Problem p02632 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <bitset>
#include <cassert>
#include <chrono>
#include <climits>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <ctime>
#include <deque>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <utility>
#include <vector>

/*
#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
#pragma GCC target("popcnt")
#pragma GCC target("avx2")
*/

#ifdef LOCAL
#define dbg(x) cout << #x << " : " << x << endl;
#else
#define dbg(x)
#endif

#define int long long
#define pb push_back
#define ppb pop_back()
#define mp make_pair
#define fi(a, b) for (int i = a; i < b; i++)
#define fj(a, b) for (int j = a; j < b; j++)
#define fk(a, b) for (int k = a; k < b; k++)
#define fi1(a, b) for (int i = a - 1; i >= b; i--)
#define fj1(a, b) for (int j = a - 1; j >= b; j--)
#define fk1(a, b) for (int k = a - 1; k >= b; k--)
#define fx(x, a) for (auto &x : a)
#define rep(i, a, b) for (int i = a; i < b; ++i)
#define rep1(i, a, b) for (int i = a - 1; i >= b; --i)
#define siz(x) (int)x.size()
#define lb lower_bound
#define ub upper_bound
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()

using namespace std;

template <typename T1, typename T2> inline void mine(T1 &x, T2 y) {
  if (x > y)
    x = y;
}
template <typename T1, typename T2> inline void maxe(T1 &x, T2 y) {
  if (x < y)
    x = y;
}

ostream &operator<<(ostream &a, const vector<int> &b) {
  for (auto k : b)
    cout << k << ' ';
  return a;
}

typedef long long ll;
typedef unsigned long long ull;
typedef char ch;
typedef string str;
typedef pair<int, int> pii;
typedef map<int, int> mii;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<pii> vpii;
typedef vector<vpii> vvpii;
typedef vector<ch> vch;
typedef vector<vch> vvch;
typedef vector<str> vs;

const int MOD = 1000000007;
const int INF = 1000000050;
const long long BIG = (long long)2e18 + 50;
const int MX = 1000010;
const double EPS = 1e-9;

mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());

ll powmod(ll a, ll b) {
  if (b == 0)
    return 1;

  ll k = powmod(a, b / 2);
  k = k * k % MOD;
  if (b & 1)
    k = k * a % MOD;
  return k;
}

int rev(int x) { return powmod(x, MOD - 2); }

int f[MX];
int f1[MX];

int C(int n, int k) {
  if (k < 0 || k > n)
    return 0;
  return f[n] * f1[k] % MOD * f1[n - k] % MOD;
}

int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);

  f[0] = 1;
  fi(1, MX) f[i] = f[i - 1] * i % MOD;
  f1[MX - 1] = rev(f[MX - 1]);
  fi1(MX - 1, 0) f1[i] = f1[i + 1] * (i + 1) % MOD;

  int res = 0;
  int n;
  cin >> n;
  str s;
  cin >> s;
  int m = siz(s);
  fi(m, m + n + 1) {
    res += C(i - 1, m - 1) * powmod(25, i - m) % MOD * powmod(26, m + n - i);
    res %= MOD;
  }
  cout << res << '\n';
}

Fixed submission:
#include <algorithm>
#include <bitset>
#include <cassert>
#include <chrono>
#include <climits>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <ctime>
#include <deque>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <utility>
#include <vector>

/*
#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
#pragma GCC target("popcnt")
#pragma GCC target("avx2")
*/

#ifdef LOCAL
#define dbg(x) cout << #x << " : " << x << endl;
#else
#define dbg(x)
#endif

#define int long long
#define pb push_back
#define ppb pop_back()
#define mp make_pair
#define fi(a, b) for (int i = a; i < b; i++)
#define fj(a, b) for (int j = a; j < b; j++)
#define fk(a, b) for (int k = a; k < b; k++)
#define fi1(a, b) for (int i = a - 1; i >= b; i--)
#define fj1(a, b) for (int j = a - 1; j >= b; j--)
#define fk1(a, b) for (int k = a - 1; k >= b; k--)
#define fx(x, a) for (auto &x : a)
#define rep(i, a, b) for (int i = a; i < b; ++i)
#define rep1(i, a, b) for (int i = a - 1; i >= b; --i)
#define siz(x) (int)x.size()
#define lb lower_bound
#define ub upper_bound
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()

using namespace std;

template <typename T1, typename T2> inline void mine(T1 &x, T2 y) {
  if (x > y)
    x = y;
}
template <typename T1, typename T2> inline void maxe(T1 &x, T2 y) {
  if (x < y)
    x = y;
}

ostream &operator<<(ostream &a, const vector<int> &b) {
  for (auto k : b)
    cout << k << ' ';
  return a;
}

typedef long long ll;
typedef unsigned long long ull;
typedef char ch;
typedef string str;
typedef pair<int, int> pii;
typedef map<int, int> mii;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<pii> vpii;
typedef vector<vpii> vvpii;
typedef vector<ch> vch;
typedef vector<vch> vvch;
typedef vector<str> vs;

const int MOD = 1000000007;
const int INF = 1000000050;
const long long BIG = (long long)2e18 + 50;
const int MX = 2000010;
const double EPS = 1e-9;

mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());

ll powmod(ll a, ll b) {
  if (b == 0)
    return 1;

  ll k = powmod(a, b / 2);
  k = k * k % MOD;
  if (b & 1)
    k = k * a % MOD;
  return k;
}

int rev(int x) { return powmod(x, MOD - 2); }

int f[MX];
int f1[MX];

int C(int n, int k) {
  if (k < 0 || k > n)
    return 0;
  return f[n] * f1[k] % MOD * f1[n - k] % MOD;
}

int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);

  f[0] = 1;
  fi(1, MX) f[i] = f[i - 1] * i % MOD;
  f1[MX - 1] = rev(f[MX - 1]);
  fi1(MX - 1, 0) f1[i] = f1[i + 1] * (i + 1) % MOD;

  int res = 0;
  int n;
  cin >> n;
  str s;
  cin >> s;
  int m = siz(s);
  fi(m, m + n + 1) {
    res += C(i - 1, m - 1) * powmod(25, i - m) % MOD * powmod(26, m + n - i);
    res %= MOD;
  }
  cout << res << '\n';
}