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

#pragma GCC target("avx")
#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
// #pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#define _USE_MATH_DEFINES
#include <algorithm>
#include <bitset>
#include <cassert>
#include <climits>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstring>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <time.h>
#include <tuple>
#include <unordered_map>
#include <vector>
using namespace std;
using ll = long long;
using ld = long double;
#define int long long
#define all(a) (a).begin(), (a).end()
#define fs first
#define sc second
#define xx first
#define yy second.first
#define zz second.second
#define H pair<int, int>
#define P pair<int, pair<int, int>>
#define Q(i, j, k) mkp(i, mkp(j, k))
#define rng(i, s, n) for (int i = (s); i < (n); i++)
#define rep(i, n) rng(i, 0, (n))
#define mkp make_pair
#define vec vector
#define vi vec<int>
#define pb emplace_back
#define siz(a) (int)(a).size()
#define crdcomp(b)                                                             \\
  sort(all((b)));                                                              \\
  (b).erase(unique(all((b))), (b).end())
#define getidx(b, i) (lower_bound(all(b), (i)) - (b).begin())
#define ssp(i, n) (i == (int)(n)-1 ? "\\n" : " ")
#define ctoi(c) (int)(c - '0')
#define itoc(c) (char)(c + '0')
#define cyes printf("Yes\\n")
#define cno printf("No\\n")
#define cdf(n)                                                                 \\
  int quetimes_ = (n);                                                         \\
  rep(qq123_, quetimes_)
#define gcj printf("Case #%lld: ", qq123_ + 1)
#define readv(a, n)                                                            \\
  a.resize(n, 0);                                                              \\
  rep(i, (n)) a[i] = read()
#define found(a, x) (a.find(x) != a.end())
// #define endl "\\n"
constexpr int mod = (ll)1e9 + 7;
constexpr int Mod = 998244353;
constexpr ld EPS = 1e-10;
constexpr ll inf = (ll)3 * 1e18;
constexpr int Inf = (ll)15 * 1e8;
constexpr int dx[] = {-1, 1, 0, 0}, dy[] = {0, 0, -1, 1};
template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}
ll read() {
  ll u, k = scanf("%lld", &u);
  return u;
}
string reads() {
  string s;
  cin >> s;
  return s;
}
H readh(short g = 0) {
  H u;
  int k = scanf("%lld %lld", &u.fs, &u.sc);
  if (g == 1)
    u.fs--, u.sc--;
  if (g == 2)
    u.fs--;
  return u;
}
bool ina(H t, int h, int w) {
  return 0 <= t.fs && t.fs < h && 0 <= t.sc && t.sc < w;
}
bool ina(int t, int l, int r) { return l <= t && t < r; }
ll gcd(ll i, ll j) { return j ? gcd(j, i % j) : i; }
ll popcount(ll x) {
  int sum = 0;
  for (int i = 0; i < 60; i++)
    if ((1ll << i) & x)
      sum++;
  return sum;
}
template <typename T> class csum {
  vec<T> v;

public:
  csum(vec<T> &a) : v(a) { build(); }
  csum() {}
  void init(vec<T> &a) {
    v = a;
    build();
  }
  void build() {
    for (int i = 1; i < v.size(); i++)
      v[i] += v[i - 1];
  }
  T a(int l, int r) {
    if (r < l)
      return 0;
    return v[r] - (l == 0 ? 0 : v[l - 1]);
  }                                         //[l,r]
  T b(int l, int r) { return a(l, r - 1); } //[l,r)
  T a(pair<int, int> t) { return a(t.first, t.second); }
  T b(pair<int, int> t) { return b(t.first, t.second); }
};
class mint {
public:
  ll v;
  mint(ll v = 0) { s(v % mod + mod); }
  constexpr static int mod = (ll)1e9 + 7;
  constexpr static int fn_ = (ll)2e6 + 5;
  static mint fact[fn_], comp[fn_];
  mint pow(int x) const {
    mint b(v), c(1);
    while (x) {
      if (x & 1)
        c *= b;
      b *= b;
      x >>= 1;
    }
    return c;
  }
  inline mint &s(int vv) {
    v = vv < mod ? vv : vv - mod;
    return *this;
  }
  inline mint inv() const { return pow(mod - 2); }
  inline mint operator-() const { return mint() - *this; }
  inline mint &operator+=(const mint b) { return s(v + b.v); }
  inline mint &operator-=(const mint b) { return s(v + mod - b.v); }
  inline mint &operator*=(const mint b) {
    v = v * b.v % mod;
    return *this;
  }
  inline mint &operator/=(const mint b) {
    v = v * b.inv().v % mod;
    return *this;
  }
  inline mint operator+(const mint b) const { return mint(v) += b; }
  inline mint operator-(const mint b) const { return mint(v) -= b; }
  inline mint operator*(const mint b) const { return mint(v) *= b; }
  inline mint operator/(const mint b) const { return mint(v) /= b; }
  friend ostream &operator<<(ostream &os, const mint &m) { return os << m.v; }
  friend istream &operator>>(istream &is, mint &m) {
    int x;
    is >> x;
    m = mint(x);
    return is;
  }
  bool operator<(const mint &r) const { return v < r.v; }
  bool operator>(const mint &r) const { return v > r.v; }
  bool operator<=(const mint &r) const { return v <= r.v; }
  bool operator>=(const mint &r) const { return v >= r.v; }
  bool operator==(const mint &r) const { return v == r.v; }
  bool operator!=(const mint &r) const { return v != r.v; }
  explicit operator bool() const { return v; }
  explicit operator int() const { return v; }
  mint comb(mint k) {
    if (k > *this)
      return mint();
    if (!fact[0])
      combinit();
    if (v >= fn_) {
      if (k > *this - k)
        k = *this - k;
      mint tmp(1);
      for (int i = v; i >= v - k.v + 1; i--)
        tmp *= mint(i);
      return tmp * comp[k.v];
    }
    return fact[v] * comp[k.v] * comp[v - k.v];
  } // nCk
  mint perm(mint k) {
    if (k > *this)
      return mint();
    if (!fact[0])
      combinit();
    if (v >= fn_) {
      mint tmp(1);
      for (int i = v; i >= v - k.v + 1; i--)
        tmp *= mint(i);
      return tmp;
    }
    return fact[v] * comp[v - k.v];
  } // nPk
  static void combinit() {
    fact[0] = 1;
    for (int i = 1; i < fn_; i++)
      fact[i] = fact[i - 1] * mint(i);
    comp[fn_ - 1] = fact[fn_ - 1].inv();
    for (int i = fn_ - 2; i >= 0; i--)
      comp[i] = comp[i + 1] * mint(i + 1);
  }
};
mint mint::fact[fn_], mint::comp[fn_];
//--------------------------------------------------------------

//---------------------------------------------------------------------

signed main() {
  int k = read();
  if (k % 2 == 0) {
    cout << -1 << endl;
    return 0;
  }
  int sum = 0, r = 1;
  for (int i = 1; i < 10000000; i++) {
    (sum += (r * 7) % k) %= k;
    ;
    if (sum == 0) {
      cout << i << endl;
      return 0;
    }
    (r *= 10) %= k;
  }
  cout << -1 << endl;
}

Error: TLE

Fixed submission:

#pragma GCC target("avx")
#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
// #pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#define _USE_MATH_DEFINES
#include <algorithm>
#include <bitset>
#include <cassert>
#include <climits>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstring>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <time.h>
#include <tuple>
#include <unordered_map>
#include <vector>
using namespace std;
using ll = long long;
using ld = long double;
#define int long long
#define all(a) (a).begin(), (a).end()
#define fs first
#define sc second
#define xx first
#define yy second.first
#define zz second.second
#define H pair<int, int>
#define P pair<int, pair<int, int>>
#define Q(i, j, k) mkp(i, mkp(j, k))
#define rng(i, s, n) for (int i = (s); i < (n); i++)
#define rep(i, n) rng(i, 0, (n))
#define mkp make_pair
#define vec vector
#define vi vec<int>
#define pb emplace_back
#define siz(a) (int)(a).size()
#define crdcomp(b)                                                             \\
  sort(all((b)));                                                              \\
  (b).erase(unique(all((b))), (b).end())
#define getidx(b, i) (lower_bound(all(b), (i)) - (b).begin())
#define ssp(i, n) (i == (int)(n)-1 ? "\\n" : " ")
#define ctoi(c) (int)(c - '0')
#define itoc(c) (char)(c + '0')
#define cyes printf("Yes\\n")
#define cno printf("No\\n")
#define cdf(n)                                                                 \\
  int quetimes_ = (n);                                                         \\
  rep(qq123_, quetimes_)
#define gcj printf("Case #%lld: ", qq123_ + 1)
#define readv(a, n)                                                            \\
  a.resize(n, 0);                                                              \\
  rep(i, (n)) a[i] = read()
#define found(a, x) (a.find(x) != a.end())
// #define endl "\\n"
constexpr int mod = (ll)1e9 + 7;
constexpr int Mod = 998244353;
constexpr ld EPS = 1e-10;
constexpr ll inf = (ll)3 * 1e18;
constexpr int Inf = (ll)15 * 1e8;
constexpr int dx[] = {-1, 1, 0, 0}, dy[] = {0, 0, -1, 1};
template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}
ll read() {
  ll u, k = scanf("%lld", &u);
  return u;
}
string reads() {
  string s;
  cin >> s;
  return s;
}
H readh(short g = 0) {
  H u;
  int k = scanf("%lld %lld", &u.fs, &u.sc);
  if (g == 1)
    u.fs--, u.sc--;
  if (g == 2)
    u.fs--;
  return u;
}
bool ina(H t, int h, int w) {
  return 0 <= t.fs && t.fs < h && 0 <= t.sc && t.sc < w;
}
bool ina(int t, int l, int r) { return l <= t && t < r; }
ll gcd(ll i, ll j) { return j ? gcd(j, i % j) : i; }
ll popcount(ll x) {
  int sum = 0;
  for (int i = 0; i < 60; i++)
    if ((1ll << i) & x)
      sum++;
  return sum;
}
template <typename T> class csum {
  vec<T> v;

public:
  csum(vec<T> &a) : v(a) { build(); }
  csum() {}
  void init(vec<T> &a) {
    v = a;
    build();
  }
  void build() {
    for (int i = 1; i < v.size(); i++)
      v[i] += v[i - 1];
  }
  T a(int l, int r) {
    if (r < l)
      return 0;
    return v[r] - (l == 0 ? 0 : v[l - 1]);
  }                                         //[l,r]
  T b(int l, int r) { return a(l, r - 1); } //[l,r)
  T a(pair<int, int> t) { return a(t.first, t.second); }
  T b(pair<int, int> t) { return b(t.first, t.second); }
};
class mint {
public:
  ll v;
  mint(ll v = 0) { s(v % mod + mod); }
  constexpr static int mod = (ll)1e9 + 7;
  constexpr static int fn_ = (ll)2e6 + 5;
  static mint fact[fn_], comp[fn_];
  mint pow(int x) const {
    mint b(v), c(1);
    while (x) {
      if (x & 1)
        c *= b;
      b *= b;
      x >>= 1;
    }
    return c;
  }
  inline mint &s(int vv) {
    v = vv < mod ? vv : vv - mod;
    return *this;
  }
  inline mint inv() const { return pow(mod - 2); }
  inline mint operator-() const { return mint() - *this; }
  inline mint &operator+=(const mint b) { return s(v + b.v); }
  inline mint &operator-=(const mint b) { return s(v + mod - b.v); }
  inline mint &operator*=(const mint b) {
    v = v * b.v % mod;
    return *this;
  }
  inline mint &operator/=(const mint b) {
    v = v * b.inv().v % mod;
    return *this;
  }
  inline mint operator+(const mint b) const { return mint(v) += b; }
  inline mint operator-(const mint b) const { return mint(v) -= b; }
  inline mint operator*(const mint b) const { return mint(v) *= b; }
  inline mint operator/(const mint b) const { return mint(v) /= b; }
  friend ostream &operator<<(ostream &os, const mint &m) { return os << m.v; }
  friend istream &operator>>(istream &is, mint &m) {
    int x;
    is >> x;
    m = mint(x);
    return is;
  }
  bool operator<(const mint &r) const { return v < r.v; }
  bool operator>(const mint &r) const { return v > r.v; }
  bool operator<=(const mint &r) const { return v <= r.v; }
  bool operator>=(const mint &r) const { return v >= r.v; }
  bool operator==(const mint &r) const { return v == r.v; }
  bool operator!=(const mint &r) const { return v != r.v; }
  explicit operator bool() const { return v; }
  explicit operator int() const { return v; }
  mint comb(mint k) {
    if (k > *this)
      return mint();
    if (!fact[0])
      combinit();
    if (v >= fn_) {
      if (k > *this - k)
        k = *this - k;
      mint tmp(1);
      for (int i = v; i >= v - k.v + 1; i--)
        tmp *= mint(i);
      return tmp * comp[k.v];
    }
    return fact[v] * comp[k.v] * comp[v - k.v];
  } // nCk
  mint perm(mint k) {
    if (k > *this)
      return mint();
    if (!fact[0])
      combinit();
    if (v >= fn_) {
      mint tmp(1);
      for (int i = v; i >= v - k.v + 1; i--)
        tmp *= mint(i);
      return tmp;
    }
    return fact[v] * comp[v - k.v];
  } // nPk
  static void combinit() {
    fact[0] = 1;
    for (int i = 1; i < fn_; i++)
      fact[i] = fact[i - 1] * mint(i);
    comp[fn_ - 1] = fact[fn_ - 1].inv();
    for (int i = fn_ - 2; i >= 0; i--)
      comp[i] = comp[i + 1] * mint(i + 1);
  }
};
mint mint::fact[fn_], mint::comp[fn_];
//--------------------------------------------------------------

//---------------------------------------------------------------------

signed main() {
  int k = read();
  if (k % 2 == 0) {
    cout << -1 << endl;
    return 0;
  }
  int sum = 0, r = 1;
  for (int i = 1; i < 100000000; i++) {
    (sum += (r * 7) % k) %= k;
    ;
    if (sum == 0) {
      cout << i << endl;
      return 0;
    }
    (r *= 10) %= k;
  }
  cout << -1 << endl;
}