Problem p02632 (C++) — buggy submission, verdict: Runtime Error

#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const int mod = 1000000007;

struct mint {
  ll x;
  mint(ll 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 += mod - a.x) >= mod)
      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 pow(ll t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }
  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 cominit {
  vector<mint> fact, ifact;
  cominit(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 r) {
    if (n < r or r < 0)
      return 0;
    return fact[n] * ifact[r] * ifact[n - r];
  }
};

int main() {
  int k;
  string s;
  cin >> k >> s;
  int ss = s.size();
  cominit nCr(k + ss);
  mint ans = 0;
  for (int i = ss; i <= k + ss; i++) {
    mint res = nCr(i - 1, ss - 1);
    res *= mint(25).pow(i - ss);
    res *= mint(26).pow(k + ss - i);
    ans += res;
  }
  cout << ans << endl;
  return 0;
}

Error: 0

Fixed submission:

#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const int mod = 1000000007;

struct mint {
  ll x;
  mint(ll 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 += mod - a.x) >= mod)
      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 pow(ll t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }
  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; }
};
ostream &operator<<(ostream &os, const mint &a) { return os << a.x; }

ll powmod(ll a, int b) {
  a %= mod;
  ll res = 1;
  while (b) {
    if (b % 2 == 1)
      res = res * a % mod;
    a = a * a % mod;
    b >>= 1;
  }
  return res;
}

ll f[200005];
void cominit() {
  f[0] = 1;
  for (int i = 0; i <= 200000; i++)
    f[i + 1] = f[i] * (i + 1) % mod;
}

ll nCr(int n, int r) {
  if (n < r or r < 0)
    return 0;
  return f[n] * powmod(f[r] * f[n - r], mod - 2) % mod;
}

int main() {
  int k;
  string s;
  cin >> k >> s;
  int ss = s.size();
  cominit();
  mint ans = 0;
  for (int i = ss; i <= k + ss; i++) {
    mint res = nCr(i - 1, ss - 1);
    res *= mint(25).pow(i - ss);
    res *= mint(26).pow(k + ss - i);
    ans += res;
  }
  cout << ans << endl;
  return 0;
}