Problem p02632 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>

using namespace std;

using ll = int64_t;
// using ll = long long;
using ull = uint64_t;
using i32 = int32_t;
using u32 = uint32_t;
using i64 = int64_t;
using u64 = uint64_t;
using f64 = double;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using ld = double;

#define X first
#define Y second

#ifndef ONLINE_JUDGE
#define FWRITE
#endif

namespace io {

#ifndef FWRITE
#include <unistd.h>
auto unistd_read = read;
auto unistd_write = write;
#endif
const int BUFSIZE = 1 << 20;

int isize, osize;
char ibuf[BUFSIZE + 10], obuf[BUFSIZE + 10];
char *is, *it, *os = obuf, *ot = obuf + BUFSIZE;

char getchar() {
  if (is == it) {
    is = ibuf;
#ifdef FWRITE
    it = ibuf + fread(ibuf, 1, BUFSIZE, stdin);
#else
    it = ibuf + unistd_read(STDIN_FILENO, ibuf, BUFSIZE);
#endif
    if (is == it)
      return EOF;
  }
  return *is++;
}

char getalpha() {
  char c = getchar();
  while (!isalpha(c))
    c = getchar();
  return c;
}

void putchar(char c) {
  *os++ = c;
  if (os == ot) {
#ifdef FWRITE
    fwrite(obuf, 1, BUFSIZE, stdout);
#else
    unistd_write(STDOUT_FILENO, obuf, BUFSIZE);
#endif
    os = obuf;
  }
}

int inp() {
  int x = 0, f = 0;
  char ch;
  for (ch = getchar(); !isdigit(ch); ch = getchar()) {
    if (ch == EOF)
      return -1;
    if (ch == '-')
      f = 1;
  }
  for (; isdigit(ch); x = x * 10 + ch - '0', ch = getchar())
    ;
  return f ? -x : x;
}

ll inp_ll() {
  ll x = 0;
  int f = 0;
  char ch;
  for (ch = getchar(); !isdigit(ch); ch = getchar())
    if (ch == '-')
      f = 1;
  for (; isdigit(ch); x = x * 10 + ch - '0', ch = getchar())
    ;
  return f ? -x : x;
}

template <class T> bool read(T &x) {
  x = 0;
  int f = 0;
  char ch;
  for (ch = getchar(); !isdigit(ch); ch = getchar()) {
    if (ch == EOF)
      return 0;
    if (ch == '-')
      f = 1;
  }
  for (; isdigit(ch); x = x * 10 + ch - '0', ch = getchar())
    ;
  if (f)
    x = -x;
  return 1;
}

bool read(char *s) {
  char *t = s;
  char ch = getchar();
  for (; ch == ' ' || ch == '\n'; ch = getchar())
    ;
  for (; ch != ' ' && ch != '\n' && ch != EOF; ch = getchar())
    *t++ = ch;
  *t = 0;
  return s != t;
}

template <class T, class... Args> bool read(T &x, Args &...args) {
  return read(x) && read(args...);
}

template <class T> bool readln(T &x) {
  x = 0;
  int f = 0;
  char ch;
  for (ch = getchar(); !isdigit(ch); ch = getchar()) {
    if (ch == EOF)
      return 0;
    if (ch == '-')
      f = 1;
  }
  for (; isdigit(ch); x = x * 10 + ch - '0', ch = getchar())
    ;
  if (f)
    x = -x;
  for (; ch != '\n' && ch != EOF; ch = getchar())
    ;
  return 1;
}

bool readln(char *s) {
  char *t = s;
  while (1) {
    char ch = getchar();
    if (ch == '\n' || ch == EOF)
      break;
    *t++ = ch;
  }
  *t = 0;
  return s != t;
}

template <class T, class... Args> bool readln(T &x, Args &...args) {
  return read(x) && readln(args...);
}

template <class T> void write(T x) {
  static char s[22];
  static char *it = s + 20;
  static char *end = s + 20;

  if (x < 0) {
    putchar('-');
    x = -x;
  }

  do {
    *--it = x % 10 + '0';
    x /= 10;
  } while (x);
  /*
  if (!x)
          *-- it = '0';
  while (x)
  {
          *-- it = x%10+'0';
          x /= 10;
  }
  */
  for (; it < end; ++it)
    putchar(*it);
}

void write(const char *s) {
  for (; *s; ++s)
    putchar(*s);
}

template <> void write(char *s) { write((const char *)s); }

template <> void write(char c) { putchar(c); }

template <class T, class... Args> void write(T x, Args... args) {
  write(x);
  write(args...);
}

void writeln() { putchar('\n'); }

template <class T, class... Args> void writeln(T x, Args... args) {
  write(x);
  writeln(args...);
}

template <class Iterator> bool input(Iterator st, Iterator ed) {
  for (; st != ed; ++st) {
    if (!read(*st))
      return false;
  }
  return true;
}

template <class T> bool input(T &a) { return input(a.begin(), a.end()); }

template <class Iterator>
void print(Iterator st, Iterator ed, const char *c = " ") {
  int flag = 0;
  for (; st != ed; ++st) {
    if (flag)
      write(c);
    flag = 1;
    write(*st);
  }
  writeln();
}

template <class T> void print(const T &a, const char *c = " ") {
  print(a.begin(), a.end(), c);
}

struct ender {
  ~ender() {
    if (os != obuf)
#ifdef FWRITE
      fwrite(obuf, 1, os - obuf, stdout);
#else
      unistd_write(STDOUT_FILENO, obuf, os - obuf);
#endif
  }
} __ender;

} // namespace io

int64_t power(int64_t a, int64_t b, int64_t p) {
  if (!b)
    return 1;
  int64_t t = power(a, b >> 1, p);
  t = t * t % p;
  if (b & 1)
    t = t * a % p;
  return t;
}

// mt19937_64 rd(chrono::steady_clock::now().time_since_epoch().count());
mt19937 rd(chrono::steady_clock::now().time_since_epoch().count());
// mt19937 rd(1);

using namespace io;

template <class T> inline void freshmin(T &a, const T &b) {
  if (a > b)
    a = b;
}

template <class T> inline void freshmax(T &a, const T &b) {
  if (a < b)
    a = b;
}

// const ll B = 31;
// const ll MOD = 998244353;
const int INF = 1000000010;
const ll INFll = 4000000000000000000LL;
const int MAXN = 500010;

int dx[] = {-1, 1, 0, 0, -1, -1, 1, 1};
int dy[] = {0, 0, -1, 1, -1, 1, -1, 1};

ld det(ld x1, ld y1, ld x2, ld y2, ld x3, ld y3) {
  return x1 * y2 - x2 * y1 + x2 * y3 - x3 * y2 + x3 * y1 - x1 * y3;
}

const int MOD = 1000000007;

struct mod_int {
  int val;

  mod_int(long long v = 0) {
    if (v < 0)
      v = v % MOD + MOD;

    if (v >= MOD)
      v %= MOD;

    val = v;
  }

  static int mod_inv(int a, int m = MOD) {
    // https://en.wikipedia.org/wiki/Extended_Euclidean_algorithm#Example
    int g = m, r = a, x = 0, y = 1;

    while (r != 0) {
      int q = g / r;
      g %= r;
      swap(g, r);
      x -= q * y;
      swap(x, y);
    }

    return x < 0 ? x + m : x;
  }

  explicit operator int() const { return val; }

  mod_int &operator+=(const mod_int &other) {
    val += other.val;
    if (val >= MOD)
      val -= MOD;
    return *this;
  }

  mod_int &operator-=(const mod_int &other) {
    val -= other.val;
    if (val < 0)
      val += MOD;
    return *this;
  }

  static unsigned fast_mod(uint64_t x, unsigned m = MOD) { return x % m; }

  mod_int &operator*=(const mod_int &other) {
    val = fast_mod((uint64_t)val * other.val);
    return *this;
  }

  mod_int &operator/=(const mod_int &other) { return *this *= other.inv(); }

  friend mod_int operator+(const mod_int &a, const mod_int &b) {
    return mod_int(a) += b;
  }
  friend mod_int operator-(const mod_int &a, const mod_int &b) {
    return mod_int(a) -= b;
  }
  friend mod_int operator*(const mod_int &a, const mod_int &b) {
    return mod_int(a) *= b;
  }
  friend mod_int operator/(const mod_int &a, const mod_int &b) {
    return mod_int(a) /= b;
  }

  mod_int &operator++() {
    val = val == MOD - 1 ? 0 : val + 1;
    return *this;
  }

  mod_int &operator--() {
    val = val == 0 ? MOD - 1 : val - 1;
    return *this;
  }

  mod_int operator++(int) {
    mod_int before = *this;
    ++*this;
    return before;
  }
  mod_int operator--(int) {
    mod_int before = *this;
    --*this;
    return before;
  }

  mod_int operator-() const { return val == 0 ? 0 : MOD - val; }

  bool operator==(const mod_int &other) const { return val == other.val; }
  bool operator!=(const mod_int &other) const { return val != other.val; }

  mod_int inv() const { return mod_inv(val); }

  mod_int pow(long long p) const {
    assert(p >= 0);
    mod_int a = *this, result = 1;

    while (p > 0) {
      if (p & 1)
        result *= a;

      a *= a;
      p >>= 1;
    }

    return result;
  }

  friend ostream &operator<<(ostream &stream, const mod_int &m) {
    return stream << m.val;
  }
};

vector<mod_int> inv, factorial, inv_factorial;

void prepare_factorials(int maximum) {
  inv.assign(maximum + 1, 1);

  // Make sure MOD is prime, which is necessary for the inverse algorithm below.
  for (int p = 2; p * p <= MOD; p++)
    assert(MOD % p != 0);

  for (int i = 2; i <= maximum; i++)
    inv[i] = inv[MOD % i] * (MOD - MOD / i);

  factorial.resize(maximum + 1);
  inv_factorial.resize(maximum + 1);
  factorial[0] = inv_factorial[0] = 1;

  for (int i = 1; i <= maximum; i++) {
    factorial[i] = i * factorial[i - 1];
    inv_factorial[i] = inv[i] * inv_factorial[i - 1];
  }
}

mod_int choose(long long n, long long r) {
  if (r < 0 || r > n)
    return 0;

  return factorial[n] * inv_factorial[r] * inv_factorial[n - r];
}

mod_int inv_choose(long long n, long long r) {
  assert(0 <= r && r <= n);
  return inv_factorial[n] * factorial[r] * factorial[n - r];
}

mod_int permute(long long n, long long k) {
  if (k < 0 || k > n)
    return 0;

  return factorial[n] * inv_factorial[n - k];
}

mod_int inv_permute(long long n, long long k) {
  assert(0 <= k && k <= n);
  return inv_factorial[n] * factorial[n - k];
}

char s[MAXN];

void solve() {
  int k;
  read(k);
  read(s);
  int n = strlen(s);
  int m = n + k;
  prepare_factorials(m);
  mod_int res = 0;
  for (int i = n; i <= m; ++i) {
    res +=
        choose(i - 1, n - 1) * mod_int(25).pow(i - n) * mod_int(26).pow(m - i);
  }
  writeln(int(res));
}

int main() {

  int T = 1;
  // read(T);
  for (int test = 1; test <= T; ++test) {
    // write("Case #", test, ": ");
    solve();
  }

  return 0;
}

Fixed submission:
#include <bits/stdc++.h>

using namespace std;

using ll = int64_t;
// using ll = long long;
using ull = uint64_t;
using i32 = int32_t;
using u32 = uint32_t;
using i64 = int64_t;
using u64 = uint64_t;
using f64 = double;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using ld = double;

#define X first
#define Y second

#ifndef ONLINE_JUDGE
#define FWRITE
#endif

namespace io {

#ifndef FWRITE
#include <unistd.h>
auto unistd_read = read;
auto unistd_write = write;
#endif
const int BUFSIZE = 1 << 20;

int isize, osize;
char ibuf[BUFSIZE + 10], obuf[BUFSIZE + 10];
char *is, *it, *os = obuf, *ot = obuf + BUFSIZE;

char getchar() {
  if (is == it) {
    is = ibuf;
#ifdef FWRITE
    it = ibuf + fread(ibuf, 1, BUFSIZE, stdin);
#else
    it = ibuf + unistd_read(STDIN_FILENO, ibuf, BUFSIZE);
#endif
    if (is == it)
      return EOF;
  }
  return *is++;
}

char getalpha() {
  char c = getchar();
  while (!isalpha(c))
    c = getchar();
  return c;
}

void putchar(char c) {
  *os++ = c;
  if (os == ot) {
#ifdef FWRITE
    fwrite(obuf, 1, BUFSIZE, stdout);
#else
    unistd_write(STDOUT_FILENO, obuf, BUFSIZE);
#endif
    os = obuf;
  }
}

int inp() {
  int x = 0, f = 0;
  char ch;
  for (ch = getchar(); !isdigit(ch); ch = getchar()) {
    if (ch == EOF)
      return -1;
    if (ch == '-')
      f = 1;
  }
  for (; isdigit(ch); x = x * 10 + ch - '0', ch = getchar())
    ;
  return f ? -x : x;
}

ll inp_ll() {
  ll x = 0;
  int f = 0;
  char ch;
  for (ch = getchar(); !isdigit(ch); ch = getchar())
    if (ch == '-')
      f = 1;
  for (; isdigit(ch); x = x * 10 + ch - '0', ch = getchar())
    ;
  return f ? -x : x;
}

template <class T> bool read(T &x) {
  x = 0;
  int f = 0;
  char ch;
  for (ch = getchar(); !isdigit(ch); ch = getchar()) {
    if (ch == EOF)
      return 0;
    if (ch == '-')
      f = 1;
  }
  for (; isdigit(ch); x = x * 10 + ch - '0', ch = getchar())
    ;
  if (f)
    x = -x;
  return 1;
}

bool read(char *s) {
  char *t = s;
  char ch = getchar();
  for (; ch == ' ' || ch == '\n'; ch = getchar())
    ;
  for (; ch != ' ' && ch != '\n' && ch != EOF; ch = getchar())
    *t++ = ch;
  *t = 0;
  return s != t;
}

template <class T, class... Args> bool read(T &x, Args &...args) {
  return read(x) && read(args...);
}

template <class T> bool readln(T &x) {
  x = 0;
  int f = 0;
  char ch;
  for (ch = getchar(); !isdigit(ch); ch = getchar()) {
    if (ch == EOF)
      return 0;
    if (ch == '-')
      f = 1;
  }
  for (; isdigit(ch); x = x * 10 + ch - '0', ch = getchar())
    ;
  if (f)
    x = -x;
  for (; ch != '\n' && ch != EOF; ch = getchar())
    ;
  return 1;
}

bool readln(char *s) {
  char *t = s;
  while (1) {
    char ch = getchar();
    if (ch == '\n' || ch == EOF)
      break;
    *t++ = ch;
  }
  *t = 0;
  return s != t;
}

template <class T, class... Args> bool readln(T &x, Args &...args) {
  return read(x) && readln(args...);
}

template <class T> void write(T x) {
  static char s[22];
  static char *it = s + 20;
  static char *end = s + 20;

  if (x < 0) {
    putchar('-');
    x = -x;
  }

  do {
    *--it = x % 10 + '0';
    x /= 10;
  } while (x);
  /*
  if (!x)
          *-- it = '0';
  while (x)
  {
          *-- it = x%10+'0';
          x /= 10;
  }
  */
  for (; it < end; ++it)
    putchar(*it);
}

void write(const char *s) {
  for (; *s; ++s)
    putchar(*s);
}

template <> void write(char *s) { write((const char *)s); }

template <> void write(char c) { putchar(c); }

template <class T, class... Args> void write(T x, Args... args) {
  write(x);
  write(args...);
}

void writeln() { putchar('\n'); }

template <class T, class... Args> void writeln(T x, Args... args) {
  write(x);
  writeln(args...);
}

template <class Iterator> bool input(Iterator st, Iterator ed) {
  for (; st != ed; ++st) {
    if (!read(*st))
      return false;
  }
  return true;
}

template <class T> bool input(T &a) { return input(a.begin(), a.end()); }

template <class Iterator>
void print(Iterator st, Iterator ed, const char *c = " ") {
  int flag = 0;
  for (; st != ed; ++st) {
    if (flag)
      write(c);
    flag = 1;
    write(*st);
  }
  writeln();
}

template <class T> void print(const T &a, const char *c = " ") {
  print(a.begin(), a.end(), c);
}

struct ender {
  ~ender() {
    if (os != obuf)
#ifdef FWRITE
      fwrite(obuf, 1, os - obuf, stdout);
#else
      unistd_write(STDOUT_FILENO, obuf, os - obuf);
#endif
  }
} __ender;

} // namespace io

int64_t power(int64_t a, int64_t b, int64_t p) {
  if (!b)
    return 1;
  int64_t t = power(a, b >> 1, p);
  t = t * t % p;
  if (b & 1)
    t = t * a % p;
  return t;
}

// mt19937_64 rd(chrono::steady_clock::now().time_since_epoch().count());
mt19937 rd(chrono::steady_clock::now().time_since_epoch().count());
// mt19937 rd(1);

using namespace io;

template <class T> inline void freshmin(T &a, const T &b) {
  if (a > b)
    a = b;
}

template <class T> inline void freshmax(T &a, const T &b) {
  if (a < b)
    a = b;
}

// const ll B = 31;
// const ll MOD = 998244353;
const int INF = 1000000010;
const ll INFll = 4000000000000000000LL;
const int MAXN = 1000010;

int dx[] = {-1, 1, 0, 0, -1, -1, 1, 1};
int dy[] = {0, 0, -1, 1, -1, 1, -1, 1};

ld det(ld x1, ld y1, ld x2, ld y2, ld x3, ld y3) {
  return x1 * y2 - x2 * y1 + x2 * y3 - x3 * y2 + x3 * y1 - x1 * y3;
}

const int MOD = 1000000007;

struct mod_int {
  int val;

  mod_int(long long v = 0) {
    if (v < 0)
      v = v % MOD + MOD;

    if (v >= MOD)
      v %= MOD;

    val = v;
  }

  static int mod_inv(int a, int m = MOD) {
    // https://en.wikipedia.org/wiki/Extended_Euclidean_algorithm#Example
    int g = m, r = a, x = 0, y = 1;

    while (r != 0) {
      int q = g / r;
      g %= r;
      swap(g, r);
      x -= q * y;
      swap(x, y);
    }

    return x < 0 ? x + m : x;
  }

  explicit operator int() const { return val; }

  mod_int &operator+=(const mod_int &other) {
    val += other.val;
    if (val >= MOD)
      val -= MOD;
    return *this;
  }

  mod_int &operator-=(const mod_int &other) {
    val -= other.val;
    if (val < 0)
      val += MOD;
    return *this;
  }

  static unsigned fast_mod(uint64_t x, unsigned m = MOD) { return x % m; }

  mod_int &operator*=(const mod_int &other) {
    val = fast_mod((uint64_t)val * other.val);
    return *this;
  }

  mod_int &operator/=(const mod_int &other) { return *this *= other.inv(); }

  friend mod_int operator+(const mod_int &a, const mod_int &b) {
    return mod_int(a) += b;
  }
  friend mod_int operator-(const mod_int &a, const mod_int &b) {
    return mod_int(a) -= b;
  }
  friend mod_int operator*(const mod_int &a, const mod_int &b) {
    return mod_int(a) *= b;
  }
  friend mod_int operator/(const mod_int &a, const mod_int &b) {
    return mod_int(a) /= b;
  }

  mod_int &operator++() {
    val = val == MOD - 1 ? 0 : val + 1;
    return *this;
  }

  mod_int &operator--() {
    val = val == 0 ? MOD - 1 : val - 1;
    return *this;
  }

  mod_int operator++(int) {
    mod_int before = *this;
    ++*this;
    return before;
  }
  mod_int operator--(int) {
    mod_int before = *this;
    --*this;
    return before;
  }

  mod_int operator-() const { return val == 0 ? 0 : MOD - val; }

  bool operator==(const mod_int &other) const { return val == other.val; }
  bool operator!=(const mod_int &other) const { return val != other.val; }

  mod_int inv() const { return mod_inv(val); }

  mod_int pow(long long p) const {
    assert(p >= 0);
    mod_int a = *this, result = 1;

    while (p > 0) {
      if (p & 1)
        result *= a;

      a *= a;
      p >>= 1;
    }

    return result;
  }

  friend ostream &operator<<(ostream &stream, const mod_int &m) {
    return stream << m.val;
  }
};

vector<mod_int> inv, factorial, inv_factorial;

void prepare_factorials(int maximum) {
  inv.assign(maximum + 1, 1);

  // Make sure MOD is prime, which is necessary for the inverse algorithm below.
  for (int p = 2; p * p <= MOD; p++)
    assert(MOD % p != 0);

  for (int i = 2; i <= maximum; i++)
    inv[i] = inv[MOD % i] * (MOD - MOD / i);

  factorial.resize(maximum + 1);
  inv_factorial.resize(maximum + 1);
  factorial[0] = inv_factorial[0] = 1;

  for (int i = 1; i <= maximum; i++) {
    factorial[i] = i * factorial[i - 1];
    inv_factorial[i] = inv[i] * inv_factorial[i - 1];
  }
}

mod_int choose(long long n, long long r) {
  if (r < 0 || r > n)
    return 0;

  return factorial[n] * inv_factorial[r] * inv_factorial[n - r];
}

mod_int inv_choose(long long n, long long r) {
  assert(0 <= r && r <= n);
  return inv_factorial[n] * factorial[r] * factorial[n - r];
}

mod_int permute(long long n, long long k) {
  if (k < 0 || k > n)
    return 0;

  return factorial[n] * inv_factorial[n - k];
}

mod_int inv_permute(long long n, long long k) {
  assert(0 <= k && k <= n);
  return inv_factorial[n] * factorial[n - k];
}

char s[MAXN];

void solve() {
  int k;
  read(k);
  read(s);
  int n = strlen(s);
  int m = n + k;
  prepare_factorials(m);
  mod_int res = 0;
  for (int i = n; i <= m; ++i) {
    res +=
        choose(i - 1, n - 1) * mod_int(25).pow(i - n) * mod_int(26).pow(m - i);
  }
  writeln(int(res));
}

int main() {

  int T = 1;
  // read(T);
  for (int test = 1; test <= T; ++test) {
    // write("Case #", test, ": ");
    solve();
  }

  return 0;
}