Problem p02550 (C++)

Buggy submission (status: Runtime Error):
#define LOCAL

#include <bits/stdc++.h>

using namespace std;

#define IOS                                                                    \
  ios::sync_with_stdio(false);                                                 \
  cin.tie(0)
#define all(x) x.begin(), x.end()
#define ff first
#define ss second
#define LLINF 0x3f3f3f3f3f3f3f3f
#define INF 0x3f3f3f3f

#define LOCAL

// Copied from Gennady-Korotkevich's template

template <typename A, typename B> string to_string(pair<A, B> p);

template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p);

template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p);

string to_string(const string &s) { return '"' + s + '"'; }

string to_string(const char *s) { return to_string((string)s); }

string to_string(bool b) { return (b ? "true" : "false"); }

string to_string(vector<bool> v) {
  bool first = true;
  string res = "{";
  for (int i = 0; i < static_cast<int>(v.size()); i++) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(v[i]);
  }
  res += "}\n";
  return res;
}

template <size_t N> string to_string(bitset<N> v) {
  string res = "";
  for (size_t i = 0; i < N; i++) {
    res += static_cast<char>('0' + v[i]);
  }
  return res;
}

template <typename A> string to_string(A v) {
  bool first = true;
  string res = "{";
  for (const auto &x : v) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(x);
  }
  res += "}\n";
  return res;
}

template <typename A, typename B> string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}

template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " +
         to_string(get<2>(p)) + ")";
}

template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " +
         to_string(get<2>(p)) + ", " + to_string(get<3>(p)) + ")";
}

template <typename A, typename B, typename C, typename D, typename E>
string to_string(tuple<A, B, C, D, E> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " +
         to_string(get<2>(p)) + ", " + to_string(get<3>(p)) + "," +
         to_string(get<4>(p)) + ")";
}

void debug_out() { cerr << endl; }

template <typename Head, typename... Tail> void debug_out(Head H, Tail... T) {
  cerr << " " << to_string(H);
  debug_out(T...);
}

#ifdef LOCAL
#define debug(...)                                                             \
  cerr << "\n[" << #__VA_ARGS__ << "]:\n", debug_out(__VA_ARGS__)
#else
#define debug(...) 42
#endif

// End of Gennady-Korotkevich's template

using llong = long long;
using PII = pair<int, int>;

const llong MOD = 998244353LL;
const double PI = acos(-1);

int main() {
  IOS;
  llong n, x, m;
  cin >> n >> x >> m;
  vector<llong> pos(m, -1);
  pos[x] = 1;
  llong cur = x;
  vector<llong> seq;
  int cnt = 0;
  seq.push_back(x);
  int s, e;
  while (1) {
    cur *= cur;
    cur %= m;
    if (pos[cur] != -1) {
      s = pos[cur];
      e = cnt;
      break;
    }
    cnt++;
    pos[cur] = cnt;
    seq.push_back(cur);
  }
  vector<llong> init, cycle;
  for (int i = 0; i < s; i++)
    init.push_back(seq[i]);
  for (int i = s; i <= e; i++)
    cycle.push_back(seq[i]);
  if (n <= (int)init.size()) {
    llong sum = 0;
    for (int i = 0; i < n; i++)
      sum += init[i];
    cout << sum;
    return 0;
  } else {
    llong sum = 0;
    for (int i = 0; i < init.size(); i++)
      sum += init[i];
    n -= init.size();
    llong cyclesum = 0;
    for (int i = 0; i < cycle.size(); i++)
      cyclesum += cycle[i];
    sum += (n / cycle.size()) * cyclesum;
    n %= (int)cycle.size();
    for (int i = 0; i < n; i++)
      sum += cycle[i];
    cout << sum;
  }
}

Fixed submission:
#define LOCAL

#include <bits/stdc++.h>

using namespace std;

#define IOS                                                                    \
  ios::sync_with_stdio(false);                                                 \
  cin.tie(0)
#define all(x) x.begin(), x.end()
#define ff first
#define ss second
#define LLINF 0x3f3f3f3f3f3f3f3f
#define INF 0x3f3f3f3f

#define LOCAL

// Copied from Gennady-Korotkevich's template

template <typename A, typename B> string to_string(pair<A, B> p);

template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p);

template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p);

string to_string(const string &s) { return '"' + s + '"'; }

string to_string(const char *s) { return to_string((string)s); }

string to_string(bool b) { return (b ? "true" : "false"); }

string to_string(vector<bool> v) {
  bool first = true;
  string res = "{";
  for (int i = 0; i < static_cast<int>(v.size()); i++) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(v[i]);
  }
  res += "}\n";
  return res;
}

template <size_t N> string to_string(bitset<N> v) {
  string res = "";
  for (size_t i = 0; i < N; i++) {
    res += static_cast<char>('0' + v[i]);
  }
  return res;
}

template <typename A> string to_string(A v) {
  bool first = true;
  string res = "{";
  for (const auto &x : v) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(x);
  }
  res += "}\n";
  return res;
}

template <typename A, typename B> string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}

template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " +
         to_string(get<2>(p)) + ")";
}

template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " +
         to_string(get<2>(p)) + ", " + to_string(get<3>(p)) + ")";
}

template <typename A, typename B, typename C, typename D, typename E>
string to_string(tuple<A, B, C, D, E> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " +
         to_string(get<2>(p)) + ", " + to_string(get<3>(p)) + "," +
         to_string(get<4>(p)) + ")";
}

void debug_out() { cerr << endl; }

template <typename Head, typename... Tail> void debug_out(Head H, Tail... T) {
  cerr << " " << to_string(H);
  debug_out(T...);
}

#ifdef LOCAL
#define debug(...)                                                             \
  cerr << "\n[" << #__VA_ARGS__ << "]:\n", debug_out(__VA_ARGS__)
#else
#define debug(...) 42
#endif

// End of Gennady-Korotkevich's template

using llong = long long;
using PII = pair<int, int>;

const llong MOD = 998244353LL;
const double PI = acos(-1);

int main() {
  IOS;
  llong n, x, m;
  cin >> n >> x >> m;
  vector<llong> pos(m, -1);
  pos[x] = 0;
  llong cur = x;
  vector<llong> seq;
  int cnt = 0;
  seq.push_back(x);
  int s, e;
  while (1) {
    cur *= cur;
    cur %= m;
    if (pos[cur] != -1) {
      s = pos[cur];
      e = cnt;
      break;
    }
    cnt++;
    pos[cur] = cnt;
    seq.push_back(cur);
  }
  vector<llong> init, cycle;
  for (int i = 0; i < s; i++)
    init.push_back(seq[i]);
  for (int i = s; i <= e; i++)
    cycle.push_back(seq[i]);
  if (n <= (int)init.size()) {
    llong sum = 0;
    for (int i = 0; i < n; i++)
      sum += init[i];
    cout << sum;
    return 0;
  } else {
    llong sum = 0;
    for (int i = 0; i < init.size(); i++)
      sum += init[i];
    n -= init.size();
    llong cyclesum = 0;
    for (int i = 0; i < cycle.size(); i++)
      cyclesum += cycle[i];
    sum += (n / cycle.size()) * cyclesum;
    n %= (int)cycle.size();
    for (int i = 0; i < n; i++)
      sum += cycle[i];
    cout << sum;
  }
}