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

#include <bits/stdc++.h>

using namespace std;

#ifndef aa
#define trace(...)
#define endl '\\n'
#endif
#define pb push_back
#define ub upper_bound
#define lb lower_bound
#define fi first
#define se second
#define int long long
typedef long long ll;
typedef long double ld;
#define double long double
#define pii pair<int, int>
#define pdd pair<double, double>
#define pll pair<ll, ll>
#define sz(x) ((long long)x.size())
#define fr(a, b, c) for (int a = b; a <= c; a++)
#define frev(a, b, c) for (int a = c; a >= b; a--)
#define rep(a, b, c) for (int a = b; a < c; a++)
#define trav(a, x) for (auto &a : x)
#define all(con) con.begin(), con.end()
#define deb(x) cout << x << endl
#define done(x)                                                                \\
  {                                                                            \\
    cout << x << endl;                                                         \\
    return;                                                                    \\
  }
const ll infl = 0x3f3f3f3f3f3f3f3fLL;
const int infi = 0x3f3f3f3f;
mt19937_64 mt(chrono::steady_clock::now().time_since_epoch().count());
// const int mod = 998244353;
const int mod = 1e9 + 7;
typedef vector<int> vi;
typedef vector<vector<int>> vvi;
typedef vector<pair<int, int>> vpii;
typedef map<int, int> mii;
typedef set<int> si;
typedef set<pair<int, int>> spii;
typedef queue<int> qi;

const int N = 2e6 + 5;
int dp[N];

void pre() {
  dp[0] = 1;
  rep(i, 1, N) dp[i] = dp[i - 1] * i % mod;
}

int powm(int a, int b) {
  int res = 1;
  while (b) {
    if (b & 1)
      res = (res * a) % mod;
    a = (a * a) % mod;
    b >>= 1;
  }
  return res;
}

int divide(int a, int b) { return (a % mod) * powm(b, mod - 2) % mod; }
int ncr(int x, int y) { return divide(dp[x], dp[y] * dp[x - y] % mod); }

void solve() {

  pre();
  int k;
  cin >> k;
  string s;
  cin >> s;
  int n = sz(s), ans = powm(26, n + k);
  rep(i, 0, n) ans =
      (ans - ncr(n + k, i) * powm(25, n + k - i) % mod + mod) % mod;
  cout << ans;
}

signed main() {
  ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);
  // srand(chrono::high_resolution_clock::now().time_since_epoch().count());
  cout << fixed << setprecision(15);
  int t = 1;
  // cin >> t;

  while (t--)
    solve();
#ifdef aa
  cout << endl
       << endl
       << endl
       << endl
       << "Time elapsed: " << (double)(clock() - clk) / CLOCKS_PER_SEC << endl;
#endif
  return 0;
}

int norm(int a) {
  while (a >= mod)
    a -= mod;
  while (a < 0)
    a += mod;
  return a;
}

int gcd(int a, int b) { return b ? gcd(b, a % b) : a; }


Error: WA

Fixed submission:

#include <bits/stdc++.h>

using namespace std;

#ifndef aa
#define trace(...)
#define endl '\\n'
#endif
#define pb push_back
#define ub upper_bound
#define lb lower_bound
#define fi first
#define se second
#define int long long
typedef long long ll;
typedef long double ld;
#define double long double
#define pii pair<int, int>
#define pdd pair<double, double>
#define pll pair<ll, ll>
#define sz(x) ((long long)x.size())
#define fr(a, b, c) for (int a = b; a <= c; a++)
#define frev(a, b, c) for (int a = c; a >= b; a--)
#define rep(a, b, c) for (int a = b; a < c; a++)
#define trav(a, x) for (auto &a : x)
#define all(con) con.begin(), con.end()
#define deb(x) cout << x << endl
#define done(x)                                                                \\
  {                                                                            \\
    cout << x << endl;                                                         \\
    return;                                                                    \\
  }
const ll infl = 0x3f3f3f3f3f3f3f3fLL;
const int infi = 0x3f3f3f3f;
mt19937_64 mt(chrono::steady_clock::now().time_since_epoch().count());
// const int mod = 998244353;
const int mod = 1e9 + 7;
typedef vector<int> vi;
typedef vector<vector<int>> vvi;
typedef vector<pair<int, int>> vpii;
typedef map<int, int> mii;
typedef set<int> si;
typedef set<pair<int, int>> spii;
typedef queue<int> qi;

const int N = 1e6 + 5;
int dp[N];

void pre() {
  dp[0] = 1;
  rep(i, 1, N) dp[i] = dp[i - 1] * i % mod;
}

int powm(int a, int b) {
  int res = 1;
  while (b) {
    if (b & 1)
      res = (res * a) % mod;
    a = (a * a) % mod;
    b >>= 1;
  }
  return res;
}

int divide(int a, int b) { return (a % mod) * powm(b, mod - 2) % mod; }
int ncr(int x, int y) { return divide(dp[x], dp[y] * dp[x - y] % mod); }

void solve() {

  pre();
  int k;
  cin >> k;
  string s;
  cin >> s;
  int n = sz(s), ans = powm(26, n + k);
  rep(i, 0, n) ans =
      (ans - ncr(n + k, i) * powm(25, n + k - i) % mod + mod) % mod;
  cout << ans;
}

signed main() {
  ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);
  // srand(chrono::high_resolution_clock::now().time_since_epoch().count());
  cout << fixed << setprecision(15);
  int t = 1;
  // cin >> t;

  while (t--)
    solve();
#ifdef aa
  cout << endl
       << endl
       << endl
       << endl
       << "Time elapsed: " << (double)(clock() - clk) / CLOCKS_PER_SEC << endl;
#endif
  return 0;
}

int norm(int a) {
  while (a >= mod)
    a -= mod;
  while (a < 0)
    a += mod;
  return a;
}

int gcd(int a, int b) { return b ? gcd(b, a % b) : a; }