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

#define _CRT_SECURE_NO_WARNINGS
#pragma GCC target("avx")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cfloat>
#include <climits>
#include <cmath>
#include <complex>
#include <ctime>
#include <deque>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <list>
#include <map>
#include <memory>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>
#define rep(i, n) for (int i = 0; i < (lint)(n); i++)
#define REP(i, n) for (int i = 1; i <= (lint)(n); i++)
#define all(V) V.begin(), V.end()
#define stackReplaceBegin(size)                                                \\
  void *p = malloc(size);                                                      \\
  esp_new = (lint)p + size - 1;                                                \\
  void *stack_extend_memory_ = malloc(size);                                   \\
  void *stack_extend_origin_memory_;                                           \\
  char *stack_extend_dummy_memory_ = (char *)alloca(                           \\
      (1 + (int)(((long long)stack_extend_memory_) & 127)) * 16);              \\
  *stack_extend_dummy_memory_ = 0;                                             \\
  asm volatile("mov %%rsp, %%rbx\\nmov %%rax, %%rsp"                            \\
               : "=b"(stack_extend_origin_memory_)                             \\
               : "a"((char *)stack_extend_memory_ + (size)-1024));
typedef long long lint;
typedef unsigned long long ulint;
typedef std::pair<lint, lint> P;
constexpr int INF = INT_MAX / 2;
constexpr lint LINF = LLONG_MAX / 2;
constexpr double eps = DBL_EPSILON;
constexpr double PI = 3.141592653589793238462643383279;
lint esp_new;
template <class T>
class prique : public std::priority_queue<T, std::vector<T>, std::greater<T>> {
};
template <class T, class U> inline bool chmax(T &lhs, const U &rhs) {
  if (lhs < rhs) {
    lhs = rhs;
    return 1;
  }
  return 0;
}
template <class T, class U> inline bool chmin(T &lhs, const U &rhs) {
  if (lhs > rhs) {
    lhs = rhs;
    return 1;
  }
  return 0;
}
inline lint gcd(lint a, lint b) {
  while (b) {
    lint c = a;
    a = b;
    b = c % b;
  }
  return a;
}
inline lint lcm(lint a, lint b) { return a / gcd(a, b) * b; }
bool isprime(lint n) {
  if (n == 1)
    return false;
  for (int i = 2; i * i <= n; i++) {
    if (n % i == 0)
      return false;
  }
  return true;
}
template <typename T> T mypow(T a, unsigned int b) {
  if (!b)
    return T(1);
  if (b & 1)
    return mypow(a, b - 1) * a;
  T memo = mypow(a, b >> 1);
  return memo * memo;
}
lint modpow(lint a, lint b, lint m) {
  if (!b)
    return 1;
  if (b & 1)
    return modpow(a, b - 1, m) * a % m;
  lint memo = modpow(a, b >> 1, m);
  return memo * memo % m;
}
template <typename T> void printArray(std::vector<T> &vec) {
  rep(i, vec.size() - 1) std::cout << vec[i] << " ";
  std::cout << vec.back() << std::endl;
}
template <typename T> void printArray(T l, T r) {
  T rprev = r;
  rprev--;
  for (T i = l; i != rprev; i++) {
    std::cout << *i << " ";
  }
  std::cout << *rprev << std::endl;
}
class ModInt {
  lint value;

public:
  static const unsigned int modulo;
  ModInt() : value(0) {}
  template <typename T> ModInt(T value = 0) : value(value) {
    if (value < 0)
      value = -(lint)(-value % modulo) + modulo;
    this->value = value % modulo;
  }
  inline operator int() const { return value; }
  inline ModInt &operator+=(const ModInt &x) {
    value += x.value;
    if (value >= modulo)
      value -= modulo;
    return *this;
  }
  inline ModInt &operator++() {
    if (value == modulo - 1)
      value = 0;
    else
      value++;
    return *this;
  }
  inline ModInt operator-() const { return ModInt(0) -= *this; }
  inline ModInt &operator-=(const ModInt &x) {
    value -= x.value;
    if (value < 0)
      value += modulo;
    return *this;
  }
  inline ModInt &operator--() {
    if (value == 0)
      value = modulo - 1;
    else
      value--;
    return *this;
  }
  inline ModInt &operator*=(const ModInt &x) {
    value = value * x.value % modulo;
    return *this;
  }
  inline ModInt &operator/=(ModInt rhs) {
    int exp = modulo - 2;
    while (exp) {
      if (exp & 1)
        *this *= rhs;
      rhs *= rhs;
      exp >>= 1;
    }
    return *this;
  }
  template <typename T> ModInt operator+(const T &rhs) const {
    return ModInt(*this) += rhs;
  }
  template <typename T> ModInt &operator+=(const T &rhs) {
    return operator+=(ModInt(rhs));
  }
  template <typename T> ModInt operator-(const T &rhs) const {
    return ModInt(*this) -= rhs;
  }
  template <typename T> ModInt &operator-=(const T &rhs) {
    return operator-=(ModInt(rhs));
  }
  template <typename T> ModInt operator*(const T &rhs) const {
    return ModInt(*this) *= rhs;
  }
  template <typename T> ModInt &operator*=(const T &rhs) {
    return operator*=(ModInt(rhs));
  }
  template <typename T> ModInt operator/(const T &rhs) const {
    return ModInt(*this) /= rhs;
  }
  template <typename T> ModInt &operator/=(const T &rhs) {
    return operator/=(ModInt(rhs));
  }
};
std::istream &operator>>(std::istream &ist, ModInt &x) {
  lint a;
  ist >> a;
  x = a;
  return ist;
}
const unsigned int ModInt::modulo = 1000000007;
int k;
std::string s;
ModInt fact[2000010], inv[2000010];
ModInt comb(int a, int b) { return fact[a] * inv[b] * inv[a - b]; }
int main() {
  std::cin >> k >> s;
  int n = s.size();
  fact[0] = 1;
  REP(i, n + k) { fact[i] = fact[i - 1] * i; }
  inv[n + k] = ModInt(1) / fact[n + k];
  for (int i = n + k - 1; i >= 0; i--)
    inv[i] = inv[i + 1] * (i + 1);
  ModInt ans = 0;
  rep(i, k + 1) {
    ans += mypow(ModInt(26), i) * mypow(ModInt(25), k - i) *
           comb(n + k - i - 1, n - 1);
  }
  std::cout << ans << std::endl;
  return 0;
}


Error: 0

Fixed submission:

#define _CRT_SECURE_NO_WARNINGS
#pragma GCC target("avx")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cfloat>
#include <climits>
#include <cmath>
#include <complex>
#include <ctime>
#include <deque>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <list>
#include <map>
#include <memory>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>
#define rep(i, n) for (int i = 0; i < (lint)(n); i++)
#define REP(i, n) for (int i = 1; i <= (lint)(n); i++)
#define all(V) V.begin(), V.end()
#define stackReplaceBegin(size)                                                \\
  void *p = malloc(size);                                                      \\
  esp_new = (lint)p + size - 1;                                                \\
  void *stack_extend_memory_ = malloc(size);                                   \\
  void *stack_extend_origin_memory_;                                           \\
  char *stack_extend_dummy_memory_ = (char *)alloca(                           \\
      (1 + (int)(((long long)stack_extend_memory_) & 127)) * 16);              \\
  *stack_extend_dummy_memory_ = 0;                                             \\
  asm volatile("mov %%rsp, %%rbx\\nmov %%rax, %%rsp"                            \\
               : "=b"(stack_extend_origin_memory_)                             \\
               : "a"((char *)stack_extend_memory_ + (size)-1024));
typedef long long lint;
typedef unsigned long long ulint;
typedef std::pair<lint, lint> P;
constexpr int INF = INT_MAX / 2;
constexpr lint LINF = LLONG_MAX / 2;
constexpr double eps = DBL_EPSILON;
constexpr double PI = 3.141592653589793238462643383279;
lint esp_new;
template <class T>
class prique : public std::priority_queue<T, std::vector<T>, std::greater<T>> {
};
template <class T, class U> inline bool chmax(T &lhs, const U &rhs) {
  if (lhs < rhs) {
    lhs = rhs;
    return 1;
  }
  return 0;
}
template <class T, class U> inline bool chmin(T &lhs, const U &rhs) {
  if (lhs > rhs) {
    lhs = rhs;
    return 1;
  }
  return 0;
}
inline lint gcd(lint a, lint b) {
  while (b) {
    lint c = a;
    a = b;
    b = c % b;
  }
  return a;
}
inline lint lcm(lint a, lint b) { return a / gcd(a, b) * b; }
bool isprime(lint n) {
  if (n == 1)
    return false;
  for (int i = 2; i * i <= n; i++) {
    if (n % i == 0)
      return false;
  }
  return true;
}
template <typename T> T mypow(T a, unsigned int b) {
  if (!b)
    return T(1);
  if (b & 1)
    return mypow(a, b - 1) * a;
  T memo = mypow(a, b >> 1);
  return memo * memo;
}
lint modpow(lint a, lint b, lint m) {
  if (!b)
    return 1;
  if (b & 1)
    return modpow(a, b - 1, m) * a % m;
  lint memo = modpow(a, b >> 1, m);
  return memo * memo % m;
}
template <typename T> void printArray(std::vector<T> &vec) {
  rep(i, vec.size() - 1) std::cout << vec[i] << " ";
  std::cout << vec.back() << std::endl;
}
template <typename T> void printArray(T l, T r) {
  T rprev = r;
  rprev--;
  for (T i = l; i != rprev; i++) {
    std::cout << *i << " ";
  }
  std::cout << *rprev << std::endl;
}
class ModInt {
  lint value;

public:
  static const unsigned int modulo;
  ModInt() : value(0) {}
  template <typename T> ModInt(T value = 0) : value(value) {
    if (value < 0)
      value = -(lint)(-value % modulo) + modulo;
    this->value = value % modulo;
  }
  inline operator int() const { return value; }
  inline ModInt &operator+=(const ModInt &x) {
    value += x.value;
    if (value >= modulo)
      value -= modulo;
    return *this;
  }
  inline ModInt &operator++() {
    if (value == modulo - 1)
      value = 0;
    else
      value++;
    return *this;
  }
  inline ModInt operator-() const { return ModInt(0) -= *this; }
  inline ModInt &operator-=(const ModInt &x) {
    value -= x.value;
    if (value < 0)
      value += modulo;
    return *this;
  }
  inline ModInt &operator--() {
    if (value == 0)
      value = modulo - 1;
    else
      value--;
    return *this;
  }
  inline ModInt &operator*=(const ModInt &x) {
    value = value * x.value % modulo;
    return *this;
  }
  inline ModInt &operator/=(ModInt rhs) {
    int exp = modulo - 2;
    while (exp) {
      if (exp & 1)
        *this *= rhs;
      rhs *= rhs;
      exp >>= 1;
    }
    return *this;
  }
  template <typename T> ModInt operator+(const T &rhs) const {
    return ModInt(*this) += rhs;
  }
  template <typename T> ModInt &operator+=(const T &rhs) {
    return operator+=(ModInt(rhs));
  }
  template <typename T> ModInt operator-(const T &rhs) const {
    return ModInt(*this) -= rhs;
  }
  template <typename T> ModInt &operator-=(const T &rhs) {
    return operator-=(ModInt(rhs));
  }
  template <typename T> ModInt operator*(const T &rhs) const {
    return ModInt(*this) *= rhs;
  }
  template <typename T> ModInt &operator*=(const T &rhs) {
    return operator*=(ModInt(rhs));
  }
  template <typename T> ModInt operator/(const T &rhs) const {
    return ModInt(*this) /= rhs;
  }
  template <typename T> ModInt &operator/=(const T &rhs) {
    return operator/=(ModInt(rhs));
  }
};
std::istream &operator>>(std::istream &ist, ModInt &x) {
  lint a;
  ist >> a;
  x = a;
  return ist;
}
const unsigned int ModInt::modulo = 1000000007;
int k;
std::string s;
ModInt fact[1000010], inv[1000010];
ModInt comb(int a, int b) { return fact[a] * inv[b] * inv[a - b]; }
int main() {
  std::cin >> k >> s;
  int n = s.size();
  fact[0] = 1;
  REP(i, 2 * (n + k)) { fact[i] = fact[i - 1] * i; }
  inv[2 * (n + k)] = ModInt(1) / fact[2 * (n + k)];
  for (int i = 2 * (n + k) - 1; i >= 0; i--)
    inv[i] = inv[i + 1] * (i + 1);
  ModInt ans = 0;
  rep(i, k + 1) {
    ans += mypow(ModInt(26), i) * mypow(ModInt(25), k - i) *
           comb(n + k - i - 1, n - 1);
  }
  std::cout << ans << std::endl;
  return 0;
}