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

// #pragma GCC optimize(2)
// #pragma G++ optimize(2)
// #pragma comment(linker,"/STACK:102400000,102400000")

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

typedef long long LL;
typedef unsigned int ui;
typedef unsigned long long ull;
typedef float fl;
typedef long double ld;
typedef pair<int, int> pii;
#if (WIN32) || (WIN64) || (__WIN32) || (__WIN64) || (_WIN32) || (_WIN64) ||    \\
    (WINDOWS)
#define lld "%I64d"
#define llu "%I64u"
#else
#define lld "%lld"
#define llu "%llu"
#endif
#define ui(n) ((unsigned int)(n))
#define LL(n) ((long long)(n))
#define ull(n) ((unsigned long long)(n))
#define fl(n) ((float)(n))
#define ld(n) ((long double)(n))
#define char(n) ((char)(n))
#define Bool(n) ((bool)(n))
#define fixpoint(n) fixed << setprecision(n)

const int INF = 1061109567;
const int NINF = -1044266559;
const LL LINF = 4557430888798830399;
const ld eps = 1e-15;
#define MOD (1000000007)
#define PI (3.1415926535897932384626433832795028841971)

#define MP make_pair
#define MT make_tuple
#define All(a) (a).begin(), (a).end()
#define pall(a) (a).rbegin(), (a).rend()
#define Log(x, y) log(x) / log(y)
#define SZ(a) ((int)(a).size())
#define rep(i, n) for (int i = 0; i < ((int)(n)); i++)
#define rep1(i, n) for (int i = 1; i <= ((int)(n)); i++)
#define repd(i, n) for (int i = ((int)(n)) - 1; i >= 0; i--)
#define repd1(i, n) for (int i = ((int)(n)); i >= 1; i--)
#define repv(itr, v)                                                           \\
  for (__typeof((v).begin()) itr = (v).begin(); itr != (v).end(); itr++)
#define repV(i, v) for (auto i : v)
#define repE(i, v) for (auto &i : v)
#define MS(x, y) memset(x, y, sizeof(x))
#define MC(x) MS(x, 0)
#define MINF(x) MS(x, 63)
#define MCP(x, y) memcpy(x, y, sizeof(y))
#define UN(v) sort(All(v)), v.erase(unique(All(v)), v.end())
#define filein(x) freopen(x, "r", stdin)
#define fileout(x) freopen(x, "w", stdout)
#define fileio(x)                                                              \\
  freopen(x ".in", "r", stdin);                                                \\
  freopen(x ".out", "w", stdout)
#define filein2(filename, name) ifstream name(filename, ios::in)
#define fileout2(filename, name) ofstream name(filename, ios::out)
#define fileio2(filename, name) fstream name(filename, ios::in | ios::out)
#define sqr(x) ((x) * (x))
#define Pause system("pause")
#define Cls system("cls")
#define fs first
#define sc second
#define SF scanf
#define PF printf

inline int Read() {
  int x = 0, w = 0;
  char ch = 0;
  while (!isdigit(ch)) {
    w |= ch == '-';
    ch = getchar();
  }
  while (isdigit(ch))
    x = (x << 3) + (x << 1) + (ch ^ 48), ch = getchar();
  return w ? -x : x;
}
inline void Write(int x) {
  if (x < 0)
    putchar('-'), x = -x;
  if (x > 9)
    Write(x / 10);
  putchar(x % 10 + '0');
}
inline LL powmod(LL a, LL b) {
  LL res = 1;
  a %= MOD;
  assert(b >= 0);
  for (; b; b >>= 1) {
    if (b & 1)
      res = res * a % MOD;
    a = a * a % MOD;
  }
  return res % MOD;
}
inline LL gcdll(LL a, LL b) { return b ? gcdll(b, a % b) : a; }

const int dx[] = {0, 1, 0, -1, 1, -1, -1, 1};
const int dy[] = {1, 0, -1, 0, -1, -1, 1, 1};
/************************************************************BEGIN************************************************************/
const int maxn = 2000010;

int n, m;
string s;
LL fac[maxn], inv[maxn], ans;

inline LL C(int a, int b) {
  if (b > a)
    return 0ll;
  else
    return fac[a] * inv[b] % MOD * inv[a - b] % MOD;
}

int main() {
  fac[0] = fac[1] = 1;
  for (int i = 2; i <= 2000000; i++)
    fac[i] = fac[i - 1] * i % MOD;

  inv[2000000] = powmod(fac[2000000], MOD - 2);
  for (int i = 2000000 - 1; i >= 0; i--)
    inv[i] = inv[i + 1] * (i + 1) % MOD;

  cin >> n >> s;
  m = SZ(s);

  for (int i = m; i <= n + m; i++)
    ans = (ans + C(i - 1, m - 1) * powmod(25, i - m) % MOD *
                     powmod(26, n + m - i) % MOD) %
          MOD;

  cout << ans;

  return 0;
}
/*************************************************************END**************************************************************/

Error: 0

Fixed submission:

// #pragma GCC optimize(2)
// #pragma G++ optimize(2)
// #pragma comment(linker,"/STACK:102400000,102400000")

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

typedef long long LL;
typedef unsigned int ui;
typedef unsigned long long ull;
typedef float fl;
typedef long double ld;
typedef pair<int, int> pii;
#if (WIN32) || (WIN64) || (__WIN32) || (__WIN64) || (_WIN32) || (_WIN64) ||    \\
    (WINDOWS)
#define lld "%I64d"
#define llu "%I64u"
#else
#define lld "%lld"
#define llu "%llu"
#endif
#define ui(n) ((unsigned int)(n))
#define LL(n) ((long long)(n))
#define ull(n) ((unsigned long long)(n))
#define fl(n) ((float)(n))
#define ld(n) ((long double)(n))
#define char(n) ((char)(n))
#define Bool(n) ((bool)(n))
#define fixpoint(n) fixed << setprecision(n)

const int INF = 1061109567;
const int NINF = -1044266559;
const LL LINF = 4557430888798830399;
const ld eps = 1e-15;
#define MOD (1000000007)
#define PI (3.1415926535897932384626433832795028841971)

#define MP make_pair
#define MT make_tuple
#define All(a) (a).begin(), (a).end()
#define pall(a) (a).rbegin(), (a).rend()
#define Log(x, y) log(x) / log(y)
#define SZ(a) ((int)(a).size())
#define rep(i, n) for (int i = 0; i < ((int)(n)); i++)
#define rep1(i, n) for (int i = 1; i <= ((int)(n)); i++)
#define repd(i, n) for (int i = ((int)(n)) - 1; i >= 0; i--)
#define repd1(i, n) for (int i = ((int)(n)); i >= 1; i--)
#define repv(itr, v)                                                           \\
  for (__typeof((v).begin()) itr = (v).begin(); itr != (v).end(); itr++)
#define repV(i, v) for (auto i : v)
#define repE(i, v) for (auto &i : v)
#define MS(x, y) memset(x, y, sizeof(x))
#define MC(x) MS(x, 0)
#define MINF(x) MS(x, 63)
#define MCP(x, y) memcpy(x, y, sizeof(y))
#define UN(v) sort(All(v)), v.erase(unique(All(v)), v.end())
#define filein(x) freopen(x, "r", stdin)
#define fileout(x) freopen(x, "w", stdout)
#define fileio(x)                                                              \\
  freopen(x ".in", "r", stdin);                                                \\
  freopen(x ".out", "w", stdout)
#define filein2(filename, name) ifstream name(filename, ios::in)
#define fileout2(filename, name) ofstream name(filename, ios::out)
#define fileio2(filename, name) fstream name(filename, ios::in | ios::out)
#define sqr(x) ((x) * (x))
#define Pause system("pause")
#define Cls system("cls")
#define fs first
#define sc second
#define SF scanf
#define PF printf

inline int Read() {
  int x = 0, w = 0;
  char ch = 0;
  while (!isdigit(ch)) {
    w |= ch == '-';
    ch = getchar();
  }
  while (isdigit(ch))
    x = (x << 3) + (x << 1) + (ch ^ 48), ch = getchar();
  return w ? -x : x;
}
inline void Write(int x) {
  if (x < 0)
    putchar('-'), x = -x;
  if (x > 9)
    Write(x / 10);
  putchar(x % 10 + '0');
}
inline LL powmod(LL a, LL b) {
  LL res = 1;
  a %= MOD;
  assert(b >= 0);
  for (; b; b >>= 1) {
    if (b & 1)
      res = res * a % MOD;
    a = a * a % MOD;
  }
  return res % MOD;
}
inline LL gcdll(LL a, LL b) { return b ? gcdll(b, a % b) : a; }

const int dx[] = {0, 1, 0, -1, 1, -1, -1, 1};
const int dy[] = {1, 0, -1, 0, -1, -1, 1, 1};
/************************************************************BEGIN************************************************************/
const int maxn = 1000010;

int n, m;
string s;
LL fac[maxn], inv[maxn], ans;

inline LL C(int a, int b) {
  if (b > a)
    return 0;
  else
    return fac[a] * inv[b] % MOD * inv[a - b] % MOD;
}

int main() {
  fac[0] = fac[1] = 1;
  for (int i = 2; i <= 1000000; i++)
    fac[i] = fac[i - 1] * i % MOD;

  inv[1000000] = powmod(fac[1000000], MOD - 2);
  for (int i = 1000000 - 1; i >= 0; i--)
    inv[i] = inv[i + 1] * (i + 1) % MOD;

  cin >> n >> s;
  m = SZ(s);

  for (int i = m; i <= n + m; i++)
    ans = (ans + C(i - 1, m - 1) * powmod(25ll, LL(i - m)) % MOD *
                     powmod(26ll, LL(n + m - i)) % MOD) %
          MOD;

  cout << ans << endl;

  return 0;
}
/*************************************************************END**************************************************************/