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

#include <bits/stdc++.h>
#define rep(i, a, b) for (i = a; i < b; i++)
#define rep2(i, a, b) for (i = a; i >= b; i--)
#define mod 1000000007
#define mod2 998244353
#define FIO                                                                    \\
  ios_base::sync_with_stdio(false);                                            \\
  cin.tie(0);                                                                  \\
  cout.tie(0)
#define INF 1e9 + 5
#define f first
#define s second
#define endl '\\n'
#define ll long long
#define ii pair<int, int>
#define pll pair<ll, ll>
#define vi vector<int>
#define vl vector<ll>
#define vvi vector<vi>
#define vii vector<ii>
#define vpl vector<pll>
#define vvii vector<vii>
#define vll vector<ll>
#define vb vector<bool>
#define pb push_back
#define mk make_pair
#define eb emplace_back
#define pg priority_queue<ll, vector<ll>, greater<ll>>
#define pq priority_queue<ll>
#define sz(a) a.size()
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define cset(a) __builtin_popcountll(a)
#define FILL(a, b, T) fill_n((T *)a, sizeof(a) / sizeof(T), b)
// #define debug(x) cerr << #x << " is " << x << endl;
using namespace std;
#define error(args...)                                                         \\
  {                                                                            \\
    string _s = #args;                                                         \\
    replace(_s.begin(), _s.end(), ',', ' ');                                   \\
    stringstream _ss(_s);                                                      \\
    istream_iterator<string> _it(_ss);                                         \\
    err(_it, args);                                                            \\
  }
#define pis pair<ll, pll>
void err(istream_iterator<string> it) {}
template <typename T, typename... Args>
void err(istream_iterator<string> it, T a, Args... args) {
  cerr << *it << " = " << a << endl;
  err(++it, args...);
}
const auto start_time = std::chrono::high_resolution_clock::now();
void timecalculater() {
#ifndef ONLINE_JUDGE
  auto end_time = std::chrono::high_resolution_clock::now();
  std::chrono::duration<double> diff = end_time - start_time;
  cerr << "Time Taken : " << diff.count() << " s\\n";
#endif
}
template <typename A, typename B> string to_string(pair<A, B> p);

string to_string(const string &s) { return '"' + s + '"'; }

string to_string(const char *s) { return to_string((string)s); }

string to_string(bool b) { return (b ? "true" : "false"); }

string to_string(vector<bool> v) {
  bool first = true;
  string res = "{";
  for (int i = 0; i < static_cast<int>(v.size()); i++) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(v[i]);
  }
  res += "}";
  return res;
}

template <size_t N> string to_string(bitset<N> v) {
  string res = "";
  for (size_t i = 0; i < N; i++) {
    res += static_cast<char>('0' + v[i]);
  }
  return res;
}

template <typename A> string to_string(A v) {
  bool first = true;
  string res = "{";
  for (const auto &x : v) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}

template <typename A, typename B> string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}

void debug_out() { cerr << endl; }

template <typename Head, typename... Tail> void debug_out(Head H, Tail... T) {
  cerr << " " << to_string(H);
  debug_out(T...);
}

#ifndef ONLINE_JUDGE
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#else
#define debug(...)
#endif
struct custom_hash {
  static uint64_t splitmix64(uint64_t x) {
    // http://xorshift.di.unimi.it/splitmix64.c
    x += 0x9e3779b97f4a7c15;
    x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9;
    x = (x ^ (x >> 27)) * 0x94d049bb133111eb;
    return x ^ (x >> 31);
  }

  size_t operator()(uint64_t x) const {
    static const uint64_t FIXED_RANDOM =
        chrono::steady_clock::now().time_since_epoch().count();
    return splitmix64(x + FIXED_RANDOM);
  }
};
// unordered_map<long long, int, custom_hash> safe_map;
// FOR VIEWERS, CODE STARTS HERE
#define N 2000005
vll fact(N);
static ll poww(ll a, ll b) {
  ll x = 1, y = a;
  while (b > 0) {
    if (b % 2) {
      x = (x * y) % mod;
    }
    y = (y * y) % mod;
    b /= 2;
  }
  return (x % mod);
}
ll add(ll x, ll y) { return (x + y + mod) % mod; }
ll mul(ll x, ll y) {
  while (x < 0)
    x += mod;
  while (y < 0)
    y += mod;
  return (x * y) % mod;
}
ll ncr(ll n, ll r) {
  ll tmp = fact[n];
  tmp = mul(tmp, poww(fact[r], mod - 2));
  tmp = mul(tmp, poww(fact[n - r], mod - 2));
  return tmp;
}
void ncrinit() {
  ll i;
  fact[0] = 1;
  rep(i, 1, N) { fact[i] = mul(fact[i - 1], i); }
}
int main() {
  FIO;
  ll t, q, n, m, prev, maxx, i, j, k1, k2, k3, k, x, y, flag, sum, cnt, ans,
      minn, cur;
  cin >> n;
  string ss;
  cin >> ss;
  ncrinit();
  ll len = n + sz(ss);
  ans = poww(26, len);
  rep(i, 0, sz(ss)) {
    cur = poww(25, len - i);
    cur = mul(cur, ncr(len, i));
    ans = add(ans, -cur);
  }
  cout << ans << endl;
  return 0;
}

Error: 0

Fixed submission:

#include <bits/stdc++.h>
#define rep(i, a, b) for (i = a; i < b; i++)
#define rep2(i, a, b) for (i = a; i >= b; i--)
#define mod 1000000007
#define mod2 998244353
#define FIO                                                                    \\
  ios_base::sync_with_stdio(false);                                            \\
  cin.tie(0);                                                                  \\
  cout.tie(0)
#define INF 1e9 + 5
#define f first
#define s second
#define endl '\\n'
#define ll long long
#define ii pair<int, int>
#define pll pair<ll, ll>
#define vi vector<int>
#define vl vector<ll>
#define vvi vector<vi>
#define vii vector<ii>
#define vpl vector<pll>
#define vvii vector<vii>
#define vll vector<ll>
#define vb vector<bool>
#define pb push_back
#define mk make_pair
#define eb emplace_back
#define pg priority_queue<ll, vector<ll>, greater<ll>>
#define pq priority_queue<ll>
#define sz(a) a.size()
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define cset(a) __builtin_popcountll(a)
#define FILL(a, b, T) fill_n((T *)a, sizeof(a) / sizeof(T), b)
// #define debug(x) cerr << #x << " is " << x << endl;
using namespace std;
#define error(args...)                                                         \\
  {                                                                            \\
    string _s = #args;                                                         \\
    replace(_s.begin(), _s.end(), ',', ' ');                                   \\
    stringstream _ss(_s);                                                      \\
    istream_iterator<string> _it(_ss);                                         \\
    err(_it, args);                                                            \\
  }
#define pis pair<ll, pll>
void err(istream_iterator<string> it) {}
template <typename T, typename... Args>
void err(istream_iterator<string> it, T a, Args... args) {
  cerr << *it << " = " << a << endl;
  err(++it, args...);
}
const auto start_time = std::chrono::high_resolution_clock::now();
void timecalculater() {
#ifndef ONLINE_JUDGE
  auto end_time = std::chrono::high_resolution_clock::now();
  std::chrono::duration<double> diff = end_time - start_time;
  cerr << "Time Taken : " << diff.count() << " s\\n";
#endif
}
template <typename A, typename B> string to_string(pair<A, B> p);

string to_string(const string &s) { return '"' + s + '"'; }

string to_string(const char *s) { return to_string((string)s); }

string to_string(bool b) { return (b ? "true" : "false"); }

string to_string(vector<bool> v) {
  bool first = true;
  string res = "{";
  for (int i = 0; i < static_cast<int>(v.size()); i++) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(v[i]);
  }
  res += "}";
  return res;
}

template <size_t N> string to_string(bitset<N> v) {
  string res = "";
  for (size_t i = 0; i < N; i++) {
    res += static_cast<char>('0' + v[i]);
  }
  return res;
}

template <typename A> string to_string(A v) {
  bool first = true;
  string res = "{";
  for (const auto &x : v) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}

template <typename A, typename B> string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}

void debug_out() { cerr << endl; }

template <typename Head, typename... Tail> void debug_out(Head H, Tail... T) {
  cerr << " " << to_string(H);
  debug_out(T...);
}

#ifndef ONLINE_JUDGE
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#else
#define debug(...)
#endif
struct custom_hash {
  static uint64_t splitmix64(uint64_t x) {
    // http://xorshift.di.unimi.it/splitmix64.c
    x += 0x9e3779b97f4a7c15;
    x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9;
    x = (x ^ (x >> 27)) * 0x94d049bb133111eb;
    return x ^ (x >> 31);
  }

  size_t operator()(uint64_t x) const {
    static const uint64_t FIXED_RANDOM =
        chrono::steady_clock::now().time_since_epoch().count();
    return splitmix64(x + FIXED_RANDOM);
  }
};
// unordered_map<long long, int, custom_hash> safe_map;
// FOR VIEWERS, CODE STARTS HERE
#define N 1000005
vll fact(N);
static ll poww(ll a, ll b) {
  ll x = 1, y = a;
  while (b > 0) {
    if (b % 2) {
      x = (x * y) % mod;
    }
    y = (y * y) % mod;
    b /= 2;
  }
  return (x % mod);
}
ll add(ll x, ll y) { return (x + y + mod) % mod; }
ll mul(ll x, ll y) {
  while (x < 0)
    x += mod;
  while (y < 0)
    y += mod;
  return (x * y) % mod;
}
ll ncr(ll n, ll r) {
  ll tmp = fact[n];
  tmp = mul(tmp, poww(fact[r], mod - 2));
  tmp = mul(tmp, poww(fact[n - r], mod - 2));
  return tmp;
}
void ncrinit() {
  ll i;
  fact[0] = 1;
  rep(i, 1, N) { fact[i] = mul(fact[i - 1], i); }
}
int main() {
  FIO;
  ll t, q, n, m, prev, maxx, i, j, k1, k2, k3, k, x, y, flag, sum, cnt, ans,
      minn, cur;
  cin >> n;
  string ss;
  cin >> ss;
  ncrinit();
  ll len = n + sz(ss);
  ans = poww(26, len);
  rep(i, 0, sz(ss)) {
    if (i > len)
      continue;
    cur = poww(25, len - i);
    cur = mul(cur, ncr(len, i));
    ans = add(ans, -cur);
  }
  cout << ans << endl;
  return 0;
}