Problem p02632 (C++)

Buggy submission (status: Runtime Error):
#pragma GCC target("avx")
// #pragma GCC target("avx512f,avx512dq,avx512cd,avx512bw,avx512vl")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#include <bits/stdc++.h>
using namespace std;
#define LL long long
#define DB double
#define LD long double
#define ST string
#define BS bitset
#define PA pair<LL, LL>
#define VE vector
#define VL vector<LL>
#define VP vector<pair<LL, LL>>
#define VVL vector<vector<LL>>
#define PQ priority_queue
#define PQS priority_queue<LL, vector<LL>, greater<LL>>
#define PB push_back
#define POB pop_back
#define PF push_front
#define POF pop_front
#define EB emplace_back
#define MP make_pair
#define TS to_string
#define TU to_ullong
#define BPL __builtin_popcountll
#define FOR(i, a, n) for (i = a; i < n; ++i)
#define FORR(i, a, n) for (i = n - 1; i >= a; --i)
#define rep(i, n) FOR(i, 0, n)
#define repr(i, n) FORR(i, 0, n)
#define ALL(a) a.begin(), a.end()
#define RALL(a) a.rbegin(), a.rend()
#define SORT(a) sort(ALL(a))
#define REV(a) reverse(ALL(a))
#define UB(a, n) *upper_bound(ALL(a), n)
#define UBn(a, n) upper_bound(ALL(a), n) - a.begin()
#define LB(a, n) *lower_bound(ALL(a), n)
#define LBn(a, n) lower_bound(ALL(a), n) - a.begin()
#define INF 1145141919810364364LL
#define PI 3.14159265358979
#define MOD 1000000007
// #define MOD 998244353
#define ERR 0.00000001
#define FAST                                                                   \
  cin.tie(0);                                                                  \
  ios::sync_with_stdio(false)

void Yn(LL a) {
  if (a)
    printf("Yes\n");
  else
    printf("No\n");
}
void YN(LL a) {
  if (a)
    printf("YES\n");
  else
    printf("NO\n");
}
LL pwmn(LL a, LL n) {
  LL ans = 1;
  while (ans < a)
    ans *= n;
  return ans;
}
LL dig(LL n) {
  LL ret = 0;
  while (n)
    n /= 10, ret++;
  return ret;
}
LL GCD(LL a, LL b) {
  LL c = 1, tmp = max(a, b);
  b = min(a, b);
  a = tmp;
  while (c != 0) {
    c = a % b;
    a = b;
    b = c;
  }
  return a;
}
LL LCM(LL a, LL b) { return a * b / GCD(a, b); }
int lcmp(const void *a, const void *b) {
  if (*(LL *)a > *(LL *)b)
    return 1;
  if (*(LL *)a < *(LL *)b)
    return -1;
  return 0;
}
int lcmpr(const void *a, const void *b) {
  if (*(LL *)a > *(LL *)b)
    return -1;
  if (*(LL *)a < *(LL *)b)
    return 1;
  return 0;
}
int ccmp(const void *a, const void *b) { return *(char *)a - *(char *)b; }
int ccmpr(const void *a, const void *b) { return *(char *)b - *(char *)a; }
int scmp(const void *a, const void *b) { return strcmp((char *)a, (char *)b); }
int scmpr(const void *a, const void *b) { return strcmp((char *)b, (char *)a); }
LL cmod(LL a, LL m) {
  if (a % m < 0)
    return a % m + abs(m);
  else
    return a % m;
}
LL DIV(LL a, LL d, LL m) {
  LL l = m, x = 1, y = 0, k;
  while (l) {
    k = d / l;
    d -= k * l;
    swap(l, d);
    x -= k * y;
    swap(x, y);
  }
  return cmod(a * cmod(x, m), m);
}
LL POW(LL a, LL n, LL m) {
  LL ans = 1;
  while (n > 0) {
    if (n & 1)
      ans = ans * a % m;
    a = a * a % m;
    n >>= 1;
  }
  return ans;
}
VL fact, finv, inv;
void comi(LL n) {
  LL i;
  fact.resize(max(2LL, n + 1));
  finv.resize(max(2LL, n + 1));
  inv.resize(max(2LL, n + 1));
  fact[0] = fact[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  FOR(i, 2, n + 1) {
    fact[i] = fact[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}
LL com(LL n, LL k) {
  if (n < k || n < 0 || k < 0)
    return 0;
  return fact[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}
bool cmps(pair<LL, LL> a, pair<LL, LL> b) {
  if (a.second != b.second)
    return a.second < b.second;
  return a.first < b.first;
}
template <typename T> bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}

#define MI modint<MOD>
template <int mod> struct modint {
  int x;
  modint() : x(0) {}
  modint(LL n) : x(n >= 0 ? n % mod : (mod - (-n) % mod) % mod) {}
  modint &operator+=(const modint &n) {
    if ((x += n.x) >= mod)
      x -= mod;
    return *this;
  }
  modint &operator-=(const modint &n) {
    if ((x += mod - n.x) >= mod)
      x -= mod;
    return *this;
  }
  modint &operator++() {
    *this += 1;
    return *this;
  }
  modint &operator--() {
    *this -= 1;
    return *this;
  }
  modint &operator*=(const modint &n) {
    x = (int)((LL)x * n.x % mod);
    return *this;
  }
  modint &operator/=(const modint &n) {
    *this *= n.inv();
    return *this;
  }
  modint operator-() const { return modint(-x); }
  modint operator+(const modint &n) const { return modint(*this) += n; }
  modint operator-(const modint &n) const { return modint(*this) -= n; }
  modint operator++(int) {
    modint ret(*this);
    *this += 1;
    return ret;
  }
  modint operator--(int) {
    modint ret(*this);
    *this -= 1;
    return ret;
  }
  modint operator*(const modint &n) const { return modint(*this) *= n; }
  modint operator/(const modint &n) const { return modint(*this) /= n; }
  bool operator<(const modint &n) const { return x < n.x; }
  bool operator>(const modint &n) const { return x > n.x; }
  bool operator<=(const modint &n) const { return x <= n.x; }
  bool operator>=(const modint &n) const { return x >= n.x; }
  bool operator!=(const modint &n) const { return x != n.x; }
  bool operator==(const modint &n) const { return x == n.x; }
  friend istream &operator>>(istream &is, modint &n) {
    LL l;
    is >> l;
    n = modint<mod>(l);
    return is;
  }
  friend ostream &operator<<(ostream &os, const modint &n) { return os << n.x; }
  int getmod() { return mod; }
  modint inv() const {
    int a = x, b = mod, c = 1, d = 0, n;
    while (b) {
      n = a / b;
      swap(a -= n * b, b);
      swap(c -= n * d, d);
    }
    return modint(c);
  }
  modint pow(LL n) const {
    modint ret(1), m(x);
    while (n) {
      if (n & 1)
        ret *= m;
      m *= m;
      n >>= 1;
    }
    return ret;
  }
};

int main() {
  FAST;
  LL K, i, j, n;
  ST S;
  cin >> K >> S;
  comi(200005);
  MI ans;
  n = S.size();
  rep(i, K + 1) {
    ans += (MI)com(K + n - i - 1, n - 1) * (MI)POW(26, i, MOD) *
           (MI)POW(25, K - i, MOD);
  }
  cout << ans << endl;
}


Fixed submission:
#pragma GCC target("avx")
// #pragma GCC target("avx512f,avx512dq,avx512cd,avx512bw,avx512vl")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#include <bits/stdc++.h>
using namespace std;
#define LL long long
#define DB double
#define LD long double
#define ST string
#define BS bitset
#define PA pair<LL, LL>
#define VE vector
#define VL vector<LL>
#define VP vector<pair<LL, LL>>
#define VVL vector<vector<LL>>
#define PQ priority_queue
#define PQS priority_queue<LL, vector<LL>, greater<LL>>
#define PB push_back
#define POB pop_back
#define PF push_front
#define POF pop_front
#define EB emplace_back
#define MP make_pair
#define TS to_string
#define TU to_ullong
#define BPL __builtin_popcountll
#define FOR(i, a, n) for (i = a; i < n; ++i)
#define FORR(i, a, n) for (i = n - 1; i >= a; --i)
#define rep(i, n) FOR(i, 0, n)
#define repr(i, n) FORR(i, 0, n)
#define ALL(a) a.begin(), a.end()
#define RALL(a) a.rbegin(), a.rend()
#define SORT(a) sort(ALL(a))
#define REV(a) reverse(ALL(a))
#define UB(a, n) *upper_bound(ALL(a), n)
#define UBn(a, n) upper_bound(ALL(a), n) - a.begin()
#define LB(a, n) *lower_bound(ALL(a), n)
#define LBn(a, n) lower_bound(ALL(a), n) - a.begin()
#define INF 1145141919810364364LL
#define PI 3.14159265358979
#define MOD 1000000007
// #define MOD 998244353
#define ERR 0.00000001
#define FAST                                                                   \
  cin.tie(0);                                                                  \
  ios::sync_with_stdio(false)

void Yn(LL a) {
  if (a)
    printf("Yes\n");
  else
    printf("No\n");
}
void YN(LL a) {
  if (a)
    printf("YES\n");
  else
    printf("NO\n");
}
LL pwmn(LL a, LL n) {
  LL ans = 1;
  while (ans < a)
    ans *= n;
  return ans;
}
LL dig(LL n) {
  LL ret = 0;
  while (n)
    n /= 10, ret++;
  return ret;
}
LL GCD(LL a, LL b) {
  LL c = 1, tmp = max(a, b);
  b = min(a, b);
  a = tmp;
  while (c != 0) {
    c = a % b;
    a = b;
    b = c;
  }
  return a;
}
LL LCM(LL a, LL b) { return a * b / GCD(a, b); }
int lcmp(const void *a, const void *b) {
  if (*(LL *)a > *(LL *)b)
    return 1;
  if (*(LL *)a < *(LL *)b)
    return -1;
  return 0;
}
int lcmpr(const void *a, const void *b) {
  if (*(LL *)a > *(LL *)b)
    return -1;
  if (*(LL *)a < *(LL *)b)
    return 1;
  return 0;
}
int ccmp(const void *a, const void *b) { return *(char *)a - *(char *)b; }
int ccmpr(const void *a, const void *b) { return *(char *)b - *(char *)a; }
int scmp(const void *a, const void *b) { return strcmp((char *)a, (char *)b); }
int scmpr(const void *a, const void *b) { return strcmp((char *)b, (char *)a); }
LL cmod(LL a, LL m) {
  if (a % m < 0)
    return a % m + abs(m);
  else
    return a % m;
}
LL DIV(LL a, LL d, LL m) {
  LL l = m, x = 1, y = 0, k;
  while (l) {
    k = d / l;
    d -= k * l;
    swap(l, d);
    x -= k * y;
    swap(x, y);
  }
  return cmod(a * cmod(x, m), m);
}
LL POW(LL a, LL n, LL m) {
  LL ans = 1;
  while (n > 0) {
    if (n & 1)
      ans = ans * a % m;
    a = a * a % m;
    n >>= 1;
  }
  return ans;
}
VL fact, finv, inv;
void comi(LL n) {
  LL i;
  fact.resize(max(2LL, n + 1));
  finv.resize(max(2LL, n + 1));
  inv.resize(max(2LL, n + 1));
  fact[0] = fact[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  FOR(i, 2, n + 1) {
    fact[i] = fact[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}
LL com(LL n, LL k) {
  if (n < k || n < 0 || k < 0)
    return 0;
  return fact[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}
bool cmps(pair<LL, LL> a, pair<LL, LL> b) {
  if (a.second != b.second)
    return a.second < b.second;
  return a.first < b.first;
}
template <typename T> bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}

#define MI modint<MOD>
template <int mod> struct modint {
  int x;
  modint() : x(0) {}
  modint(LL n) : x(n >= 0 ? n % mod : (mod - (-n) % mod) % mod) {}
  modint &operator+=(const modint &n) {
    if ((x += n.x) >= mod)
      x -= mod;
    return *this;
  }
  modint &operator-=(const modint &n) {
    if ((x += mod - n.x) >= mod)
      x -= mod;
    return *this;
  }
  modint &operator++() {
    *this += 1;
    return *this;
  }
  modint &operator--() {
    *this -= 1;
    return *this;
  }
  modint &operator*=(const modint &n) {
    x = (int)((LL)x * n.x % mod);
    return *this;
  }
  modint &operator/=(const modint &n) {
    *this *= n.inv();
    return *this;
  }
  modint operator-() const { return modint(-x); }
  modint operator+(const modint &n) const { return modint(*this) += n; }
  modint operator-(const modint &n) const { return modint(*this) -= n; }
  modint operator++(int) {
    modint ret(*this);
    *this += 1;
    return ret;
  }
  modint operator--(int) {
    modint ret(*this);
    *this -= 1;
    return ret;
  }
  modint operator*(const modint &n) const { return modint(*this) *= n; }
  modint operator/(const modint &n) const { return modint(*this) /= n; }
  bool operator<(const modint &n) const { return x < n.x; }
  bool operator>(const modint &n) const { return x > n.x; }
  bool operator<=(const modint &n) const { return x <= n.x; }
  bool operator>=(const modint &n) const { return x >= n.x; }
  bool operator!=(const modint &n) const { return x != n.x; }
  bool operator==(const modint &n) const { return x == n.x; }
  friend istream &operator>>(istream &is, modint &n) {
    LL l;
    is >> l;
    n = modint<mod>(l);
    return is;
  }
  friend ostream &operator<<(ostream &os, const modint &n) { return os << n.x; }
  int getmod() { return mod; }
  modint inv() const {
    int a = x, b = mod, c = 1, d = 0, n;
    while (b) {
      n = a / b;
      swap(a -= n * b, b);
      swap(c -= n * d, d);
    }
    return modint(c);
  }
  modint pow(LL n) const {
    modint ret(1), m(x);
    while (n) {
      if (n & 1)
        ret *= m;
      m *= m;
      n >>= 1;
    }
    return ret;
  }
};

int main() {
  FAST;
  LL K, i, j, n;
  ST S;
  cin >> K >> S;
  comi(2000005);
  MI ans;
  n = S.size();
  rep(i, K + 1) {
    ans += (MI)com(K + n - i - 1, n - 1) * (MI)POW(26, i, MOD) *
           (MI)POW(25, K - i, MOD);
  }
  cout << ans << endl;
}