Problem p02716 (C++)

Buggy submission (status: Runtime Error):
#define _CRT_SECURE_NO_WARNINGS
#pragma GCC target("avx")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cfloat>
#include <climits>
#include <cmath>
#include <complex>
#include <ctime>
#include <deque>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <memory>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>
#define rep(i, n) for (int i = 0; i < (n); i++)
#define REP(i, n) for (int i = 1; i <= (n); i++)
#define all(V) V.begin(), V.end()
typedef long long lint;
typedef std::pair<lint, lint> P;
constexpr int INF = INT_MAX / 10;
constexpr lint LINF = LLONG_MAX / 10;
constexpr double eps = 1e-9;
template <class T>
class prique : public std::priority_queue<T, std::vector<T>, std::greater<T>> {
};
template <class T, class U> inline bool chmax(T &lhs, const U &rhs) {
  if (lhs < rhs) {
    lhs = rhs;
    return 1;
  }
  return 0;
}
template <class T, class U> inline bool chmin(T &lhs, const U &rhs) {
  if (lhs > rhs) {
    lhs = rhs;
    return 1;
  }
  return 0;
}
inline lint gcd(lint a, lint b) {
  while (b) {
    lint c = a;
    a = b;
    b = c % b;
  }
  return a;
}
inline lint lcm(lint a, lint b) { return a / gcd(a, b) * b; }
bool isprime(lint n) {
  if (n == 1)
    return false;
  for (int i = 2; i * i <= n; i++) {
    if (n % i == 0)
      return false;
  }
  return true;
}
lint mypow(lint a, lint b) {
  if (!b)
    return 1;
  if (b & 1)
    return mypow(a, b - 1) * a;
  lint memo = mypow(a, b >> 1);
  return memo * memo;
}
lint modpow(lint a, lint b, lint m) {
  if (!b)
    return 1;
  if (b & 1)
    return modpow(a, b - 1, m) * a % m;
  lint memo = modpow(a, b >> 1, m);
  return memo * memo % m;
}
void printArray(std::vector<int> &vec) {
  rep(i, vec.size() - 1) std::cout << vec[i] << " ";
  std::cout << vec.back() << std::endl;
}
template <typename T> void printArray(T l, T r) {
  T rprev = r;
  rprev--;
  for (T i = l; i != rprev; i++) {
    std::cout << *i << " ";
  }
  std::cout << *rprev << std::endl;
}
std::string to_string(std::vector<int> &vec) {
  std::string res = "[";
  rep(i, vec.size() - 1) res += std::to_string(vec[i]) + ", ";
  res += std::to_string(vec.back()) + "]";
  return res;
}
int n, a[100010], b[100010][2], c[100010][2];
lint dp[100010][2][2];
signed main() {
  std::cin >> n;
  rep(i, n) std::cin >> a[i];
  rep(i, n) {
    if (n % 2) {
      b[i][0] = i / 2;
      c[i][0] = (i + 1) / 2;
      b[i][1] = (i + 1) / 2;
      c[i][1] = i / 2 + 1;
      if (i == 0)
        b[i][1] = 1;
      if (i == n - 1)
        c[i][1] = n / 2;
    } else {
      b[i][0] = c[i][0] = (i + 1) / 2;
      b[i][1] = c[i][1] = i / 2 + 1;
    }
  }
  rep(i, n) {
    rep(j, 2) { rep(k, 2) dp[i][j][k] = -LINF; }
  }
  dp[0][0][0] = 0;
  dp[0][1][0] = a[0];
  REP(i, n - 1) {
    // use
    if (b[i - 1][0] + 1 == b[i][1])
      chmax(dp[i][1][0], dp[i - 1][0][0] + a[i]);
    if (b[i - 1][0] + 2 == b[i][1])
      chmax(dp[i][1][0], dp[i - 1][0][1] + a[i]);
    if (b[i][1] != c[i][1] && b[i - 1][0] + 1 == b[i][1] + 1)
      chmax(dp[i][1][1], dp[i - 1][0][0] + a[i]);
    if (b[i][1] != c[i][1] && b[i - 1][0] + 2 == b[i][1] + 1)
      chmax(dp[i][1][1], dp[i - 1][0][1] + a[i]);
    // not
    if (b[i - 1][0] == b[i][0])
      chmax(dp[i][0][0], dp[i - 1][0][0]);
    if (b[i - 1][0] + 1 == b[i][0])
      chmax(dp[i][0][0], dp[i - 1][0][1]);
    if (b[i][0] != c[i][0] && b[i - 1][0] == b[i][0] + 1)
      chmax(dp[i][0][1], dp[i - 1][0][0]);
    if (b[i][0] != c[i][0] && b[i - 1][0] + 1 == b[i][0] + 1)
      chmax(dp[i][0][1], dp[i - 1][0][1]);
    if (b[i - 1][1] == b[i][0])
      chmax(dp[i][0][0], dp[i - 1][1][0]);
    if (b[i - 1][1] + 1 == b[i][0])
      chmax(dp[i][0][0], dp[i - 1][1][1]);
    if (b[i][0] != c[i][0] && b[i - 1][1] == b[i][0] + 1)
      chmax(dp[i][0][1], dp[i - 1][1][0]);
    if (b[i][0] != c[i][0] && b[i - 1][1] + 1 == b[i][0] + 1)
      chmax(dp[i][0][1], dp[i - 1][1][1]);
  }
  lint ans = -LINF;
  rep(i, 2) { rep(j, 2) chmax(ans, dp[n - 1][i][j]); }
  std::cout << ans << std::endl;
  return 0;
}

Fixed submission:
#define _CRT_SECURE_NO_WARNINGS
#pragma GCC target("avx")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cfloat>
#include <climits>
#include <cmath>
#include <complex>
#include <ctime>
#include <deque>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <memory>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>
#define rep(i, n) for (int i = 0; i < (n); i++)
#define REP(i, n) for (int i = 1; i <= (n); i++)
#define all(V) V.begin(), V.end()
typedef long long lint;
typedef std::pair<lint, lint> P;
constexpr int INF = INT_MAX / 10;
constexpr lint LINF = LLONG_MAX / 10;
constexpr double eps = 1e-9;
template <class T>
class prique : public std::priority_queue<T, std::vector<T>, std::greater<T>> {
};
template <class T, class U> inline bool chmax(T &lhs, const U &rhs) {
  if (lhs < rhs) {
    lhs = rhs;
    return 1;
  }
  return 0;
}
template <class T, class U> inline bool chmin(T &lhs, const U &rhs) {
  if (lhs > rhs) {
    lhs = rhs;
    return 1;
  }
  return 0;
}
inline lint gcd(lint a, lint b) {
  while (b) {
    lint c = a;
    a = b;
    b = c % b;
  }
  return a;
}
inline lint lcm(lint a, lint b) { return a / gcd(a, b) * b; }
bool isprime(lint n) {
  if (n == 1)
    return false;
  for (int i = 2; i * i <= n; i++) {
    if (n % i == 0)
      return false;
  }
  return true;
}
lint mypow(lint a, lint b) {
  if (!b)
    return 1;
  if (b & 1)
    return mypow(a, b - 1) * a;
  lint memo = mypow(a, b >> 1);
  return memo * memo;
}
lint modpow(lint a, lint b, lint m) {
  if (!b)
    return 1;
  if (b & 1)
    return modpow(a, b - 1, m) * a % m;
  lint memo = modpow(a, b >> 1, m);
  return memo * memo % m;
}
void printArray(std::vector<int> &vec) {
  rep(i, vec.size() - 1) std::cout << vec[i] << " ";
  std::cout << vec.back() << std::endl;
}
template <typename T> void printArray(T l, T r) {
  T rprev = r;
  rprev--;
  for (T i = l; i != rprev; i++) {
    std::cout << *i << " ";
  }
  std::cout << *rprev << std::endl;
}
std::string to_string(std::vector<int> &vec) {
  std::string res = "[";
  rep(i, vec.size() - 1) res += std::to_string(vec[i]) + ", ";
  res += std::to_string(vec.back()) + "]";
  return res;
}
int n, a[200010], b[200010][2], c[200010][2];
lint dp[200010][2][2];
signed main() {
  std::cin >> n;
  rep(i, n) std::cin >> a[i];
  rep(i, n) {
    if (n % 2) {
      b[i][0] = i / 2;
      c[i][0] = (i + 1) / 2;
      b[i][1] = (i + 1) / 2;
      c[i][1] = i / 2 + 1;
      if (i == 0)
        b[i][1] = 1;
      if (i == n - 1)
        c[i][1] = n / 2;
    } else {
      b[i][0] = c[i][0] = (i + 1) / 2;
      b[i][1] = c[i][1] = i / 2 + 1;
    }
  }
  rep(i, n) {
    rep(j, 2) { rep(k, 2) dp[i][j][k] = -LINF; }
  }
  dp[0][0][0] = 0;
  dp[0][1][0] = a[0];
  REP(i, n - 1) {
    // use
    if (b[i - 1][0] + 1 == b[i][1])
      chmax(dp[i][1][0], dp[i - 1][0][0] + a[i]);
    if (b[i - 1][0] + 2 == b[i][1])
      chmax(dp[i][1][0], dp[i - 1][0][1] + a[i]);
    if (b[i][1] != c[i][1] && b[i - 1][0] + 1 == b[i][1] + 1)
      chmax(dp[i][1][1], dp[i - 1][0][0] + a[i]);
    if (b[i][1] != c[i][1] && b[i - 1][0] + 2 == b[i][1] + 1)
      chmax(dp[i][1][1], dp[i - 1][0][1] + a[i]);
    // not
    if (b[i - 1][0] == b[i][0])
      chmax(dp[i][0][0], dp[i - 1][0][0]);
    if (b[i - 1][0] + 1 == b[i][0])
      chmax(dp[i][0][0], dp[i - 1][0][1]);
    if (b[i][0] != c[i][0] && b[i - 1][0] == b[i][0] + 1)
      chmax(dp[i][0][1], dp[i - 1][0][0]);
    if (b[i][0] != c[i][0] && b[i - 1][0] + 1 == b[i][0] + 1)
      chmax(dp[i][0][1], dp[i - 1][0][1]);
    if (b[i - 1][1] == b[i][0])
      chmax(dp[i][0][0], dp[i - 1][1][0]);
    if (b[i - 1][1] + 1 == b[i][0])
      chmax(dp[i][0][0], dp[i - 1][1][1]);
    if (b[i][0] != c[i][0] && b[i - 1][1] == b[i][0] + 1)
      chmax(dp[i][0][1], dp[i - 1][1][0]);
    if (b[i][0] != c[i][0] && b[i - 1][1] + 1 == b[i][0] + 1)
      chmax(dp[i][0][1], dp[i - 1][1][1]);
  }
  lint ans = -LINF;
  rep(i, 2) { rep(j, 2) chmax(ans, dp[n - 1][i][j]); }
  std::cout << ans << std::endl;
  return 0;
}