
}
template <typename T> istream &operator>>(istream &is, vector<T> &v) {
  for (T &in : v)
    is >> in;
  return is;
}
template <class S, class T>
pair<S, T> operator+(const pair<S, T> &s, const pair<S, T> &t) {
  return pair<S, T>(s.first + t.first, s.second + t.second);
}
template <class S, class T>
pair<S, T> operator-(const pair<S, T> &s, const pair<S, T> &t) {
  return pair<S, T>(s.first - t.first, s.second - t.second);
}
template <typename T> void Exit(T first) {
  cout << first << endl;
  exit(0);
};
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>;
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};
const pii dxy[4] = {pii(1, 0), pii(0, 1), pii(-1, 0), pii(0, -1)};
const int calender[13] = {0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
const int ucalender[13] = {0, 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
bool range(int a, int b, int x) {
  if (a <= x and x < b)
    return true;
  else
    return false;
}
bool range(int a, int b, int c, int d, pii p) {
  if (a <= p.first and p.first < b and c <= p.second and p.second < d)
    return true;
  else
    return false;
}

int main() {
  iosetup();
  int N;
  cin >> N;
  vector<int> A(N);
  cin >> A;
  vector<vector<ll>> dp(N, vector<ll>(3, -INFLL));
  dp[0][0] = A[0];
  dp[1][1] = A[1];
  dp[2][2] = A[2];
  rep(i, 0, N) {
    if (i + 2 < N)
      chmax(dp[i + 2][0], dp[i][0] + A[i + 2]);
    if (i + 2 < N)
      chmax(dp[i + 2][1], dp[i][1] + A[i + 2]);
    if (i + 2 < N)
      chmax(dp[i + 2][2], dp[i][2] + A[i + 2]);
    if (i + 3 < N)
      chmax(dp[i + 3][1], dp[i][0] + A[i + 3]);
    if (i + 3 < N)
      chmax(dp[i + 3][2], dp[i][1] + A[i + 3]);
    if (i + 4 < N)
      chmax(dp[i + 4][2], dp[i][0] + A[i + 4]);
  }
  // rep(i, 0, N){
  //     cerr << dp[i] << endl;
  // }
  if (N % 2 == 0)
    cout << dp[N - 1][1] << endl;
  else
    cout << dp[N - 1][2] << endl;

  return 0;
}

Fixed submission:
#include <algorithm>
#include <array>
#include <bitset>
#include <cassert>
#include <complex>
#include <cstdio>
#include <cstring>
#include <functional>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <limits.h>
#include <map>
#include <math.h>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <time.h>
#include <unordered_map>
#include <unordered_set>
#include <vector>
using namespace std;
using ll = long long;
#define rep(i, a, b) for (int i = a; i < b; i++)
#define rrep(i, a, b) for (int i = b - 1; i >= a; i--)
#define ALL(a) a.begin(), a.end()
using pii = pair<int, int>;
using piii = pair<pii, int>;
using pll = pair<long long, long long>;
using plll = pair<pll, long long>;
// #pragma GCC optimize("Ofast")
#define pcnt __builtin_popcount
#define buli(x) __builtin_popcountll(x)
#define pb push_back
#define mp make_pair
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end());
#define isSquare(x) (sqrt(x) * sqrt(x) == x)
template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
};
template <class T> bool chmin(T &a, const T &b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
};
inline void in(void) { return; }
template <typename First, typename... Rest>
void in(First &first, Rest &...rest) {
  cin >> first;
  in(rest...);
  return;
}
inline void out(void) {
  cout << "\n";
  return;
}
template <typename First, typename... Rest>
void out(First first, Rest... rest) {
  cout << first << " ";
  out(rest...);
  return;
}
const double EPS = 1e-9;
const int mod = 1e9 + 7;
const int INF = 1e9;
const long long INFLL = 1e18;
void iosetup() {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(10);
  cerr << fixed << setprecision(10);
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) {
  os << p.first << " " << p.second;
  return os;
}
template <typename T1, typename T2>
istream &operator>>(istream &is, pair<T1, T2> &p) {
  is >> p.first >> p.second;
  return is;
}
template <typename T> ostream &operator<<(ostream &os, const vector<T> &v) {
  for (int i = 0; i < (int)v.size(); i++) {
    os << v[i] << (i + 1 != v.size() ? " " : "");
  }
  return os;
}
template <typename T> istream &operator>>(istream &is, vector<T> &v) {
  for (T &in : v)
    is >> in;
  return is;
}
template <class S, class T>
pair<S, T> operator+(const pair<S, T> &s, const pair<S, T> &t) {
  return pair<S, T>(s.first + t.first, s.second + t.second);
}
template <class S, class T>
pair<S, T> operator-(const pair<S, T> &s, const pair<S, T> &t) {
  return pair<S, T>(s.first - t.first, s.second - t.second);
}
template <typename T> void Exit(T first) {
  cout << first << endl;
  exit(0);
};
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>;
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};
const pii dxy[4] = {pii(1, 0), pii(0, 1), pii(-1, 0), pii(0, -1)};
const int calender[13] = {0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
const int ucalender[13] = {0, 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
bool range(int a, int b, int x) {
  if (a <= x and x < b)
    return true;
  else
    return false;
}
bool range(int a, int b, int c, int d, pii p) {
  if (a <= p.first and p.first < b and c <= p.second and p.second < d)
    return true;
  else
    return false;
}

int main() {
  iosetup();
  int N;
  cin >> N;
  vector<int> A(N);
  cin >> A;
  vector<vector<ll>> dp(N, vector<ll>(3, -INFLL));
  if (N <= 3) {
    cout << *max_element(ALL(A)) << endl;
    return 0;
  }
  dp[0][0] = A[0];
  dp[1][1] = A[1];
  dp[2][2] = A[2];
  rep(i, 0, N) {
    if (i + 2 < N)
      chmax(dp[i + 2][0], dp[i][0] + A[i + 2]);
    if (i + 2 < N)
      chmax(dp[i + 2][1], dp[i][1] + A[i + 2]);
    if (i + 2 < N)
      chmax(dp[i + 2][2], dp[i][2] + A[i + 2]);
    if (i + 3 < N)
      chmax(dp[i + 3][1], dp[i][0] + A[i + 3]);
    if (i + 3 < N)
      chmax(dp[i + 3][2], dp[i][1] + A[i + 3]);
    if (i + 4 < N)
      chmax(dp[i + 4][2], dp[i][0] + A[i + 4]);
  }
  // rep(i, 0, N){
  //     cerr << dp[i] << endl;
  // }
  if (N % 2 == 0)
    cout << dp[N - 1][1] << endl;
  else
    cout << dp[N - 1][2] << endl;

  return 0;
}