Problem p02875 (C++)

Buggy submission (status: Runtime Error):
// #include <bits/stdc++.h>
#include "bits/stdc++.h"
using namespace std;
typedef long long ll;
// #include "boost/multiprecision/cpp_int.hpp"
// typedef boost::multiprecision::cpp_int LL;
typedef long double dd;
// #define i_7 (ll)(1E9+7)
#define i_7 998244353
#define i_5 i_7 - 2
ll mod(ll a) {
  ll c = a % i_7;
  if (c >= 0)
    return c;
  return c + i_7;
}
typedef pair<ll, ll> l_l;
typedef pair<dd, dd> d_d;
ll inf = (ll)1E16;
#define rep(i, l, r) for (ll i = l; i <= r; i++)
#define pb push_back
ll max(ll a, ll b) {
  if (a < b)
    return b;
  else
    return a;
}
ll min(ll a, ll b) {
  if (a > b)
    return b;
  else
    return a;
}
void Max(ll &pos, ll val) { pos = max(pos, val); } // Max(dp[n],dp[n-1]);
void Min(ll &pos, ll val) { pos = min(pos, val); }
void Add(ll &pos, ll val) { pos = mod(pos + val); }
dd EPS = 1E-9;
#define fastio                                                                 \
  ios::sync_with_stdio(false);                                                 \
  cin.tie(0);                                                                  \
  cout.tie(0);
#define fi first
#define se second
#define endl "\n"
#define SORT(v) sort(v.begin(), v.end())
#define ERASE(v) v.erase(unique(v.begin(), v.end()), v.end())
#define POSL(v, x) (lower_bound(v.begin(), v.end(), x) - v.begin())
#define POSU(v, x) (upper_bound(v.begin(), v.end(), x) - v.begin())
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}

//////////////////////////

ll po(ll i, ll p) {
  if (p == 0)
    return 1;
  else {
    i = mod(i);
    if (p == 1)
      return i;
    if (p % 2 == 0)
      return po(mod(i * i), p / 2);
    return mod(i * po(i, p - 1));
  }
}
ll bunbo(ll n) { return po(n, i_5); }

#define N 2000004
ll kai[N];
ll kai2[N];
void calc() {
  kai[0] = 1;
  kai2[0] = 1;
  rep(i, 1, N - 1) { kai[i] = mod(kai[i - 1] * i); }
  kai2[N - 1] = po(kai[N - 1], i_5);
  for (ll i = N - 2; i >= 0; i--) {
    kai2[i] = mod(kai2[i + 1] * (i + 1));
  }
}
ll comb(ll n, ll k) {
  if (n < k)
    return 0;
  if (n == 0)
    return 1;
  return mod(mod(kai[n] * kai2[n - k]) * kai2[k]);
}

int main() {
  fastio calc();
  ll n;
  cin >> n;
  ll ans = po(3, n);
  ll gen = 0;
  rep(a, n / 2 + 1, n) { Add(gen, comb(n, a) * po(2, n - a)); }
  cout << mod(ans - gen * 2) << endl;

  return 0;
}


Fixed submission:
// #include <bits/stdc++.h>
#include "bits/stdc++.h"
using namespace std;
typedef long long ll;
// #include "boost/multiprecision/cpp_int.hpp"
// typedef boost::multiprecision::cpp_int LL;
typedef long double dd;
// #define i_7 (ll)(1E9+7)
#define i_7 998244353
#define i_5 i_7 - 2
ll mod(ll a) {
  ll c = a % i_7;
  if (c >= 0)
    return c;
  return c + i_7;
}
typedef pair<ll, ll> l_l;
typedef pair<dd, dd> d_d;
ll inf = (ll)1E16;
#define rep(i, l, r) for (ll i = l; i <= r; i++)
#define pb push_back
ll max(ll a, ll b) {
  if (a < b)
    return b;
  else
    return a;
}
ll min(ll a, ll b) {
  if (a > b)
    return b;
  else
    return a;
}
void Max(ll &pos, ll val) { pos = max(pos, val); } // Max(dp[n],dp[n-1]);
void Min(ll &pos, ll val) { pos = min(pos, val); }
void Add(ll &pos, ll val) { pos = mod(pos + val); }
dd EPS = 1E-9;
#define fastio                                                                 \
  ios::sync_with_stdio(false);                                                 \
  cin.tie(0);                                                                  \
  cout.tie(0);
#define fi first
#define se second
#define endl "\n"
#define SORT(v) sort(v.begin(), v.end())
#define ERASE(v) v.erase(unique(v.begin(), v.end()), v.end())
#define POSL(v, x) (lower_bound(v.begin(), v.end(), x) - v.begin())
#define POSU(v, x) (upper_bound(v.begin(), v.end(), x) - v.begin())
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}

//////////////////////////

ll po(ll i, ll p) {
  if (p == 0)
    return 1;
  else {
    i = mod(i);
    if (p == 1)
      return i;
    if (p % 2 == 0)
      return po(mod(i * i), p / 2);
    return mod(i * po(i, p - 1));
  }
}
ll bunbo(ll n) { return po(n, i_5); }

#define N 25000004
ll kai[N];
ll kai2[N];
void calc() {
  kai[0] = 1;
  kai2[0] = 1;
  rep(i, 1, N - 1) { kai[i] = mod(kai[i - 1] * i); }
  kai2[N - 1] = po(kai[N - 1], i_5);
  for (ll i = N - 2; i >= 0; i--) {
    kai2[i] = mod(kai2[i + 1] * (i + 1));
  }
}
ll comb(ll n, ll k) {
  if (n < k)
    return 0;
  if (n == 0)
    return 1;
  return mod(mod(kai[n] * kai2[n - k]) * kai2[k]);
}

int main() {
  fastio calc();
  ll n;
  cin >> n;
  ll ans = po(3, n);
  ll gen = 0;
  rep(a, n / 2 + 1, n) { Add(gen, comb(n, a) * po(2, n - a)); }
  cout << mod(ans - gen * 2) << endl;

  return 0;
}