Problem p02629 (C++) — buggy submission, verdict: Runtime Error

#include <bits/stdc++.h>
typedef long long ll;
#define fr(i, a, b) for (ll i = a; i < b; i++)
#define rf(i, a, b) for (ll i = a; i >= b; i--)
typedef std::vector<long long> vi;
#define F first
#define S second
#define fast                                                                   \\
  ios_base::sync_with_stdio(0);                                                \\
  cin.tie(0);                                                                  \\
  cout.tie(0);
#define mod 1000000007
#define PB push_back
#define MP make_pair
#define PI 3.14159265358979323846
#define all(a) a.begin(), a.end()
#define mx(a) *max_element(all(a))
#define mn(a) *min_element(all(a))
#define endl '\\n'
#define show(a)                                                                \\
  for (auto el : a)                                                            \\
  cout << el << " "
const ll INF = LLONG_MAX / 2;
const ll N = 2e5 + 1;
#define yesno(f)                                                               \\
  yes(f);                                                                      \\
  no(f);
#define yes(f)                                                                 \\
  if (f)                                                                       \\
    cout << "YES\\n";
#define no(f)                                                                  \\
  if (!f)                                                                      \\
    cout << "NO\\n";
using namespace std;
int main() {
  fast;
  ll n;
  std::cin >> n;
  ll a[100], j = 0;
  for (;;) {
    n--;
    a[j++] = n % 26;
    n /= 26;
    if (n == 0)
      break;
  }
  rf(i, j - 1, 0) {
    char c = 'a' + a[i];
    cout << c;
  }
  cout << "\\n";
}

Error: 0

Fixed submission:

#include <bits/stdc++.h>
typedef long long ll;
#define fr(i, a, b) for (ll i = a; i < b; i++)
#define rf(i, a, b) for (ll i = a; i >= b; i--)
typedef std::vector<long long> vi;
#define F first
#define S second
#define fast                                                                   \\
  ios_base::sync_with_stdio(0);                                                \\
  cin.tie(0);                                                                  \\
  cout.tie(0);
#define mod 1000000007
#define PB push_back
#define MP make_pair
#define PI 3.14159265358979323846
#define all(a) a.begin(), a.end()
#define mx(a) *max_element(all(a))
#define mn(a) *min_element(all(a))
#define endl '\\n'
#define show(a)                                                                \\
  for (auto el : a)                                                            \\
  cout << el << " "
const ll INF = LLONG_MAX / 2;
const ll N = 2e5 + 1;
#define yesno(f)                                                               \\
  yes(f);                                                                      \\
  no(f);
#define yes(f)                                                                 \\
  if (f)                                                                       \\
    cout << "YES\\n";
#define no(f)                                                                  \\
  if (!f)                                                                      \\
    cout << "NO\\n";
using namespace std;
long long fact[100005], ifact[100005];
void pre();
ll ncr(ll, ll);
ll pow1(ll n, ll p) {
  if (p == 0)
    return 1;
  ll x = pow1(n, p / 2);
  x = (x * x) % mod;
  if (p % 2 == 0)
    return x;
  else
    return (x * n) % mod;
}
ll binarySearch(ll arr[], ll l, ll r, ll x) {
  if (r >= l) {
    ll mid = l + (r - l) / 2;
    if (arr[mid] == x)
      return mid;
    if (arr[mid] > x)
      return binarySearch(arr, l, mid - 1, x);
    return binarySearch(arr, mid + 1, r, x);
  }
  return -1;
}
ll gcd(ll a, ll b, ll &x, ll &y) {
  if (a == 0) {
    x = 0;
    y = 1;
    return b;
  }
  ll x1, y1;
  ll d = gcd(b % a, a, x1, y1);
  x = y1 - (b / a) * x1;
  y = x1;
  return d;
}

bool find_any_solution(ll a, ll b, ll c, ll &x0, ll &y0, ll &g) {
  g = gcd(llabs(a), llabs(b), x0, y0);
  if (c % g) {
    return false;
  }

  x0 *= c / g;
  y0 *= c / g;
  if (a < 0)
    x0 = -x0;
  if (b < 0)
    y0 = -y0;
  return true;
}
void shift_solution(ll &x, ll &y, ll a, ll b, ll cnt) {
  x += cnt * b;
  y -= cnt * a;
}

ll find_all_solutions(ll a, ll b, ll c, ll minx, ll maxx, ll miny, ll maxy) {
  ll x, y, g;
  if (!find_any_solution(a, b, c, x, y, g))
    return 0;
  a /= g;
  b /= g;

  ll sign_a = a > 0 ? +1 : -1;
  ll sign_b = b > 0 ? +1 : -1;

  shift_solution(x, y, a, b, (minx - x) / b);
  if (x < minx)
    shift_solution(x, y, a, b, sign_b);
  if (x > maxx)
    return 0;
  ll lx1 = x;
  shift_solution(x, y, a, b, (maxx - x) / b);
  if (x > maxx)
    shift_solution(x, y, a, b, -sign_b);
  ll rx1 = x;
  shift_solution(x, y, a, b, -(miny - y) / a);
  if (y < miny)
    shift_solution(x, y, a, b, -sign_a);
  if (y > maxy)
    return 0;
  ll lx2 = x;

  shift_solution(x, y, a, b, -(maxy - y) / a);
  if (y > maxy)
    shift_solution(x, y, a, b, sign_a);
  ll rx2 = x;
  if (lx2 > rx2)
    swap(lx2, rx2);
  ll lx = max(lx1, lx2);
  ll rx = min(rx1, rx2);
  if (lx > rx)
    return 0;
  return (rx - lx) / llabs(b) + 1;
}
bool sortbysec(const pair<int, int> &a, const pair<int, int> &b) {
  return (a.second < b.second);
}

bool compare(const pair<ll, ll> &p1, const pair<ll, ll> &p2) {
  if (p1.F < p2.F)
    return true;
  if (p1.F == p2.F)
    return p1.S < p2.S;
  return false;
}
int main() {
  fast;
  ll n;
  std::cin >> n;
  ll a[n], j = 0;
  for (;;) {
    n--;
    a[j++] = n % 26;
    n /= 26;
    if (n == 0)
      break;
  }
  rf(i, j - 1, 0) {
    char c = 'a' + a[i];
    cout << c;
  }
  cout << "\\n";
}
void pre() {
  ll n = 100000;
  fact[0] = fact[1] = 1;
  for (long long i = 2; i <= n; i++)
    fact[i] = (fact[i - 1] * i) % mod;
  ifact[n] = pow1(fact[n], mod - 2);
  for (long long i = n - 1; i >= 0; i--) {
    ifact[i] = (ifact[i + 1] * (i + 1)) % mod;
  }
}
ll ncr(ll n, ll r) {
  ll res = (fact[n] * ifact[r]) % mod;
  res = (res * ifact[n - r]) % mod;
  return res;
}