Problem p02596 (C++)

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

typedef long long ll;
typedef long double ld;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef pair<ld, ld> pdd;
typedef vector<ll> vll;
typedef vector<ld> vld;
typedef vector<pll> vpl;
typedef vector<vll> vvll;

#define ALL(a) a.begin(), a.end()
#define SZ(a) ((int)a.size())
#define FI first
#define SE second
#define REP(i, n) for (int i = 0; i < ((int)n); i++)
#define REP1(i, n) for (int i = 1; i < ((int)n); i++)
#define FOR(i, a, b) for (int i = (a); i < (b); i++)
#define PB push_back
#define EB emplace_back
#define MP(a, b) make_pair(a, b)
#define SORT_UNIQUE(c)                                                         \
  (sort(c.begin(), c.end()),                                                   \
   c.resize(distance(c.begin(), unique(c.begin(), c.end()))))
#define GET_POS(c, x) (lower_bound(c.begin(), c.end(), x) - c.begin())
#define yes cout << "Yes" << endl
#define YES cout << "YES" << endl
#define no cout << "No" << endl
#define NO cout << "NO" << endl
#define Decimal fixed << setprecision(20)
#define INF 1000000000
#define LLINF 1000000000000000000LL

const int inf = 1e9;
const ll linf = 1LL << 50;
const double eps = 1e-10;
const int MOD = 1e9 + 7;
const int dx[4] = {-1, 0, 1, 0};
const int dy[4] = {0, -1, 0, 1};

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

  ll k;
  cin >> k;

  ll cnt = 0;
  ll rem = 0;
  ll power = 7;
  while (true) {
    cnt++;
    rem = (rem + power) % k;
    if (rem == 0)
      break;

    power = power * 10 % k;
    if (power == 0) {
      cnt = -1;
      break;
    }
  }

  cout << cnt << endl;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;

typedef long long ll;
typedef long double ld;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef pair<ld, ld> pdd;
typedef vector<ll> vll;
typedef vector<ld> vld;
typedef vector<pll> vpl;
typedef vector<vll> vvll;

#define ALL(a) a.begin(), a.end()
#define SZ(a) ((int)a.size())
#define FI first
#define SE second
#define REP(i, n) for (int i = 0; i < ((int)n); i++)
#define REP1(i, n) for (int i = 1; i < ((int)n); i++)
#define FOR(i, a, b) for (int i = (a); i < (b); i++)
#define PB push_back
#define EB emplace_back
#define MP(a, b) make_pair(a, b)
#define SORT_UNIQUE(c)                                                         \
  (sort(c.begin(), c.end()),                                                   \
   c.resize(distance(c.begin(), unique(c.begin(), c.end()))))
#define GET_POS(c, x) (lower_bound(c.begin(), c.end(), x) - c.begin())
#define yes cout << "Yes" << endl
#define YES cout << "YES" << endl
#define no cout << "No" << endl
#define NO cout << "NO" << endl
#define Decimal fixed << setprecision(20)
#define INF 1000000000
#define LLINF 1000000000000000000LL

const int inf = 1e9;
const ll linf = 1LL << 50;
const double eps = 1e-10;
const int MOD = 1e9 + 7;
const int dx[4] = {-1, 0, 1, 0};
const int dy[4] = {0, -1, 0, 1};

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

  ll k;
  cin >> k;

  ll cnt = 0;
  ll rem = 0;
  ll power = 7;
  while (true) {
    cnt++;
    rem = (rem + power) % k;
    if (rem == 0)
      break;

    power = power * 10 % k;
    if (power == 0 || cnt >= 1e6 + 1) {
      cnt = -1;
      break;
    }
  }

  cout << cnt << endl;
}