Problem p02875 (C++)

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

struct ModInt {
  using M = ModInt;
  long long v;
  ModInt(long long _v = 0) : v(_v % MOD + MOD) { norm(); }
  M &norm() {
    v = (v < MOD) ? v : v - MOD;
    return *this;
  }
  M operator+(const M &x) const { return M(v + x.v); }
  M operator-(const M &x) const { return M(v + MOD - x.v); }
  M operator*(const M &x) const { return M(v * x.v % MOD); }
  M operator/(const M &x) const { return M(v * x.inv().v); }
  M &operator+=(const M &x) { return *this = *this + x; }
  M &operator-=(const M &x) { return *this = *this - x; }
  M &operator*=(const M &x) { return *this = *this * x; }
  M &operator/=(const M &x) { return *this = *this / x; }
  friend istream &operator>>(istream &input, M &x) {
    return input >> x.v, x.norm(), input;
  }
  friend ostream &operator<<(ostream &output, const M &x) {
    return output << x.v;
  }
  M pow(long long n) const {
    M x(v), res(1);
    while (n) {
      if (n & 1)
        res *= x;
      x *= x;
      n >>= 1;
    }
    return res;
  }
  M inv() const { return this->pow(MOD - 2); }

  static long long MOD;
  static vector<M> fact, finv;
  static void build(int n) {
    fact.assign(n + 1, 1);
    for (int i = 1; i < n + 1; i++)
      fact[i] = fact[i - 1] * M(i);
    finv.assign(n + 1, fact[n].inv());
    for (int i = n; i > 0; i--)
      finv[i - 1] = finv[i] * M(i);
  }
  static M comb(int n, int k) {
    if (n < k || k < 0)
      return M(0);
    return fact[n] * finv[n - k] * finv[k];
  }
  static M extgcd(int a, int b, int *x, int *y) {
    M d(a);
    if (b) {
      d = extgcd(b, a % b, y, x);
      *y -= (a / b) * *x;
    } else {
      *x = 1, *y = 0;
    }
    return d;
  }
};
vector<ModInt> ModInt::fact = vector<ModInt>();
vector<ModInt> ModInt::finv = vector<ModInt>();
long long ModInt::MOD = 998244353;

int main() {
  int n;
  cin >> n;
  ModInt::build(n);
  ModInt u = ModInt(3).pow(n);
  ModInt c = 0;
  for (int i = n / 2 + 1; i <= n; i++) {
    c += ModInt::comb(n, i) * ModInt(2).pow(n - i);
  }
  cout << u - c * 2 << endl;
}


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

struct ModInt {
  using M = ModInt;
  long long v;
  ModInt(long long _v = 0) : v(_v % MOD + MOD) { norm(); }
  M &norm() {
    v = (v < MOD) ? v : v - MOD;
    return *this;
  }
  M operator+(const M &x) const { return M(v + x.v); }
  M operator-(const M &x) const { return M(v + MOD - x.v); }
  M operator*(const M &x) const { return M(v * x.v % MOD); }
  M operator/(const M &x) const { return M(v * x.inv().v); }
  M &operator+=(const M &x) { return *this = *this + x; }
  M &operator-=(const M &x) { return *this = *this - x; }
  M &operator*=(const M &x) { return *this = *this * x; }
  M &operator/=(const M &x) { return *this = *this / x; }
  friend istream &operator>>(istream &input, M &x) {
    return input >> x.v, x.norm(), input;
  }
  friend ostream &operator<<(ostream &output, const M &x) {
    return output << x.v;
  }
  M pow(long long n) const {
    M x(v), res(1);
    while (n) {
      if (n & 1)
        res *= x;
      x *= x;
      n >>= 1;
    }
    return res;
  }
  M inv() const { return this->pow(MOD - 2); }

  static long long MOD;
  static vector<M> fact, finv;
  static void build(int n) {
    fact.assign(n + 1, 1);
    for (int i = 1; i < n + 1; i++)
      fact[i] = fact[i - 1] * M(i);
    finv.assign(n + 1, fact[n].inv());
    for (int i = n; i > 0; i--)
      finv[i - 1] = finv[i] * M(i);
  }
  static M comb(int n, int k) {
    if (n < k || k < 0)
      return M(0);
    return fact[n] * finv[n - k] * finv[k];
  }
  static M extgcd(int a, int b, int *x, int *y) {
    M d(a);
    if (b) {
      d = extgcd(b, a % b, y, x);
      *y -= (a / b) * *x;
    } else {
      *x = 1, *y = 0;
    }
    return d;
  }
};
vector<ModInt> ModInt::fact = vector<ModInt>();
vector<ModInt> ModInt::finv = vector<ModInt>();
long long ModInt::MOD = 998244353;

int main() {
  int n;
  cin >> n;
  ModInt::build(n);
  ModInt u = ModInt(3).pow(n);
  ModInt c = 0;
  ModInt p = 1;
  for (int i = n; i > n / 2; i--) {
    c += ModInt::comb(n, i) * p;
    p *= 2;
  }
  cout << u - c * 2 << endl;
}