Problem p02632 (C++) — buggy submission, verdict: Time Limit Exceeded

#include <bits/stdc++.h>
#define rep(i, n) for (long long i = 0; i < (n); ++i)
#define orep(i, n) for (long long i = 1; i <= (n); ++i)     // one rep
#define drep(i, n) for (long long i = (n)-1; i >= 0; --i)   // down rep
#define srep(i, s, t) for (long long i = (s); i < (t); ++i) // set rep
#define rng(x) (x).begin(), (x).end()

// range
#define rrng(a) (a).rbegin(), (a).rend() // reverse range
#define isin(x, l, r) ((l) <= (x) && (x) < (r))
#define ru(x, y) (((x) + (y)-1) / (y)) // round up
#define fi first
#define se second
#define eb emplace_back
#define fcout cout << fixed << setprecision(15)

using namespace std;
using ll = long long;
using P = pair<int, int>;
using vi = vector<int>;
using vvi = vector<vi>;
using vp = vector<P>;
using vl = vector<ll>;

template <typename T> void Yes(T flag) {
  cout << (flag ? "Yes" : "No") << endl;
}

template <typename T> void pv(vector<T> vec) {
  cout << "[";
  for (auto &v : vec) {
    cout << v << ",";
  }
  cout << "]" << endl;
}

template <> void pv(vector<P> vec) {
  cout << "[";
  for (auto &v : vec) {
    cout << "<" << v.fi << "," << v.se << ">"
         << ",";
  }
  cout << "]" << endl;
}

template <typename T> void pv2(vector<vector<T>> vec) {
  for (auto &v : vec)
    pv(v);
}

const int mod = 1000000007;
// const int mod = 998244353;

struct mint {
  long long x;
  mint(long long x = 0) : x((x % mod + mod) % mod) {}
  mint operator-() const { return mint(-x); }
  mint &operator+=(const mint a) {
    if ((x += a.x) >= mod)
      x -= mod;
    return *this;
  }
  mint &operator-=(const mint a) {
    if ((x -= a.x) < 0)
      x += mod;
    return *this;
  }
  mint &operator*=(const mint a) {
    (x *= a.x) %= mod;
    return *this;
  }
  mint operator+(const mint a) const { return mint(*this) += a; }
  mint operator-(const mint a) const { return mint(*this) -= a; }
  mint operator*(const mint a) const { return mint(*this) *= a; }
  mint operator++() const { return mint(*this) += mint(1); }
  mint operator--() const { return mint(*this) -= mint(1); }
  mint pow(long long t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }

  // for prime mod
  mint inv() const { return pow(mod - 2); }
  mint &operator/=(const mint a) { return *this *= a.inv(); }
  mint operator/(const mint a) const { return mint(*this) /= a; }
};
istream &operator>>(istream &is, const mint &a) { return is >> a.x; }
ostream &operator<<(ostream &os, const mint &a) { return os << a.x; }

struct combination {
  vector<mint> fact, ifact;
  combination(int n) : fact(n + 1), ifact(n + 1) {
    assert(n < mod);
    fact[0] = 1;
    for (int i = 1; i <= n; ++i)
      fact[i] = fact[i - 1] * i;
    ifact[n] = fact[n].inv();
    for (int i = n; i >= 1; --i)
      ifact[i - 1] = ifact[i] * i;
  }
  mint operator()(int n, int k) {
    if (k < 0 || k > n)
      return 0;
    return fact[n] * ifact[k] * ifact[n - k];
  }
};

int main() {
  int k;
  cin >> k;
  string s;
  cin >> s;
  int n = s.size();
  combination c(n + k);
  mint ans;
  rep(i, k + 1) {
    ans += mint(26).pow(k - i) * c(i + n - 1, n - 1) * mint(25).pow(i);
  }
  cout << ans << endl;

  return 0;
}


Error: TLE

Fixed submission:

#include <bits/stdc++.h>
#define rep(i, n) for (long long i = 0; i < (n); ++i)
#define orep(i, n) for (long long i = 1; i <= (n); ++i)     // one rep
#define drep(i, n) for (long long i = (n)-1; i >= 0; --i)   // down rep
#define srep(i, s, t) for (long long i = (s); i < (t); ++i) // set rep
#define rng(x) (x).begin(), (x).end()

// range
#define rrng(a) (a).rbegin(), (a).rend() // reverse range
#define isin(x, l, r) ((l) <= (x) && (x) < (r))
#define ru(x, y) (((x) + (y)-1) / (y)) // round up
#define fi first
#define se second
#define eb emplace_back
#define fcout cout << fixed << setprecision(15)

using namespace std;
using ll = long long;
using P = pair<int, int>;
using vi = vector<int>;
using vvi = vector<vi>;
using vp = vector<P>;
using vl = vector<ll>;

template <typename T> void Yes(T flag) {
  cout << (flag ? "Yes" : "No") << endl;
}

template <typename T> void pv(vector<T> vec) {
  cout << "[";
  for (auto &v : vec) {
    cout << v << ",";
  }
  cout << "]" << endl;
}

template <> void pv(vector<P> vec) {
  cout << "[";
  for (auto &v : vec) {
    cout << "<" << v.fi << "," << v.se << ">"
         << ",";
  }
  cout << "]" << endl;
}

template <typename T> void pv2(vector<vector<T>> vec) {
  for (auto &v : vec)
    pv(v);
}

const int mod = 1000000007;
// const int mod = 998244353;

struct mint {
  long long x;
  mint(long long x = 0) : x((x % mod + mod) % mod) {}
  mint operator-() const { return mint(-x); }
  mint &operator+=(const mint a) {
    if ((x += a.x) >= mod)
      x -= mod;
    return *this;
  }
  mint &operator-=(const mint a) {
    if ((x -= a.x) < 0)
      x += mod;
    return *this;
  }
  mint &operator*=(const mint a) {
    (x *= a.x) %= mod;
    return *this;
  }
  mint operator+(const mint a) const { return mint(*this) += a; }
  mint operator-(const mint a) const { return mint(*this) -= a; }
  mint operator*(const mint a) const { return mint(*this) *= a; }
  mint operator++() const { return mint(*this) += mint(1); }
  mint operator--() const { return mint(*this) -= mint(1); }
  mint pow(long long t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }

  // for prime mod
  mint inv() const { return pow(mod - 2); }
  mint &operator/=(const mint a) { return *this *= a.inv(); }
  mint operator/(const mint a) const { return mint(*this) /= a; }
};
istream &operator>>(istream &is, const mint &a) { return is >> a.x; }
ostream &operator<<(ostream &os, const mint &a) { return os << a.x; }

struct combination {
  vector<mint> fact, ifact;
  combination(int n) : fact(n + 1), ifact(n + 1) {
    assert(n < mod);
    fact[0] = 1;
    for (int i = 1; i <= n; ++i)
      fact[i] = fact[i - 1] * i;
    ifact[n] = fact[n].inv();
    for (int i = n; i >= 1; --i)
      ifact[i - 1] = ifact[i] * i;
  }
  mint operator()(int n, int k) {
    if (k < 0 || k > n)
      return 0;
    return fact[n] * ifact[k] * ifact[n - k];
  }
};

int main() {
  int k;
  cin >> k;
  string s;
  cin >> s;
  int n = s.size();

  mint ans;
  rep(i, k + 1) {
    ans += mint(26).pow(k - i) * combination(i + n - 1)(i + n - 1, n - 1) *
           mint(25).pow(i);
  }
  cout << ans << endl;

  return 0;
}