Problem p02632 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
#define int long long
#define x first
#define y second
#define getbit(x, i) (((x) >> (i)) & 1)
using namespace std;
typedef pair<int, int> pii;
#define hashset unordered_set
#define hashmap unordered_map
#define newline fast_writechar('\n')
#define unify(arr) arr.resize(unique(arr.begin(), arr.end()) - arr.begin())
#define getbit(x, i) (((x) >> (i)) & 1)

template <typename T> vector<T> readvector(size_t sz) {
  vector<T> res(sz);
  for (size_t i = 0; i < sz; ++i) {
    cin >> res[i];
  }
  return res;
}
template <typename T>
std::ostream &operator<<(std::ostream &out, const std::vector<T> &v) {
  std::copy(v.begin(), v.end(), std::ostream_iterator<T>(out, " "));
  return out;
}

int mylog(int n) { return 63 - __builtin_clzll(n); }

inline int binPow(int x, int deg, int mod) {
  int ans = 1;
  for (int i = sizeof(deg) * CHAR_BIT - 1; i >= 0; i--) {
    ans *= ans;
    ans %= mod;
    if (getbit(deg, i))
      ans *= x;
    ans %= mod;
  }
  return ans;
}

/** Interface */

inline int readInt();
inline int readUInt();
inline void readWord(char *s);
inline int fast_readchar(); // you may use readchar() instead of it

inline void writeInt(int x);
inline void fast_writechar(int x); // you may use putchar() instead of it
inline void writeWord(const char *s);
inline void fast_flush();

// ====================== END ======================
const int MAXN = 5e5 + 10;
const int MOD = 1e9 + 7;
const int INF = 1e18;

int fact[MAXN], inv_fact[MAXN];
int fpow1[MAXN], fpow2[MAXN];

void precalc() {
  fpow1[0] = fpow2[0] = 1;
  for (int i = 1; i < MAXN; i++) {
    fpow1[i] = (fpow1[i - 1] * 26) % MOD;
    fpow2[i] = (fpow2[i - 1] * 25) % MOD;
  }
  fact[0] = 1;
  for (int i = 1; i < MAXN; i++) {
    fact[i] = (fact[i - 1] * i) % MOD;
  }
  inv_fact[MAXN - 1] = binPow(fact[MAXN - 1], MOD - 2, MOD);
  for (int i = MAXN - 1; i > 0; i--) {
    inv_fact[i - 1] = (inv_fact[i] * i) % MOD;
  }
}

int c(int n, int k) {
  return ((fact[n] * inv_fact[k]) % MOD * inv_fact[n - k]) % MOD;
}

int f(int n, int m) {
  int ans = 0;
  for (int i = 0; i <= n; i++) {
    ans += ((i % 2 == 0 ? 1 : -1) * c(n, i) * fact[m - i]) % MOD;
    // cout << ans << endl;
  }
  ans = (ans % MOD) + MOD;
  ans %= MOD;
  // cout << ans << endl;
  return (ans * inv_fact[m - n]) % MOD;
}

void solve() {
  precalc();
  // for (int i = 1; i < 100; i++) {
  //     cout << f(1, i) << endl;
  // }
  int n;
  string s;
  cin >> n >> s;
  int ans = 0;
  for (int i = 0; i <= n; i++) {
    ans +=
        ((fpow1[n - i] * fpow2[i]) % MOD) * c(i + s.size() - 1, s.size() - 1);
    ans %= MOD;
  }
  cout << ans << endl;
}

signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int t = 1;
  // t = readInt();
  // cin >> t;
  for (int i = 1; i <= t; ++i) {
    // cout << "Case #" << i << ": ";
    solve();
  }
  // fast_flush();
  return 0;
}

/** Read */

static const int buf_size = 4096;

inline int fast_readchar() {
  static char buf[buf_size];
  static int len = 0, pos = 0;
  if (pos == len)
    pos = 0, len = fread(buf, 1, buf_size, stdin);
  if (pos == len)
    return -1;
  return buf[pos++];
}

inline int readUInt() {
  int c = fast_readchar(), x = 0;
  while (c <= 32)
    c = fast_readchar();
  while ('0' <= c && c <= '9')
    x = x * 10 + c - '0', c = fast_readchar();
  return x;
}

inline int readInt() {
  int s = 1, c = fast_readchar();
  int x = 0;
  while (c <= 32)
    c = fast_readchar();
  if (c == '-')
    s = -1, c = fast_readchar();
  while ('0' <= c && c <= '9')
    x = x * 10 + c - '0', c = fast_readchar();
  return x * s;
}

inline void readWord(char *s) {
  int c = fast_readchar();
  while (c <= 32)
    c = fast_readchar();
  while (c > 32)
    *s++ = c, c = fast_readchar();
  *s = 0;
}

/** Write */

static int write_pos = 0;
static char write_buf[buf_size];

inline void fast_writechar(int x) {
  if (write_pos == buf_size)
    fwrite(write_buf, 1, buf_size, stdout), write_pos = 0;
  write_buf[write_pos++] = x;
}

inline void fast_flush() {
  if (write_pos)
    fwrite(write_buf, 1, write_pos, stdout), write_pos = 0;
}

inline void writeInt(int x) {
  if (x < 0)
    fast_writechar('-'), x = -x;

  char s[24];
  int n = 0;
  while (x || !n)
    s[n++] = '0' + x % 10, x /= 10;
  while (n--)
    fast_writechar(s[n]);
}

inline void writeWord(const char *s) {
  while (*s)
    fast_writechar(*s++);
}


Fixed submission:
#include <bits/stdc++.h>
#define int long long
#define x first
#define y second
#define getbit(x, i) (((x) >> (i)) & 1)
using namespace std;
typedef pair<int, int> pii;
#define hashset unordered_set
#define hashmap unordered_map
#define newline fast_writechar('\n')
#define unify(arr) arr.resize(unique(arr.begin(), arr.end()) - arr.begin())
#define getbit(x, i) (((x) >> (i)) & 1)

template <typename T> vector<T> readvector(size_t sz) {
  vector<T> res(sz);
  for (size_t i = 0; i < sz; ++i) {
    cin >> res[i];
  }
  return res;
}
template <typename T>
std::ostream &operator<<(std::ostream &out, const std::vector<T> &v) {
  std::copy(v.begin(), v.end(), std::ostream_iterator<T>(out, " "));
  return out;
}

int mylog(int n) { return 63 - __builtin_clzll(n); }

inline int binPow(int x, int deg, int mod) {
  int ans = 1;
  for (int i = sizeof(deg) * CHAR_BIT - 1; i >= 0; i--) {
    ans *= ans;
    ans %= mod;
    if (getbit(deg, i))
      ans *= x;
    ans %= mod;
  }
  return ans;
}

/** Interface */

inline int readInt();
inline int readUInt();
inline void readWord(char *s);
inline int fast_readchar(); // you may use readchar() instead of it

inline void writeInt(int x);
inline void fast_writechar(int x); // you may use putchar() instead of it
inline void writeWord(const char *s);
inline void fast_flush();

// ====================== END ======================
const int MAXN = 2e6 + 10;
const int MOD = 1e9 + 7;
const int INF = 1e18;

int fact[MAXN], inv_fact[MAXN];
int fpow1[MAXN], fpow2[MAXN];

void precalc() {
  fpow1[0] = fpow2[0] = 1;
  for (int i = 1; i < MAXN; i++) {
    fpow1[i] = (fpow1[i - 1] * 26) % MOD;
    fpow2[i] = (fpow2[i - 1] * 25) % MOD;
  }
  fact[0] = 1;
  for (int i = 1; i < MAXN; i++) {
    fact[i] = (fact[i - 1] * i) % MOD;
  }
  inv_fact[MAXN - 1] = binPow(fact[MAXN - 1], MOD - 2, MOD);
  for (int i = MAXN - 1; i > 0; i--) {
    inv_fact[i - 1] = (inv_fact[i] * i) % MOD;
  }
}

int c(int n, int k) {
  return ((fact[n] * inv_fact[k]) % MOD * inv_fact[n - k]) % MOD;
}

int f(int n, int m) {
  int ans = 0;
  for (int i = 0; i <= n; i++) {
    ans += ((i % 2 == 0 ? 1 : -1) * c(n, i) * fact[m - i]) % MOD;
    // cout << ans << endl;
  }
  ans = (ans % MOD) + MOD;
  ans %= MOD;
  // cout << ans << endl;
  return (ans * inv_fact[m - n]) % MOD;
}

void solve() {
  precalc();
  // for (int i = 1; i < 100; i++) {
  //     cout << f(1, i) << endl;
  // }
  int n;
  string s;
  cin >> n >> s;
  int ans = 0;
  for (int i = 0; i <= n; i++) {
    ans +=
        ((fpow1[n - i] * fpow2[i]) % MOD) * c(i + s.size() - 1, s.size() - 1);
    ans %= MOD;
  }
  cout << ans << endl;
}

signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int t = 1;
  // t = readInt();
  // cin >> t;
  for (int i = 1; i <= t; ++i) {
    // cout << "Case #" << i << ": ";
    solve();
  }
  // fast_flush();
  return 0;
}

/** Read */

static const int buf_size = 4096;

inline int fast_readchar() {
  static char buf[buf_size];
  static int len = 0, pos = 0;
  if (pos == len)
    pos = 0, len = fread(buf, 1, buf_size, stdin);
  if (pos == len)
    return -1;
  return buf[pos++];
}

inline int readUInt() {
  int c = fast_readchar(), x = 0;
  while (c <= 32)
    c = fast_readchar();
  while ('0' <= c && c <= '9')
    x = x * 10 + c - '0', c = fast_readchar();
  return x;
}

inline int readInt() {
  int s = 1, c = fast_readchar();
  int x = 0;
  while (c <= 32)
    c = fast_readchar();
  if (c == '-')
    s = -1, c = fast_readchar();
  while ('0' <= c && c <= '9')
    x = x * 10 + c - '0', c = fast_readchar();
  return x * s;
}

inline void readWord(char *s) {
  int c = fast_readchar();
  while (c <= 32)
    c = fast_readchar();
  while (c > 32)
    *s++ = c, c = fast_readchar();
  *s = 0;
}

/** Write */

static int write_pos = 0;
static char write_buf[buf_size];

inline void fast_writechar(int x) {
  if (write_pos == buf_size)
    fwrite(write_buf, 1, buf_size, stdout), write_pos = 0;
  write_buf[write_pos++] = x;
}

inline void fast_flush() {
  if (write_pos)
    fwrite(write_buf, 1, write_pos, stdout), write_pos = 0;
}

inline void writeInt(int x) {
  if (x < 0)
    fast_writechar('-'), x = -x;

  char s[24];
  int n = 0;
  while (x || !n)
    s[n++] = '0' + x % 10, x /= 10;
  while (n--)
    fast_writechar(s[n]);
}

inline void writeWord(const char *s) {
  while (*s)
    fast_writechar(*s++);
}