Problem p02541 (C++) — buggy submission, verdict: Time Limit Exceeded

#include <bits/stdc++.h>
#pragma GCC target("avx")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
using namespace std;
#define rep(i, n) for (long long i = 0; i < (long long)(n); i++)
#define REP(i, k, n) for (long long i = k; i < (long long)(n); i++)
#define all(a) a.begin(), a.end()
#define pb emplace_back
#define eb emplace_back
#define lb(v, k) (lower_bound(all(v), k) - v.begin())
#define ub(v, k) (upper_bound(all(v), k) - v.begin())
#define fi first
#define se second
#define pi M_PI
#define PQ(T) priority_queue<T>
#define SPQ(T) priority_queue<T, vector<T>, greater<T>>
#define dame(a)                                                                \\
  {                                                                            \\
    out(a);                                                                    \\
    return 0;                                                                  \\
  }
#define decimal cout << fixed << setprecision(15);
#define dupli(a)                                                               \\
  {                                                                            \\
    sort(all(a));                                                              \\
    a.erase(unique(all(a)), a.end());                                          \\
  }
typedef long long ll;
typedef pair<ll, ll> P;
typedef tuple<ll, ll, ll> PP;
typedef tuple<ll, ll, ll, ll> PPP;
typedef multiset<ll> S;
using vi = vector<ll>;
using vvi = vector<vi>;
using vvvi = vector<vvi>;
using vvvvi = vector<vvvi>;
using vp = vector<P>;
using vvp = vector<vp>;
using vb = vector<bool>;
using vvb = vector<vb>;
const ll inf = 1001001001001001001;
const ll INF = 1001001001;
const ll mod = 1000000007;
const double eps = 1e-10;
template <class T> bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> void out(T a) { cout << a << '\\n'; }
template <class T> void outp(T a) {
  cout << '(' << a.fi << ',' << a.se << ')' << '\\n';
}
template <class T> void outvp(T v) {
  rep(i, v.size()) cout << '(' << v[i].fi << ',' << v[i].se << ')';
  cout << '\\n';
}
template <class T> void outvvp(T v) { rep(i, v.size()) outvp(v[i]); }
template <class T> void outv(T v) {
  rep(i, v.size()) {
    if (i)
      cout << ' ';
    cout << v[i];
  }
  cout << '\\n';
}
template <class T> void outvv(T v) { rep(i, v.size()) outv(v[i]); }
template <class T> bool isin(T x, T l, T r) { return (l) <= (x) && (x) <= (r); }
template <class T> void yesno(T b) {
  if (b)
    out("yes");
  else
    out("no");
}
template <class T> void YesNo(T b) {
  if (b)
    out("Yes");
  else
    out("No");
}
template <class T> void YESNO(T b) {
  if (b)
    out("YES");
  else
    out("NO");
}
template <class T> void noyes(T b) {
  if (b)
    out("no");
  else
    out("yes");
}
template <class T> void NoYes(T b) {
  if (b)
    out("No");
  else
    out("Yes");
}
template <class T> void NOYES(T b) {
  if (b)
    out("NO");
  else
    out("YES");
}
void outs(ll a, ll b) {
  if (a >= inf - 100)
    out(b);
  else
    out(a);
}
ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}
ll modpow(ll a, ll b) {
  ll res = 1;
  a %= mod;
  while (b) {
    if (b & 1)
      res = res * a % mod;
    a = a * a % mod;
    b >>= 1;
  }
  return res;
}
ll MX = 1000005;
vi primenumbers, f;
void initprime() { // O(loglogMX)
  f = vi(MX + 5, -1);
  vb b(MX - 1, true);
  for (ll i = 2; i <= sqrt(MX); i++)
    if (b[i - 2]) {
      for (ll j = 2; i * j <= MX; j++) {
        if (b[i * j - 2])
          f[i * j] = i;
        b[i * j - 2] = false;
      }
    }
  rep(i, MX - 1) if (b[i]) primenumbers.pb(i + 2);
}
bool is_prime(ll n) {
  if (n <= 1)
    return false;
  REP(i, 2, sqrt(n) + 1) if (n % i == 0) return false;
  return true;
}
vi x, y;
vp v;
int main() {
  ll n;
  cin >> n;
  n *= 2;
  /*initprime();
  ll s=1;
  rep(i,primenumbers.size()){
      s*=primenumbers[i];
      if(s>1000000000000000)dame(i);
  }*/
  ll ans = n;
  REP(i, 1, n) {
    if (i * i > n)
      break;
    if (n % i == 0) {
      ll k = n / i;
      if (gcd(k, i) == 1) {
        REP(j, 1, i + 1) {
          ll x = k * j + 1;
          if (k * j >= ans)
            break;
          if (x % i == 0) {
            chmin(ans, k * j);
            break;
          }
        }
        REP(j, 1, i + 1) {
          ll x = k * j - 1;
          if (k * j - 1 >= ans)
            break;
          if (x % i == 0) {
            chmin(ans, k * j - 1);
            break;
          }
        }
      }
    }
  }
  out(ans);
}

Error: TLE

Fixed submission:

#include <bits/stdc++.h>
#pragma GCC target("avx")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
using namespace std;
#define rep(i, n) for (long long i = 0; i < (long long)(n); i++)
#define REP(i, k, n) for (long long i = k; i < (long long)(n); i++)
#define all(a) a.begin(), a.end()
#define pb emplace_back
#define eb emplace_back
#define lb(v, k) (lower_bound(all(v), k) - v.begin())
#define ub(v, k) (upper_bound(all(v), k) - v.begin())
#define fi first
#define se second
#define pi M_PI
#define PQ(T) priority_queue<T>
#define SPQ(T) priority_queue<T, vector<T>, greater<T>>
#define dame(a)                                                                \\
  {                                                                            \\
    out(a);                                                                    \\
    return 0;                                                                  \\
  }
#define decimal cout << fixed << setprecision(15);
#define dupli(a)                                                               \\
  {                                                                            \\
    sort(all(a));                                                              \\
    a.erase(unique(all(a)), a.end());                                          \\
  }
typedef long long ll;
typedef pair<ll, ll> P;
typedef tuple<ll, ll, ll> PP;
typedef tuple<ll, ll, ll, ll> PPP;
typedef multiset<ll> S;
using vi = vector<ll>;
using vvi = vector<vi>;
using vvvi = vector<vvi>;
using vvvvi = vector<vvvi>;
using vp = vector<P>;
using vvp = vector<vp>;
using vb = vector<bool>;
using vvb = vector<vb>;
const ll inf = 1001001001001001001;
const ll INF = 1001001001;
const ll mod = 1000000007;
const double eps = 1e-10;
template <class T> bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> void out(T a) { cout << a << '\\n'; }
template <class T> void outp(T a) {
  cout << '(' << a.fi << ',' << a.se << ')' << '\\n';
}
template <class T> void outvp(T v) {
  rep(i, v.size()) cout << '(' << v[i].fi << ',' << v[i].se << ')';
  cout << '\\n';
}
template <class T> void outvvp(T v) { rep(i, v.size()) outvp(v[i]); }
template <class T> void outv(T v) {
  rep(i, v.size()) {
    if (i)
      cout << ' ';
    cout << v[i];
  }
  cout << '\\n';
}
template <class T> void outvv(T v) { rep(i, v.size()) outv(v[i]); }
template <class T> bool isin(T x, T l, T r) { return (l) <= (x) && (x) <= (r); }
template <class T> void yesno(T b) {
  if (b)
    out("yes");
  else
    out("no");
}
template <class T> void YesNo(T b) {
  if (b)
    out("Yes");
  else
    out("No");
}
template <class T> void YESNO(T b) {
  if (b)
    out("YES");
  else
    out("NO");
}
template <class T> void noyes(T b) {
  if (b)
    out("no");
  else
    out("yes");
}
template <class T> void NoYes(T b) {
  if (b)
    out("No");
  else
    out("Yes");
}
template <class T> void NOYES(T b) {
  if (b)
    out("NO");
  else
    out("YES");
}
void outs(ll a, ll b) {
  if (a >= inf - 100)
    out(b);
  else
    out(a);
}
ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}
ll modpow(ll a, ll b) {
  ll res = 1;
  a %= mod;
  while (b) {
    if (b & 1)
      res = res * a % mod;
    a = a * a % mod;
    b >>= 1;
  }
  return res;
}

int main() {
  ll n;
  cin >> n;
  n *= 2;
  /*initprime();
  ll s=1;
  rep(i,primenumbers.size()){
      s*=primenumbers[i];
      if(s>1000000000000000)dame(i);
  }*/
  ll ans = n;
  REP(i, 1, n) {
    if (i * i > n)
      break;
    if (n % i == 0) {
      ll k = n / i;
      if (gcd(k, i) == 1) {
        REP(j, 1, i + 1) {
          ll x = k * j + 1;
          if (x % i == 0) {
            chmin(ans, k * j);
            break;
          }
        }
        REP(j, 1, i + 1) {
          ll x = k * j - 1;
          if (x % i == 0) {
            chmin(ans, k * j - 1);
            break;
          }
        }
      }
    }
  }
  out(ans);
}