Language: C++

#pragma GCC optimize("Ofast")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#include <bits/stdc++.h>
#define int long long
#define ll long long
#define vec vector
#define vi vec<int>
#define vvi vec<vec<int>>
#define ALL(a) begin(a), end(a)
#define RALL(a) rbegin(a), rend(a)
#define SORT(a) stable_sort(ALL(a))
#define RSORT(a) stable_sort(RALL(a))
#define REV(a) reverse(ALL(a))
#define UNIQUE(a) a.erase(unique(ALL(a)), end(a))
#define lambda(i) [=](auto i)
#define compare(i, j) [=](auto i, auto j)
#define min_of(a) *min_element(ALL(a))
#define max_of(a) *max_element(ALL(a))
#define sum(a) accumulate(ALL(a), 0)
#define pb push_back
#define mp make_pair
#define mt make_tuple
#define SZ(a) (int)(a).size()
#define REP(i, n) for (decltype(+n) i = 0; i < (n); i++)
#define RREP(i, n) for (auto i = n - 1; i != static_cast<decltype(i)>(-1); i--)
#define FOR(i, a, b) for (auto i = (a); i < (b); i++)
#define RFOR(i, a, b) for (auto i = b - 1; i != a; i--)
#define EACH(i, a) for (auto&& i: (a))
using namespace std;

const int INF = 1e18;
const int MOD = 1e9 + 7;

int ceil_div(int X, int Y) {
  return (X - 1) / Y + 1;
}

struct union_find {
  vi data;
  union_find(int size) : data(size, -1) { }
  bool union_set(int x, int y) {
    x = root(x); y = root(y);
    if (x != y) {
      if (data[y] < data[x]) swap(x, y);
      data[x] += data[y]; data[y] = x;
    }
    return x != y;
  }
  bool find_set(int x, int y) {
    return root(x) == root(y);
  }
  int root(int x) {
    return data[x] < 0 ? x : data[x] = root(data[x]);
  }
  int size(int x) {
    return -data[root(x)];
  }
};

vi cum(vi V) {
  vi U(SZ(V));
  partial_sum(ALL(V), begin(U));
  return U;
}

void solve() {
  int n;

  cin >> n;
  vvi dp(n, vi(3)); //select i-th and skipped j-times.
  cin >> dp[0][0];
  cin >> dp[1][1];
  FOR(i, 2, n) {
    int a;
    cin >> a;
    if (i % 2 == 0) {
      dp[i][0] = dp[i - 2][0] + a;
      if(n % 2 == 1) {
        dp[i][2] = dp[i - 2][2] + a;
        if(i > 3) {
          dp[i][2] = max(dp[i][2], dp[i - 3][1] + a);
          dp[i][2] = max(dp[i][2], dp[i - 4][0] + a);
        }
      }
    } else {
      dp[i][1] = dp[i - 2][1] + a;
      if(i > 2) dp[i][1] = max(dp[i][1], dp[i - 3][0] + a);
    }
  }
  if (n % 2 == 0) cout << max(dp[n - 1][1], dp[n - 2][0]) << endl;
  else cout << max(dp[n - 1][2], max(dp[n - 2][1], dp[n - 3][0])) << endl;
}

signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout << fixed << setprecision(15);
  solve();
}


Compiler output:
In file included from /usr/include/c++/14/string:43,
                 from /usr/include/c++/14/bitset:52,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:52,
                 from a.cc:3:
/usr/include/c++/14/bits/allocator.h: In destructor 'std::_Vector_base<long long int, std::allocator<long long int> >::_Vector_impl::~_Vector_impl()':
/usr/include/c++/14/bits/allocator.h:182:7: error: inlining failed in call to 'always_inline' 'std::allocator< <template-parameter-1-1> >::~allocator() noexcept [with _Tp = long long int]': target specific option mismatch
  182 |       ~allocator() _GLIBCXX_NOTHROW { }
      |       ^
In file included from /usr/include/c++/14/vector:66,
                 from /usr/include/c++/14/functional:64,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:53:
/usr/include/c++/14/bits/stl_vector.h:132:14: note: called from here
  132 |       struct _Vector_impl
      |              ^~~~~~~~~~~~