Problem p02596 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>

#define f(i, a, b) for (ll i = a; i < b; i++)
#define af(i, a, b) for (ll i = a; i >= b; i--)
#define rep(i, a, b, k) for (ll i = a; i < b; i += k)
#define arep(i, a, b, k) for (ll i = a; i >= b; i -= k)
#define ff first
#define ss second
#define pb push_back
#define mp make_pair
#define sz(a) (ll) a.size()
#define all(a) a.begin(), a.end()
#define sor(a) sort(a.begin(), a.end())
#define fastio                                                                 \
  ios_base::sync_with_stdio(false);                                            \
  cin.tie(NULL)
#define inter                                                                  \
  ios::sync_with_stdio(false);                                                 \
  cin.tie(nullptr);                                                            \
  cout.tie(nullptr)

// policy-based
// #include <ext/pb_ds/assoc_container.hpp>
// #include <ext/pb_ds/tree_policy.hpp>
// using namespace __gnu_pbds;

using namespace std;

typedef long long ll; // int or long long
typedef long double ld;
typedef pair<ll, ll> ii;
typedef vector<ll> vi;
typedef vector<ii> vii;

/*
typedef tree<
ll,
null_type,
less<ll>,
rb_tree_tag,
tree_order_statistics_node_update>
ordered_set;
*/

const ll MAX = 3e5 + 100;
const ll inf = 1e12;
const ll mod = 1e9 + 7;

int main() {
  fastio;
  ll tc, n, x, y, k;
  cin >> k;

  ll ans = 0, pot = 7, sum = 0;
  if (k % 2 == 0 || k == 5) {
    cout << "-1\n";
    return 0;
  }
  while (1) {
    ans++;
    (sum += (pot) % k) %= k;
    (pot *= 10) %= k;
    if (sum % k == 0)
      break;
  }
  cout << ans << endl;
}

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

#define f(i, a, b) for (ll i = a; i < b; i++)
#define af(i, a, b) for (ll i = a; i >= b; i--)
#define rep(i, a, b, k) for (ll i = a; i < b; i += k)
#define arep(i, a, b, k) for (ll i = a; i >= b; i -= k)
#define ff first
#define ss second
#define pb push_back
#define mp make_pair
#define sz(a) (ll) a.size()
#define all(a) a.begin(), a.end()
#define sor(a) sort(a.begin(), a.end())
#define fastio                                                                 \
  ios_base::sync_with_stdio(false);                                            \
  cin.tie(NULL)
#define inter                                                                  \
  ios::sync_with_stdio(false);                                                 \
  cin.tie(nullptr);                                                            \
  cout.tie(nullptr)

// policy-based
// #include <ext/pb_ds/assoc_container.hpp>
// #include <ext/pb_ds/tree_policy.hpp>
// using namespace __gnu_pbds;

using namespace std;

typedef long long ll; // int or long long
typedef long double ld;
typedef pair<ll, ll> ii;
typedef vector<ll> vi;
typedef vector<ii> vii;

/*
typedef tree<
ll,
null_type,
less<ll>,
rb_tree_tag,
tree_order_statistics_node_update>
ordered_set;
*/

const ll MAX = 3e5 + 100;
const ll inf = 1e12;
const ll mod = 1e9 + 7;

int main() {
  fastio;
  ll tc, n, x, y, k;
  cin >> k;

  ll ans = 0, pot = 7, sum = 0;
  if (k % 2 == 0 || k % 5 == 0) {
    cout << "-1\n";
    return 0;
  }
  while (1) {
    ans++;
    (sum += (pot) % k) %= k;
    (pot *= 10) %= k;
    if (sum % k == 0)
      break;
  }
  cout << ans << endl;
}