Problem p02596 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> P;

#define all(v) v.begin(), v.end()

const ll mod = 1000000007;

ll countBits(ll in) {
  int res = 0;
  for (; in > 0; in >>= 1) {
    if ((in & 0x01) != 0) {
      res++;
    }
  }
  return res;
}

template <typename T> void show2dVector(vector<vector<T>> &v) {
  for (int i = 0; i < v.size(); i++) {
    int m = v[i].size();
    cout << i << " : ";
    for (int j = 0; i < m; i++) {
      cout << v[i][j] << " ";
    }
    cout << endl;
  }
}

void bfs(const vector<vector<int>> &g, int v, vector<bool> &seen) {
  seen[v] = true;
  cout << v << " ";
  for (auto next : g[v]) {
    if (seen[next]) {
      continue;
    }
    bfs(g, next, seen);
  }
}

bool dfs(vector<vector<int>> &g, int start, int goal, vector<bool> &seen) {
  bool res = false;
  seen[start] = true;
  if (start == goal) {
    return true;
  }
  for (auto next : g[start]) {
    if (seen[next]) {
      continue;
    }
    res = dfs(g, next, goal, seen);
    if (res) {
      break;
    }
  }
  return res;
}

ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }
ll lcm(ll a, ll b) { return a * b / gcd(a, b); }
bool isLowerCase(char c) { return (c >= 'a' && c <= 'z'); }

ll powm(ll a, ll n, ll m) {
  ll ret = 1;
  while (n > 0) {
    if (n % 2 == 1) {
      ret = (ret * a) % m;
    }
    n >>= 1;
    a = (a * a) % m;
  }
  return ret;
}

void primeFactor(ll val, map<ll, ll> &primeList) {
  if (val < 1) {
    return;
  }
  for (int i = 2; i * i < val; i++) {
    while (val % i == 0) {
      primeList[i]++;
      val /= i;
    }
  }
  if (val != 1) {
    primeList[val] = 1;
  }
  return;
}

const string yesno(bool ans) { return (ans ? "Yes" : "No"); }
int main() {
  ll k;
  cin >> k;
  int ans = 0;
  ll a = 0;
  if (k == 1 || k == 7) {
    cout << 1 << endl;
    return 0;
  } else if (k % 2 == 0) {
    cout << -1 << endl;
    return 0;
  }
  for (int i = 1; i < INT_MAX; i++) {
    a = a * 10 + 7;
    a %= k;
    if (a == 0) {
      ans = i;
      break;
    }
  }
  cout << ans << endl;
  return 0;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> P;

#define all(v) v.begin(), v.end()

const ll mod = 1000000007;

ll countBits(ll in) {
  int res = 0;
  for (; in > 0; in >>= 1) {
    if ((in & 0x01) != 0) {
      res++;
    }
  }
  return res;
}

template <typename T> void show2dVector(vector<vector<T>> &v) {
  for (int i = 0; i < v.size(); i++) {
    int m = v[i].size();
    cout << i << " : ";
    for (int j = 0; i < m; i++) {
      cout << v[i][j] << " ";
    }
    cout << endl;
  }
}

void bfs(const vector<vector<int>> &g, int v, vector<bool> &seen) {
  seen[v] = true;
  cout << v << " ";
  for (auto next : g[v]) {
    if (seen[next]) {
      continue;
    }
    bfs(g, next, seen);
  }
}

bool dfs(vector<vector<int>> &g, int start, int goal, vector<bool> &seen) {
  bool res = false;
  seen[start] = true;
  if (start == goal) {
    return true;
  }
  for (auto next : g[start]) {
    if (seen[next]) {
      continue;
    }
    res = dfs(g, next, goal, seen);
    if (res) {
      break;
    }
  }
  return res;
}

ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }
ll lcm(ll a, ll b) { return a * b / gcd(a, b); }
bool isLowerCase(char c) { return (c >= 'a' && c <= 'z'); }

ll powm(ll a, ll n, ll m) {
  ll ret = 1;
  while (n > 0) {
    if (n % 2 == 1) {
      ret = (ret * a) % m;
    }
    n >>= 1;
    a = (a * a) % m;
  }
  return ret;
}

void primeFactor(ll val, map<ll, ll> &primeList) {
  if (val < 1) {
    return;
  }
  for (int i = 2; i * i < val; i++) {
    while (val % i == 0) {
      primeList[i]++;
      val /= i;
    }
  }
  if (val != 1) {
    primeList[val] = 1;
  }
  return;
}

const string yesno(bool ans) { return (ans ? "Yes" : "No"); }
int main() {
  ll k;
  cin >> k;
  int ans = 0;
  ll a = 0;
  if (k == 1 || k == 7) {
    cout << 1 << endl;
    return 0;
  } else if (k % 2 == 0 || k % 5 == 0) {
    cout << -1 << endl;
    return 0;
  }
  for (int i = 1; i < INT_MAX; i++) {
    a = a * 10 + 7;
    a %= k;
    if (a == 0) {
      ans = i;
      break;
    }
  }
  cout << ans << endl;
  return 0;
}