Problem p02550 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define endl '\n'
#define mod 998244353
#define pb push_back
#define ff first
#define ss second
#define con continue
#define ub upper_bound
#define lb lower_bound
#define sum(a) (accumulate((a).begin(), (a).end(), 0ll))
#define mine(a) (*min_element((a).begin(), (a).end()))
#define maxe(a) (*max_element((a).begin(), (a).end()))
#define mini(a) (min_element((a).begin(), (a).end()) - (a).begin())
#define maxi(a) (max_element((a).begin(), (a).end()) - (a).begin())
#define lowb(a, x) (lower_bound((a).begin(), (a).end(), (x)) - (a).begin())
#define uppb(a, x) (upper_bound((a).begin(), (a).end(), (x)) - (a).begin())

const int dx[] = {-1, 0, 1, 0};
const int dy[] = {0, -1, 0, 1};

const int dx8[] = {-1, 0, 1, 0, 1, 1, -1, -1};
const int dy8[] = {0, -1, 0, 1, 1, -1, 1, -1};

unordered_map<ll, ll> m1;
void solve() {
  m1.clear();
  ll n, x, m;
  cin >> n >> x >> m;
  vector<ll> arr1;
  vector<ll> arr2;
  arr1.pb(x);
  arr2.pb(x);
  m1[x] = 0;
  for (ll i = 1; i < n; i++) {
    arr1.pb(0);
    arr2.pb(0);
    arr1[i] = (arr1[i - 1] * arr1[i - 1]) % m;
    arr2[i] = arr2[i - 1] + arr1[i];
    if (m1.count(arr1[i]) != 0) {
      // cout <<"hello" << endl;
      ll len = i - m1[arr1[i]];
      ll sum = arr2[i] - arr2[m1[arr1[i]]];
      ll rem = n - i;
      ll ans = 0;
      ans = arr2[i - 1] + (rem / len) * sum;
      ll x2 = m1[arr1[i]];
      ll y = x + (rem % len) - 1;
      ll x1 = x2 - 1 >= 0 ? arr2[x2 - 1] : 0;
      ans += arr2[y] - x1;
      cout << ans << endl;

      return;
    }
    m1[arr1[i]] = i;
  }

  cout << arr2[n - 1] << endl;
}

int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  solve();
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define endl '\n'
#define mod 998244353
#define pb push_back
#define ff first
#define ss second
#define con continue
#define ub upper_bound
#define lb lower_bound
#define sum(a) (accumulate((a).begin(), (a).end(), 0ll))
#define mine(a) (*min_element((a).begin(), (a).end()))
#define maxe(a) (*max_element((a).begin(), (a).end()))
#define mini(a) (min_element((a).begin(), (a).end()) - (a).begin())
#define maxi(a) (max_element((a).begin(), (a).end()) - (a).begin())
#define lowb(a, x) (lower_bound((a).begin(), (a).end(), (x)) - (a).begin())
#define uppb(a, x) (upper_bound((a).begin(), (a).end(), (x)) - (a).begin())

const int dx[] = {-1, 0, 1, 0};
const int dy[] = {0, -1, 0, 1};

const int dx8[] = {-1, 0, 1, 0, 1, 1, -1, -1};
const int dy8[] = {0, -1, 0, 1, 1, -1, 1, -1};

unordered_map<ll, ll> m1;
void solve() {
  m1.clear();
  ll n, x, m;
  cin >> n >> x >> m;
  vector<ll> arr1;
  vector<ll> arr2;
  arr1.pb(x);
  arr2.pb(x);
  m1[x] = 0;
  for (ll i = 1; i < n; i++) {
    arr1.pb(0);
    arr2.pb(0);
    arr1[i] = (arr1[i - 1] * arr1[i - 1]) % m;
    arr2[i] = arr2[i - 1] + arr1[i];
    if (m1.count(arr1[i]) != 0) {
      // cout <<"hello" << endl;
      ll len = i - m1[arr1[i]];
      ll sum = arr2[i] - arr2[m1[arr1[i]]];
      ll rem = n - i;
      ll ans = 0;
      ans = arr2[i - 1] + (rem / len) * sum;
      ll a = m1[arr1[i]];
      ll b = a + (rem % len) - 1;
      for (ll j = a; j <= b; j++)
        ans += arr1[j];
      cout << ans << endl;

      return;
    }
    m1[arr1[i]] = i;
  }

  cout << arr2[n - 1] << endl;
}

int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  solve();
}