Problem p02541 (C++) — buggy submission, verdict: Runtime Error

#ifdef DEBUG
#define _GLIBCXX_DEBUG
#endif
// #pragma GCC optimize("O3")
#include <bits/stdc++.h>
using namespace std;
typedef long double ld;
typedef long long ll;
ll m;
int k;
mt19937_64 rnd(228);

const int BUBEN = (int)1e6 + 228;
const int BUBEN2 = 15;

long long multModulo(long long a, long long b, long long m) {
  long long q = (long long)((long double)a * (long double)b / (long double)m);
  long long r = a * b - q * m;
  while (r < 0)
    r += m;
  while (r >= m)
    r -= m;
  return r;
}

ll sumModulo(ll a, ll b, ll mod) {
  ll s = a + b;
  if (s >= mod)
    s -= mod;
  return s;
}

ll powModulo(ll a, ll b, ll mod) {
  if (b == 0)
    return 1;
  if (b & 1)
    return multModulo(a, powModulo(a, b - 1, mod), mod);
  ll res = powModulo(a, b / 2, mod);
  return multModulo(res, res, mod);
}

ll pollard_rho(ll n) {
  ll y = rnd() % (n - 1) + 1;
  ll c = rnd() % (n - 1) + 1;
  ll m = rnd() % (n - 1) + 1;
  ll g = 1;
  ll r = 1;
  ll q = 1;
  ll x = 0;
  ll ys = 0;
  ll k = 0;
  while (g == 1) {
    x = y;
    for (int i = 0; i < r; i++) {
      y = sumModulo(multModulo(y, y, n), c, n);
      k = 0;
    }
    while (k < r && g == 1) {
      ys = y;
      for (int i = 0; i < min(m, r - k); i++) {
        y = sumModulo(multModulo(y, y, n), c, n);
        q = multModulo(q, abs(x - y), n);
        g = __gcd(q, n);
      }
      k = k + m;
    }
    r *= 2;
  }
  if (g == n) {
    while (true) {
      ys = sumModulo(multModulo(ys, ys, n), c, n);
      g = __gcd(abs(x - ys), n);
      if (g > 1)
        break;
    }
  }
  return g;
}

bool is_prime(ll n) {
  if (n == 2 || n == 3 || n == 5 || n == 7) {
    return true;
  }
  if (n % 2 == 0 || n % 3 == 0 || n % 5 == 0 || n % 7 == 0) {
    return false;
  }
  ll d = n - 1;
  int r = 0;
  // can add
  while (d % 2 == 0) {
    d /= 2;
    r++;
  }
  for (int j = 0; j < BUBEN2; j++) {
    ll a = rnd() % (n - 3) + 2;
    ll x = powModulo(a, d, n);
    if (x == 1 || x == n - 1) {
      continue;
    }
    for (int it = 0; it < r - 1; it++) {
      x = multModulo(x, x, n);
      if (x == n - 1) {
        break;
      }
    }
    if (x != (n - 1)) {
      return false;
    }
  }
  return true;
}

int lp[BUBEN];
vector<int> small_primes;

void init() {
  for (int i = 2; i < BUBEN; i++) {
    if (lp[i] == 0) {
      lp[i] = i;
      if (i < BUBEN)
        small_primes.push_back(i);
      for (int j = 2 * i; j < BUBEN; j += i)
        lp[j] = i;
    }
  }
}

vector<pair<ll, int>> get_fact(ll n) {
  vector<pair<ll, int>> ans;
  if (n < BUBEN) {
    // factorize
    while (n > 1) {
      int p = lp[n];
      int ex = 0;
      while (n % p == 0) {
        n /= p;
        ex++;
      }
      ans.emplace_back(p, ex);
    }
    sort(ans.begin(), ans.end());
    return ans;
  }
  if (n == 1)
    return {};
  ll fact = 0;
  for (ll v : small_primes) {
    if (n % v == 0) {
      fact = v;
      break;
    }
  }
  if (fact != 0) {
    int ex = 0;
    while (n % fact == 0) {
      n /= fact;
      ex++;
    }
    ans.emplace_back(fact, ex);
    auto ans2 = get_fact(n);
    for (auto it : ans2)
      ans.push_back(it);
    return ans;
  }

  if (is_prime(n)) {
    ans.emplace_back(n, 1);
    return ans;
  }
  if (fact == 0) {
    fact = 1;
    while (fact == 1 || fact == n)
      fact = pollard_rho(n);
  }
  auto cur_ans = get_fact(fact);
  n /= fact;
  for (auto &t : cur_ans) {
    ll x = t.first;
    int y = t.second;
    if (n % x == 0) {
      while (n % x == 0) {
        n /= x;
        y++;
      }
    }
    ans.emplace_back(x, y);
  }
  if (n > 1) {
    auto ans2 = get_fact(n);
    for (auto it : ans2)
      ans.push_back(it);
    return ans;
  }
  sort(ans.begin(), ans.end());
  return ans;
}
ll do_gcd(ll a, ll b, ll &x, ll &y) {
  if (a == 0) {
    x = 0;
    y = 1;
    return b;
  }
  ll p = b / a;
  ll g = do_gcd(b - p * a, a, y, x);
  x -= p * y;
  return g;
}
ll getInv(ll a, ll M) {
  ll c1, c2;
  ll d = do_gcd(a, M, c1, c2);
  assert(d == 1);
  c1 %= M;
  if (c1 < 0)
    c1 += M;
  return c1;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  //    freopen("input.txt", "r", stdin);
  init();
  ll n;
  cin >> n;
  n *= 2;
  auto it = get_fact(n);
  ll best = n;
  for (int mask = 0; mask < (1 << it.size()); mask++) {
    ll mod1 = 1;
    ll mod2 = 1;
    for (int i = 0; i < it.size(); i++) {
      ll p = 1;
      for (int j = 0; j < it[i].second; j++)
        p *= it[i].first;
      if (mask & (1 << i)) {
        mod1 *= p;
      } else {
        mod2 *= p;
      }
    }
    ll f = getInv(mod2, mod1);
    if (f == 0)
      f += mod1;
    if (f * mod2 == 1) {
      f += mod1;
    }
    best = min(best, f * mod2 - 1);
  }
  cout << best;
  return 0;
}

Error: WA

Fixed submission:

#ifdef DEBUG
#define _GLIBCXX_DEBUG
#endif
// #pragma GCC optimize("O3")
#include <bits/stdc++.h>
using namespace std;
typedef long double ld;
typedef long long ll;
ll m;
int k;
mt19937_64 rnd(228);

const int BUBEN = (int)1e6 + 228;
const int BUBEN2 = 15;

long long multModulo(long long a, long long b, long long m) {
  long long q = (long long)((long double)a * (long double)b / (long double)m);
  long long r = a * b - q * m;
  while (r < 0)
    r += m;
  while (r >= m)
    r -= m;
  return r;
}

ll sumModulo(ll a, ll b, ll mod) {
  ll s = a + b;
  if (s >= mod)
    s -= mod;
  return s;
}

ll powModulo(ll a, ll b, ll mod) {
  if (b == 0)
    return 1;
  if (b & 1)
    return multModulo(a, powModulo(a, b - 1, mod), mod);
  ll res = powModulo(a, b / 2, mod);
  return multModulo(res, res, mod);
}

ll pollard_rho(ll n) {
  ll y = rnd() % (n - 1) + 1;
  ll c = rnd() % (n - 1) + 1;
  ll m = rnd() % (n - 1) + 1;
  ll g = 1;
  ll r = 1;
  ll q = 1;
  ll x = 0;
  ll ys = 0;
  ll k = 0;
  while (g == 1) {
    x = y;
    for (int i = 0; i < r; i++) {
      y = sumModulo(multModulo(y, y, n), c, n);
      k = 0;
    }
    while (k < r && g == 1) {
      ys = y;
      for (int i = 0; i < min(m, r - k); i++) {
        y = sumModulo(multModulo(y, y, n), c, n);
        q = multModulo(q, abs(x - y), n);
        g = __gcd(q, n);
      }
      k = k + m;
    }
    r *= 2;
  }
  if (g == n) {
    while (true) {
      ys = sumModulo(multModulo(ys, ys, n), c, n);
      g = __gcd(abs(x - ys), n);
      if (g > 1)
        break;
    }
  }
  return g;
}

bool is_prime(ll n) {
  if (n == 2 || n == 3 || n == 5 || n == 7) {
    return true;
  }
  if (n % 2 == 0 || n % 3 == 0 || n % 5 == 0 || n % 7 == 0) {
    return false;
  }
  ll d = n - 1;
  int r = 0;
  // can add
  while (d % 2 == 0) {
    d /= 2;
    r++;
  }
  for (int j = 0; j < BUBEN2; j++) {
    ll a = rnd() % (n - 3) + 2;
    ll x = powModulo(a, d, n);
    if (x == 1 || x == n - 1) {
      continue;
    }
    for (int it = 0; it < r - 1; it++) {
      x = multModulo(x, x, n);
      if (x == n - 1) {
        break;
      }
    }
    if (x != (n - 1)) {
      return false;
    }
  }
  return true;
}

int lp[BUBEN];
vector<int> small_primes;

void init() {
  for (int i = 2; i < BUBEN; i++) {
    if (lp[i] == 0) {
      lp[i] = i;
      if (i < BUBEN)
        small_primes.push_back(i);
      for (int j = 2 * i; j < BUBEN; j += i)
        lp[j] = i;
    }
  }
}

vector<pair<ll, int>> get_fact(ll n) {
  vector<pair<ll, int>> ans;
  if (n < BUBEN) {
    // factorize
    while (n > 1) {
      int p = lp[n];
      int ex = 0;
      while (n % p == 0) {
        n /= p;
        ex++;
      }
      ans.emplace_back(p, ex);
    }
    sort(ans.begin(), ans.end());
    return ans;
  }
  if (n == 1)
    return {};
  ll fact = 0;
  for (ll v : small_primes) {
    if (n % v == 0) {
      fact = v;
      break;
    }
  }
  if (fact != 0) {
    int ex = 0;
    while (n % fact == 0) {
      n /= fact;
      ex++;
    }
    ans.emplace_back(fact, ex);
    auto ans2 = get_fact(n);
    for (auto it : ans2)
      ans.push_back(it);
    return ans;
  }

  if (is_prime(n)) {
    ans.emplace_back(n, 1);
    return ans;
  }
  if (fact == 0) {
    fact = 1;
    while (fact == 1 || fact == n)
      fact = pollard_rho(n);
  }
  auto cur_ans = get_fact(fact);
  n /= fact;
  for (auto &t : cur_ans) {
    ll x = t.first;
    int y = t.second;
    if (n % x == 0) {
      while (n % x == 0) {
        n /= x;
        y++;
      }
    }
    ans.emplace_back(x, y);
  }
  if (n > 1) {
    auto ans2 = get_fact(n);
    for (auto it : ans2)
      ans.push_back(it);
    return ans;
  }
  sort(ans.begin(), ans.end());
  return ans;
}
ll do_gcd(ll a, ll b, ll &x, ll &y) {
  if (a == 0) {
    x = 0;
    y = 1;
    return b;
  }
  int p = b / a;
  int g = do_gcd(b - p * a, a, y, x);
  x -= p * y;
  return g;
}
ll getInv(ll a, ll M) {
  ll c1, c2;
  ll d = do_gcd(a, M, c1, c2);
  assert(d == 1);
  c1 %= M;
  if (c1 < 0)
    c1 += M;
  return c1;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  //    freopen("input.txt", "r", stdin);
  init();
  ll n;
  cin >> n;
  n *= 2;
  auto it = get_fact(n);
  ll best = n;
  for (int mask = 0; mask < (1 << it.size()); mask++) {
    ll mod1 = 1;
    ll mod2 = 1;
    for (int i = 0; i < it.size(); i++) {
      ll p = 1;
      for (int j = 0; j < it[i].second; j++)
        p *= it[i].first;
      if (mask & (1 << i)) {
        mod1 *= p;
      } else {
        mod2 *= p;
      }
    }
    ll f = getInv(mod2, mod1);
    if (f == 0)
      f += mod1;
    if (f * mod2 == 1) {
      f += mod1;
    }
    best = min(best, f * mod2 - 1);
  }
  cout << best;
  return 0;
}