Problem p02596 (C++) — buggy submission, verdict: Time Limit Exceeded

#include <bits/stdc++.h>
#define int int64_t
#define REP(i, n) for (int i = 0; (i) < (int)(n); ++(i))
#define REP3(i, m, n) for (int i = (m); (i) < (int)(n); ++(i))
#define REP_R(i, n) for (int i = (int)(n)-1; (i) >= 0; --(i))
#define REP3R(i, m, n) for (int i = (int)(n)-1; (i) >= (int)(m); --(i))
#define REPP(i, n) for (int i = 1; (i) < (int)(n + 1); ++(i))
#define REPR(i, n) for (int i = (int)(n)-1; (i) >= 0; --(i))
#define FOR(i, m, n) for (int i = m; i < n; i++)
#define FORR(i, m, n) for (int i = (m)-1; i >= (n); i--)
#define ALL(v) v.begin(), v.end()
#define SZ(a) int((a).size())
#define SORT(c) sort((c).begin(), (c).end())
#define SORTR(c) sort((c).begin(), (c).end(), greater<int>());
#define BIT(n) (1LL << (n))
#define PCNT(x) __builtin_popcountll(x)
#define GCD(x, y) __gcd(x, y)
#define LCM(x, y) x / __gcd(x, y) * y
#define PB push_back
#define MP make_pair
#define endl "\\n"
#define yes cout << "Yes" << endl
#define no cout << "No" << endl
using namespace std;
using Graph = vector<vector<int>>;
using WeightedGraph = vector<vector<pair<int, int>>>;
typedef long double LD;
typedef vector<int> VI;
typedef vector<vector<int>> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef vector<PII> VPII;
const int INF = 2000000000;
const long double PI = acosl(-1);
const int MOD = 1000000007;
void print() { cout << endl; }
template <class Head, class... Tail> void print(Head &&head, Tail &&...tail) {
  cout << head;
  if (sizeof...(tail) != 0)
    cout << " ";
  print(forward<Tail>(tail)...);
}
template <class T> void print(vector<T> &vec) {
  for (auto &a : vec) {
    cout << a;
    if (&a != &vec.back())
      cout << " ";
  }
  cout << endl;
}
template <class T> void print(vector<vector<T>> &df) {
  for (auto &vec : df) {
    print(vec);
  }
}

int power(int N, int M) {
  return M ? power(N * N, M / 2) * (M % 2 ? N : 1) : 1;
}

int mystoi(string S) {
  int num = 0;
  for (int i = (int)S.length() - 1; i >= 0; i--) {
    num += (S[i] - '0') * power(10, S.length() - 1 - i);
  }
  return num;
}

signed main() {
  std::ios::sync_with_stdio(false);
  std::cin.tie(nullptr);
  cout << fixed << setprecision(10);
  int ans = 0;

  int K;
  cin >> K;

  if (K % 2 == 0) {
    print(-1);
    return 0;
  }

  int S = 7;
  ans = 1;
  while (S % K != 0) {
    S = S * 10 + 7;
    S %= K;
    ans++;
    if (ans > K) {
      ans = -1;
      break;
    }
  }

  cout << ans << endl;

  return 0;
}

/*
cd ~/Desktop/Programming/AtCoder/contest/abc174/abc174_c && sh
shellscripts/test_cpp.sh cd
~/Desktop/Programming/AtCoder/contest/abc174/abc174_c && sh
shellscripts/etest_cpp.sh cd
~/Desktop/Programming/AtCoder/contest/abc174/abc174_c && sh
shellscripts/submit_cpp.sh
*/


Error: TLE

Fixed submission:

#include <bits/stdc++.h>
#define int int64_t
#define REP(i, n) for (int i = 0; (i) < (int)(n); ++(i))
#define REP3(i, m, n) for (int i = (m); (i) < (int)(n); ++(i))
#define REP_R(i, n) for (int i = (int)(n)-1; (i) >= 0; --(i))
#define REP3R(i, m, n) for (int i = (int)(n)-1; (i) >= (int)(m); --(i))
#define REPP(i, n) for (int i = 1; (i) < (int)(n + 1); ++(i))
#define REPR(i, n) for (int i = (int)(n)-1; (i) >= 0; --(i))
#define FOR(i, m, n) for (int i = m; i < n; i++)
#define FORR(i, m, n) for (int i = (m)-1; i >= (n); i--)
#define ALL(v) v.begin(), v.end()
#define SZ(a) int((a).size())
#define SORT(c) sort((c).begin(), (c).end())
#define SORTR(c) sort((c).begin(), (c).end(), greater<int>());
#define BIT(n) (1LL << (n))
#define PCNT(x) __builtin_popcountll(x)
#define GCD(x, y) __gcd(x, y)
#define LCM(x, y) x / __gcd(x, y) * y
#define PB push_back
#define MP make_pair
#define endl "\\n"
#define yes cout << "Yes" << endl
#define no cout << "No" << endl
using namespace std;
using Graph = vector<vector<int>>;
using WeightedGraph = vector<vector<pair<int, int>>>;
typedef long double LD;
typedef vector<int> VI;
typedef vector<vector<int>> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef vector<PII> VPII;
const int INF = 2000000000;
const long double PI = acosl(-1);
const int MOD = 1000000007;
void print() { cout << endl; }
template <class Head, class... Tail> void print(Head &&head, Tail &&...tail) {
  cout << head;
  if (sizeof...(tail) != 0)
    cout << " ";
  print(forward<Tail>(tail)...);
}
template <class T> void print(vector<T> &vec) {
  for (auto &a : vec) {
    cout << a;
    if (&a != &vec.back())
      cout << " ";
  }
  cout << endl;
}
template <class T> void print(vector<vector<T>> &df) {
  for (auto &vec : df) {
    print(vec);
  }
}

int power(int N, int M) {
  return M ? power(N * N, M / 2) * (M % 2 ? N : 1) : 1;
}

int mystoi(string S) {
  int num = 0;
  for (int i = (int)S.length() - 1; i >= 0; i--) {
    num += (S[i] - '0') * power(10, S.length() - 1 - i);
  }
  return num;
}

signed main() {
  std::ios::sync_with_stdio(false);
  std::cin.tie(nullptr);
  cout << fixed << setprecision(10);
  int ans = 0;

  int K;
  cin >> K;

  if (K % 2 == 0) {
    print(-1);
    return 0;
  }

  int S = 7;
  ans = 1;
  while (S % K != 0) {
    S = S * 10 + 7;
    S %= K;
    ans++;
  }
  cout << ans << endl;

  return 0;
}

/*
cd ~/Desktop/Programming/AtCoder/contest/abc174/abc174_c && sh
shellscripts/test_cpp.sh cd
~/Desktop/Programming/AtCoder/contest/abc174/abc174_c && sh
shellscripts/etest_cpp.sh cd
~/Desktop/Programming/AtCoder/contest/abc174/abc174_c && sh
shellscripts/submit_cpp.sh
*/