Problem p02550 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
// #include <atcoder/all>
using namespace std;
// using namespace atcoder;
#define ll long long
#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define rep(i, n) FOR(i, 0, n)
#define rep1(i, n) FOR(i, 1, n + 1)
#define rrep(i, n) for (int i = ((int)(n)-1); i >= 0; --i)
#define whole(x) (x).begin(), (x).end()
#define rwhole(x) (x).rbegin(), (x).rend()
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end())
#define P pair<ll, ll>
#define debug(var) cerr << "[" << #var << "] " << var << endl
template <typename T1, typename T2> bool chmax(T1 &a, T2 b) {
  if (a < b) {
    a = b;
    return true;
  } else
    return false;
}
template <typename T1, typename T2> bool chmin(T1 &a, T2 b) {
  if (a > b) {
    a = b;
    return true;
  } else
    return false;
}
#define vi vector<int>
#define vl vector<ll>
#define pb push_back
#define eb emplace_back
#define fi first
#define se second
#define pr(s) cout << (s) << '\n'
const ll mod = 1000000007;
const int dx[] = {-1, 0, 1, 0};
const int dy[] = {0, -1, 0, 1};
const int INF = 1001001001;
const ll INFll = 1E+18;

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);

  ll n, x, m;
  cin >> n >> x >> m;
  ll ans = 0;
  vector<P> memo(m, P(-1, -1));
  for (ll i = 0; i < n; i++) {
    if (memo[x].second > 0) {
      ll loop = i - memo[x].first;
      // debug(loop);
      ll sum_loop = ans - memo[x].second + x;
      ll num = (n - i - 1) / loop;
      ans += sum_loop * num;
      i += loop * num;
    } else {
      memo[x] = {i, ans + x};
    }
    ans += x;
    x = (x * x) % m;
    if (x == 0)
      return -1;
  }

  pr(ans);
  return 0;
}


Fixed submission:
#include <bits/stdc++.h>
// #include <atcoder/all>
using namespace std;
// using namespace atcoder;
#define ll long long
#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define rep(i, n) FOR(i, 0, n)
#define rep1(i, n) FOR(i, 1, n + 1)
#define rrep(i, n) for (int i = ((int)(n)-1); i >= 0; --i)
#define whole(x) (x).begin(), (x).end()
#define rwhole(x) (x).rbegin(), (x).rend()
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end())
#define P pair<ll, ll>
#define debug(var) cerr << "[" << #var << "] " << var << endl
template <typename T1, typename T2> bool chmax(T1 &a, T2 b) {
  if (a < b) {
    a = b;
    return true;
  } else
    return false;
}
template <typename T1, typename T2> bool chmin(T1 &a, T2 b) {
  if (a > b) {
    a = b;
    return true;
  } else
    return false;
}
#define vi vector<int>
#define vl vector<ll>
#define pb push_back
#define eb emplace_back
#define fi first
#define se second
#define pr(s) cout << (s) << '\n'
const ll mod = 1000000007;
const int dx[] = {-1, 0, 1, 0};
const int dy[] = {0, -1, 0, 1};
const int INF = 1001001001;
const ll INFll = 1E+18;

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);

  ll n, x, m;
  cin >> n >> x >> m;
  ll ans = 0;
  vector<P> memo(m, P(-1, -1));
  for (ll i = 0; i < n; i++) {
    if (memo[x].second > 0) {
      ll loop = i - memo[x].first;
      // debug(loop);
      ll sum_loop = ans - memo[x].second + x;
      ll num = (n - i - 1) / loop;
      ans += sum_loop * num;
      i += loop * num;
    } else {
      memo[x] = {i, ans + x};
    }
    ans += x;
    x = (x * x) % m;
    if (x == 0)
      break;
  }

  pr(ans);
  return 0;
}