Problem p02716 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;

// #define int long long
#define all(v) (v).begin(), (v).end()
#define rep(i, n) for (int i = 0; i < n; ++i)
#define rep1(i, n) for (int i = 1; i < n; ++i)
#define exrep(i, a, b) for (ll i = a; i < b; i++)
#define out(x) cout << x << endl
#define EPS (1e-7)
#define gearup                                                                 \
  ios_base::sync_with_stdio(0);                                                \
  cin.tie(0);                                                                  \
  cout.tie(0);
typedef long double ld;
typedef long long int ll;
typedef unsigned long long int ull;
typedef vector<int> vi;
typedef vector<char> vc;
typedef vector<bool> vb;
typedef vector<double> vd;
typedef vector<string> vs;
typedef vector<pair<int, int>> vpii;
typedef vector<vector<int>> vvi;
typedef vector<vector<char>> vvc;
typedef vector<vector<string>> vvs;
typedef vector<ll> vl;
typedef vector<vector<ll>> vvl;
typedef vector<vector<vector<ll>>> vvvl;
ll MOD = 1000000007;
const long long L_INF = 1LL << 60;
const int INF = 2147483647; // 2^31-1
const double PI = acos(-1);
// cout<<fixed<<setprecision(10);

template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> void debug(T v) {
  rep(i, v.size()) cout << v[i] << " ";
  cout << endl;
}
const ll dx[8] = {1, 1, 0, -1, -1, -1, 0, 1};
const ll dy[8] = {0, 1, 1, 1, 0, -1, -1, -1};

signed main() {
  gearup;
  int n;
  cin >> n;
  vl a(n);
  rep(i, n) cin >> a[i];
  vvl dp(n + 10, vl(3, -L_INF));
  int m = n % 2 + 1; // 奇数なら2こ飛ばせる
  dp[0][0] = 0;
  rep(i, n) {
    rep(j, m + 1) {
      chmax(dp[i + 1][j + 1], dp[i][j]);
      ll now = dp[i][j];
      if ((i + j) % 2 == 0)
        now += a[i];
      chmax(dp[i + 1][j], now);
    }
  }
  out(dp[n][m]);
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;

// #define int long long
#define all(v) (v).begin(), (v).end()
#define rep(i, n) for (int i = 0; i < n; ++i)
#define rep1(i, n) for (int i = 1; i < n; ++i)
#define exrep(i, a, b) for (ll i = a; i < b; i++)
#define out(x) cout << x << endl
#define EPS (1e-7)
#define gearup                                                                 \
  ios_base::sync_with_stdio(0);                                                \
  cin.tie(0);                                                                  \
  cout.tie(0);
typedef long double ld;
typedef long long int ll;
typedef unsigned long long int ull;
typedef vector<int> vi;
typedef vector<char> vc;
typedef vector<bool> vb;
typedef vector<double> vd;
typedef vector<string> vs;
typedef vector<pair<int, int>> vpii;
typedef vector<vector<int>> vvi;
typedef vector<vector<char>> vvc;
typedef vector<vector<string>> vvs;
typedef vector<ll> vl;
typedef vector<vector<ll>> vvl;
typedef vector<vector<vector<ll>>> vvvl;
ll MOD = 1000000007;
const long long L_INF = 1LL << 60;
const int INF = 2147483647; // 2^31-1
const double PI = acos(-1);
// cout<<fixed<<setprecision(10);

template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> void debug(T v) {
  rep(i, v.size()) cout << v[i] << " ";
  cout << endl;
}
const ll dx[8] = {1, 1, 0, -1, -1, -1, 0, 1};
const ll dy[8] = {0, 1, 1, 1, 0, -1, -1, -1};

signed main() {
  gearup;
  int n;
  cin >> n;
  vl a(n);
  rep(i, n) cin >> a[i];
  vvl dp(n + 10, vl(4, -L_INF));
  int m = n % 2 + 1; // 奇数なら2こ飛ばせる
  dp[0][0] = 0;
  rep(i, n) {
    rep(j, m + 1) {
      chmax(dp[i + 1][j + 1], dp[i][j]);
      ll now = dp[i][j];
      if ((i + j) % 2 == 0)
        now += a[i];
      chmax(dp[i + 1][j], now);
    }
  }
  out(dp[n][m]);
}