Problem p02716 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
typedef pair<ll, ll> P;
typedef pair<int, int> Pi;
#define rep(i, n) for (ll i = 0; i < n; i++)
#define FOR(i, a, b) for (ll i = a; i < b; i++)
#define fi first
#define se second
#define endl "\n"

template <typename T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> ostream &operator<<(ostream &s, const complex<T> &d) {
  return s << "(" << d.real() << ", " << d.imag() << ")";
}
template <typename T1, typename T2>
ostream &operator<<(ostream &s, const pair<T1, T2> &d) {
  return s << "(" << d.first << ", " << d.second << ")";
}
template <typename T> ostream &operator<<(ostream &s, const vector<T> &d) {
  int len = d.size();
  rep(i, len) {
    s << d[i];
    if (i < len - 1)
      s << " ";
  }
  return s;
}
template <typename T>
ostream &operator<<(ostream &s, const vector<vector<T>> &d) {
  int len = d.size();
  rep(i, len) { s << d[i] << endl; }
  return s;
}
template <typename T> ostream &operator<<(ostream &s, const set<T> &v) {
  s << "{ ";
  for (auto itr = v.begin(); itr != v.end(); ++itr) {
    if (itr != v.begin()) {
      s << ", ";
    }
    s << (*itr);
  }
  s << " }";
  return s;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &s, const map<T1, T2> &m) {
  s << "{" << endl;
  for (auto itr = m.begin(); itr != m.end(); ++itr) {
    s << " " << (*itr).first << " : " << (*itr).second << endl;
  }
  s << "}" << endl;
  return s;
}

const ll mod = 1e9 + 7;
const ll inf = 1e17;
const int INF = 1e9;
const double PI = acos(-1);
const double EPS = 1e-10;

ll dp[30][5][2];

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  int n;
  cin >> n;
  vector<ll> a(n);
  rep(i, n) { cin >> a[i]; }
  rep(i, n + 1) rep(j, 5) rep(k, 2) dp[i][j][k] = -inf;
  dp[1][0][1] = a[0];
  dp[2][0][0] = a[1];
  dp[2][0][1] = a[1];
  dp[3][0][0] = a[2];
  FOR(i, 1, n) rep(j, 5) rep(k, 2) {
    if (dp[i][j][k] == -inf)
      continue;
    int now = i / 2;
    if (k)
      now = (i + 1) / 2;
    FOR(l, 2, 5) {
      int ind = i + l;
      int nk;
      if (ind / 2 == now + 1)
        nk = 0;
      else if ((ind + 1) / 2 == now + 1)
        nk = 1;
      else
        continue;
      if (ind <= n) {
        chmax(dp[ind][l][nk], dp[i][j][k] + a[ind - 1]);
        if (ind % 2 == 0) {
          chmax(dp[ind][l][!nk], dp[i][j][k] + a[ind - 1]);
        }
      }
      // cout<<i<<' '<<ind<<' '<<nk<<' '<<dp[i][j][k]<<'
      // '<<dp[ind][l][nk]<<endl;
    }
  }
  ll ans = -inf;
  rep(i, n + 1) {
    if (i / 2 == n / 2) {
      rep(j, 5) chmax(ans, dp[i][j][0]);
    }
    if ((i + 1) / 2 == n / 2) {
      rep(j, 5) chmax(ans, dp[i][j][1]);
    }
  }
  cout << ans << endl;
}

Fixed submission:
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
typedef pair<ll, ll> P;
typedef pair<int, int> Pi;
#define rep(i, n) for (ll i = 0; i < n; i++)
#define FOR(i, a, b) for (ll i = a; i < b; i++)
#define fi first
#define se second
#define endl "\n"

template <typename T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> ostream &operator<<(ostream &s, const complex<T> &d) {
  return s << "(" << d.real() << ", " << d.imag() << ")";
}
template <typename T1, typename T2>
ostream &operator<<(ostream &s, const pair<T1, T2> &d) {
  return s << "(" << d.first << ", " << d.second << ")";
}
template <typename T> ostream &operator<<(ostream &s, const vector<T> &d) {
  int len = d.size();
  rep(i, len) {
    s << d[i];
    if (i < len - 1)
      s << " ";
  }
  return s;
}
template <typename T>
ostream &operator<<(ostream &s, const vector<vector<T>> &d) {
  int len = d.size();
  rep(i, len) { s << d[i] << endl; }
  return s;
}
template <typename T> ostream &operator<<(ostream &s, const set<T> &v) {
  s << "{ ";
  for (auto itr = v.begin(); itr != v.end(); ++itr) {
    if (itr != v.begin()) {
      s << ", ";
    }
    s << (*itr);
  }
  s << " }";
  return s;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &s, const map<T1, T2> &m) {
  s << "{" << endl;
  for (auto itr = m.begin(); itr != m.end(); ++itr) {
    s << " " << (*itr).first << " : " << (*itr).second << endl;
  }
  s << "}" << endl;
  return s;
}

const ll mod = 1e9 + 7;
const ll inf = 1e17;
const int INF = 1e9;
const double PI = acos(-1);
const double EPS = 1e-10;

ll dp[200020][5][2];

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  int n;
  cin >> n;
  vector<ll> a(n);
  rep(i, n) { cin >> a[i]; }
  rep(i, n + 1) rep(j, 5) rep(k, 2) dp[i][j][k] = -inf;
  dp[1][0][1] = a[0];
  dp[2][0][0] = a[1];
  dp[2][0][1] = a[1];
  dp[3][0][0] = a[2];
  FOR(i, 1, n) rep(j, 5) rep(k, 2) {
    if (dp[i][j][k] == -inf)
      continue;
    int now = i / 2;
    if (k)
      now = (i + 1) / 2;
    FOR(l, 2, 5) {
      int ind = i + l;
      int nk;
      if (ind / 2 == now + 1)
        nk = 0;
      else if ((ind + 1) / 2 == now + 1)
        nk = 1;
      else
        continue;
      if (ind <= n) {
        chmax(dp[ind][l][nk], dp[i][j][k] + a[ind - 1]);
        if (ind % 2 == 0) {
          chmax(dp[ind][l][!nk], dp[i][j][k] + a[ind - 1]);
        }
      }
      // cout<<i<<' '<<ind<<' '<<nk<<' '<<dp[i][j][k]<<'
      // '<<dp[ind][l][nk]<<endl;
    }
  }
  ll ans = -inf;
  rep(i, n + 1) {
    if (i / 2 == n / 2) {
      rep(j, 5) chmax(ans, dp[i][j][0]);
    }
    if ((i + 1) / 2 == n / 2) {
      rep(j, 5) chmax(ans, dp[i][j][1]);
    }
  }
  cout << ans << endl;
}