Problem p02696 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <algorithm>
#include <climits>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <vector>
// #include <boost/multiprecision/cpp_dec_float.hpp>
// #include <boost/multiprecision/cpp_int.hpp>

///////////////////////////////////////////////////////////////
// namespace
///////////////////////////////////////////////////////////////
using namespace std;
// namespace mp = boost::multiprecision;

///////////////////////////////////////////////////////////////
// types
///////////////////////////////////////////////////////////////
using ll = long long int;
using ld = long double;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
// using mpi = mp::cpp_int;
// using mpf = mp::number<mp::cpp_dec_float<1024>>;

///////////////////////////////////////////////////////////////
// mod https://qiita.com/drken/items/3b4fdf0a78e7a138cd9a
///////////////////////////////////////////////////////////////
ll mod(ll val, ll MOD) {
  long long res = val % MOD;
  if (res < 0)
    res += MOD;
  return res;
}

ll modinv(ll a, ll MOD) {
  ll b = MOD, u = 1, v = 0;
  while (b) {
    ll t = a / b;
    a -= t * b;
    swap(a, b);
    u -= t * v;
    swap(u, v);
  }
  u %= MOD;
  if (u < 0)
    u += MOD;
  return u;
}

// const int MAX = -1;
// ll fac[MAX], finv[MAX], inv[MAX];
// void com_init() {  // initialize nCk table with O(N)
//     fac[0] = fac[1] = 1;
//     finv[0] = finv[1] = 1;
//     inv[1] = 1;
//     for (int i = 2; i < MAX; i++){
//         fac[i] = fac[i - 1] * i % MOD;
//         inv[i] = MOD - inv[MOD%i] * (MOD / i) % MOD;
//         finv[i] = finv[i - 1] * inv[i] % MOD;
//     }
// }
// ll com(int n, int k){  // nCk
//     if (n < k) return 0;
//     if (n < 0 || k < 0) return 0;
//     return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
// }

ll f(ll a, ll b, ll x) { return floor(a * x / b) - a * floor(x / b); }

int main() {
  ll a, b, n;
  cin >> a >> b >> n;
  unordered_set<ll> s;
  ll ans = 0;
  for (int x = 0; x <= n; ++x) {
    ll y = a * x % b;
    if (s.find(y) != s.end())
      break;
    ans = max(ans, f(a, b, x));
  }
  cout << ans << endl;
  return 0;
}


Fixed submission:
#include <algorithm>
#include <climits>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <vector>
// #include <boost/multiprecision/cpp_dec_float.hpp>
// #include <boost/multiprecision/cpp_int.hpp>

///////////////////////////////////////////////////////////////
// namespace
///////////////////////////////////////////////////////////////
using namespace std;
// namespace mp = boost::multiprecision;

///////////////////////////////////////////////////////////////
// types
///////////////////////////////////////////////////////////////
using ll = long long int;
using ld = long double;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
// using mpi = mp::cpp_int;
// using mpf = mp::number<mp::cpp_dec_float<1024>>;

///////////////////////////////////////////////////////////////
// mod https://qiita.com/drken/items/3b4fdf0a78e7a138cd9a
///////////////////////////////////////////////////////////////
ll mod(ll val, ll MOD) {
  long long res = val % MOD;
  if (res < 0)
    res += MOD;
  return res;
}

ll modinv(ll a, ll MOD) {
  ll b = MOD, u = 1, v = 0;
  while (b) {
    ll t = a / b;
    a -= t * b;
    swap(a, b);
    u -= t * v;
    swap(u, v);
  }
  u %= MOD;
  if (u < 0)
    u += MOD;
  return u;
}

// const int MAX = -1;
// ll fac[MAX], finv[MAX], inv[MAX];
// void com_init() {  // initialize nCk table with O(N)
//     fac[0] = fac[1] = 1;
//     finv[0] = finv[1] = 1;
//     inv[1] = 1;
//     for (int i = 2; i < MAX; i++){
//         fac[i] = fac[i - 1] * i % MOD;
//         inv[i] = MOD - inv[MOD%i] * (MOD / i) % MOD;
//         finv[i] = finv[i - 1] * inv[i] % MOD;
//     }
// }
// ll com(int n, int k){  // nCk
//     if (n < k) return 0;
//     if (n < 0 || k < 0) return 0;
//     return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
// }

ll f(ll a, ll b, ll x) { return floor(a * x / b) - a * floor(x / b); }

int main() {
  ll a, b, n;
  cin >> a >> b >> n;
  cout << f(a, b, min(n, b - 1)) << endl;
  return 0;
}