Problem p02550 (C++) — buggy submission, verdict: Runtime Error

#include <bits/stdc++.h>

using namespace std;

#define REP(i, n) for (int i = 0; i < (n); ++i)
#define FOR(i, a, n) for (int i = (a); i < (n); ++i)

#define SZ(x) ((int)(x).size())
#define ALL(x) (x).begin(), (x).end()

#define DUMP(x) cerr << #x << " = " << (x) << endl
#define DEBUG(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ")" << endl;

template <class T> ostream &operator<<(ostream &os, const vector<T> &v) {
  os << "[";
  REP(i, SZ(v)) {
    if (i)
      os << ", ";
    os << v[i];
  }
  return os << "]";
}

template <class T, class U>
ostream &operator<<(ostream &os, const pair<T, U> &p) {
  return os << "(" << p.first << " " << p.second << ")";
}

template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}

template <class T> bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return true;
  }
  return false;
}

using ll = long long;
using ull = unsigned long long;
using ld = long double;
using P = pair<int, int>;
using vi = vector<int>;
using vll = vector<ll>;
using vvi = vector<vi>;
using vvll = vector<vll>;

const ll MOD = 1e9 + 7;
const int INF = INT_MAX / 2;
const ll LINF = LLONG_MAX / 2;
const ld eps = 1e-9;

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(10);

  ll n, x, m;
  cin >> n >> x >> m;
  ll orx = x;

  ll ans = 0;
  ll cnt = 0;
  vector<int> v(m, -1);
  if (x < m)
    v[x] = 0;
  REP(i, n) {
    ans += x;
    x %= m;
    x = x * x % m;
    if (x == 0) {
      cout << ans << endl;
      return 0;
    }
    // cout << x << endl;
    if (v[x] != -1)
      break;
    cnt++;
    v[x] = cnt;
  }
  if (cnt == n) {
    cout << ans << endl;
    return 0;
  }
  // cout << cnt << endl;
  // cout << x << endl;

  ll be = v[x], ed = cnt + 1;
  // cout << be << " " << ed << endl;
  ll su = 0;
  ll tmpx = x;
  while (1) {
    su += x;
    x = x * x % m;
    if (x == tmpx)
      break;
  }
  ans -= su;

  // cout << ans << ":" << su << endl;

  ll add = 0;
  x = tmpx;
  REP(i, (n - be) % (ed - be)) {
    add += x;
    x = x * x % m;
  }
  cout << ans + su * ((n - be) / (ed - be)) + add << endl;

  return 0;
}


Error: 0

Fixed submission:

#include <bits/stdc++.h>

using namespace std;

#define REP(i, n) for (int i = 0; i < (n); ++i)
#define FOR(i, a, n) for (int i = (a); i < (n); ++i)

#define SZ(x) ((int)(x).size())
#define ALL(x) (x).begin(), (x).end()

#define DUMP(x) cerr << #x << " = " << (x) << endl
#define DEBUG(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ")" << endl;

template <class T> ostream &operator<<(ostream &os, const vector<T> &v) {
  os << "[";
  REP(i, SZ(v)) {
    if (i)
      os << ", ";
    os << v[i];
  }
  return os << "]";
}

template <class T, class U>
ostream &operator<<(ostream &os, const pair<T, U> &p) {
  return os << "(" << p.first << " " << p.second << ")";
}

template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}

template <class T> bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return true;
  }
  return false;
}

using ll = long long;
using ull = unsigned long long;
using ld = long double;
using P = pair<int, int>;
using vi = vector<int>;
using vll = vector<ll>;
using vvi = vector<vi>;
using vvll = vector<vll>;

const ll MOD = 1e9 + 7;
const int INF = INT_MAX / 2;
const ll LINF = LLONG_MAX / 2;
const ld eps = 1e-9;

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(10);

  ll n, x, m;
  cin >> n >> x >> m;
  ll orx = x;

  ll ans = 0;
  ll cnt = 0;
  vector<int> v(m, -1);
  if (x < m)
    v[x] = 0;
  ll prev = x;
  REP(i, n) {
    ans += x;
    x %= m;
    x = x * x % m;
    if (x == 0) {
      cout << ans << endl;
      return 0;
    }
    /*
    if(x == prev) {
      cout << ans + (n-cnt) * x << endl;
      return 0;
    }
     */
    prev = x;
    // cout << x << endl;
    cnt++;
    if (v[x] != -1)
      break;
    v[x] = cnt;
  }
  if (v[x] == -1) {
    cout << ans << endl;
    return 0;
  }
  // cout << cnt << endl;
  // cout << x << endl;

  ll be = v[x], ed = cnt;
  // cout << be << " " << ed << endl;
  ll su = 0;
  ll tmpx = x;
  while (1) {
    su += x;
    x = x * x % m;
    if (x == tmpx)
      break;
  }
  ans -= su;

  // cout << ans << ":" << su << endl;

  ll add = 0;
  x = tmpx;
  REP(i, (n - be) % (ed - be)) {
    add += x;
    x = x * x % m;
  }
  cout << ans + su * ((n - be) / (ed - be)) + add << endl;

  return 0;
}