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

#include <bits/stdc++.h>
// #include <ext/pb_ds/assoc_container.hpp>
// #include <ext/pb_ds/tree_policy.hpp>
using namespace std;
// #pragma GCC optimize("Ofast,no-stack-protector,unroll-loops,fast-math,O3")
// #pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
// using namespace __gnu_pbds;
#define fastio()                                                               \\
  ios_base::sync_with_stdio(0);                                                \\
  cin.tie(0);                                                                  \\
  cout.tie(0)
#define pb push_back
#define show(x) cout << (#x) << " : " << x << endl;
#define ll long long
#define ld long double
#define fill(a, val) memset(a, val, sizeof(a))
#define mp make_pair
#define ff first
#define ss second
#define pii pair<ll, ll>
#define sq(x) ((x) * (x))
#define all(v) v.begin(), v.end()
#define rall(v) v.rbegin(), v.rend()
#define endl "\\n"
#define int long long
#define printclock                                                             \\
  cerr << "Time : " << 1000 * (ld)clock() / (ld)CLOCKS_PER_SEC << "ms\\n";
const ll MOD = 1000 * 1000 * 1000 + 7;
const ll INF = 1ll * 1000 * 1000 * 1000 * 1000 * 1000 * 1000 + 7;
const ll MOD2 = 998244353;
const ll N = 1000 * 100 + 10;
const ll N2 = 70;
const ld PI = 3.141592653589793;
// template<class T> using oset=tree<T, null_type, less<T>, rb_tree_tag,
// tree_order_statistics_node_update>;
ll gcd(ll a, ll b) {
  if (!b)
    return a;
  return gcd(b, a % b);
}
ll power(ll x, ll y, ll p = LLONG_MAX) {
  ll res = 1;
  x %= p;
  while (y > 0) {
    if (y & 1)
      res = (res * x) % p;
    y = y >> 1;
    x = (x * x) % p;
  }
  return res;
}
ll lcm(ll a, ll b) { return (a * b) / gcd(a, b); }

signed main() {
  fastio();
// cout<<fixed<<setprecision(20);
// CHECK for LONG LONG and LONG DOUBLE
//*comment for all except cc/cf
#ifndef ONLINE_JUDGE
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
#endif //*/
  int n, x, m;
  cin >> n >> x >> m;
  map<int, int> store;
  std::vector<int> v(1e6);
  int c = 1;
  v[0] = x;
  store[x] = 0;
  int rep = -1;
  bool ok = true;
  for (int i = 1; i < n; ++i) {
    int temp = sq(v[i - 1]);
    temp %= m;
    v[i] = temp;
    if (!temp) {
      ok = false;
      break;
    }
    if (store.find(temp) == store.end()) {
      store[temp] = i;
    } else {
      rep = store[temp];
      break;
    }
    c++;
  }

  for (int i = 1; i < c; i++)
    v[i] += v[i - 1];

  if (!ok || rep == -1) {
    cout << v[c - 1];
    exit(0);
  }

  int ans = v[c - 1];
  n -= c;

  int len = c - rep;
  int req = n / (len);
  int tempsum = v[c - 1];
  if (rep)
    tempsum -= v[rep - 1];

  ans += (req * tempsum);

  int left = n % len;

  if ((rep + left) > 1)
    ans += v[rep + left - 1];
  if (rep)
    ans -= v[rep - 1];

  cout << ans;

  printclock;
  return 0;
}


Error: WA

Fixed submission:

#include <bits/stdc++.h>
// #include <ext/pb_ds/assoc_container.hpp>
// #include <ext/pb_ds/tree_policy.hpp>
using namespace std;
// #pragma GCC optimize("Ofast,no-stack-protector,unroll-loops,fast-math,O3")
// #pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
// using namespace __gnu_pbds;
#define fastio()                                                               \\
  ios_base::sync_with_stdio(0);                                                \\
  cin.tie(0);                                                                  \\
  cout.tie(0)
#define pb push_back
#define show(x) cout << (#x) << " : " << x << endl;
#define ll long long
#define ld long double
#define fill(a, val) memset(a, val, sizeof(a))
#define mp make_pair
#define ff first
#define ss second
#define pii pair<ll, ll>
#define sq(x) ((x) * (x))
#define all(v) v.begin(), v.end()
#define rall(v) v.rbegin(), v.rend()
#define endl "\\n"
#define int long long
#define printclock                                                             \\
  cerr << "Time : " << 1000 * (ld)clock() / (ld)CLOCKS_PER_SEC << "ms\\n";
const ll MOD = 1000 * 1000 * 1000 + 7;
const ll INF = 1ll * 1000 * 1000 * 1000 * 1000 * 1000 * 1000 + 7;
const ll MOD2 = 998244353;
const ll N = 1000 * 100 + 10;
const ll N2 = 70;
const ld PI = 3.141592653589793;
// template<class T> using oset=tree<T, null_type, less<T>, rb_tree_tag,
// tree_order_statistics_node_update>;
ll gcd(ll a, ll b) {
  if (!b)
    return a;
  return gcd(b, a % b);
}
ll power(ll x, ll y, ll p = LLONG_MAX) {
  ll res = 1;
  x %= p;
  while (y > 0) {
    if (y & 1)
      res = (res * x) % p;
    y = y >> 1;
    x = (x * x) % p;
  }
  return res;
}
ll lcm(ll a, ll b) { return (a * b) / gcd(a, b); }

signed main() {
  fastio();
// cout<<fixed<<setprecision(20);
// CHECK for LONG LONG and LONG DOUBLE
//*comment for all except cc/cf
#ifndef ONLINE_JUDGE
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
#endif //*/
  int n, x, m;
  cin >> n >> x >> m;
  map<int, int> store;
  std::vector<int> v(m + 1);
  int c = 1;
  v[0] = x;
  store[x]++;
  int rep = -1;
  bool ok = true;
  for (int i = 1; i < n; ++i) {
    int temp = sq(v[i - 1]);
    temp %= m;
    v[i] = temp;
    if (!temp) {
      ok = false;
      break;
    }
    if (store.find(temp) == store.end()) {
      store[temp] = i;
    } else {
      rep = store[temp];
      break;
    }
    c++;
  }

  for (int i = 1; i < c; i++)
    v[i] += v[i - 1];

  if (!ok || rep == -1) {
    cout << v[c - 1];
    exit(0);
  }
  int ans = v[c - 1];
  n -= c;
  int len = c - rep;
  int req = n / (len);
  int tempsum = v[c - 1];
  if (rep)
    tempsum -= v[rep - 1];

  ans += (req * tempsum);

  int left = n % len;

  ans += v[rep + left - 1];
  if (rep)
    ans -= v[rep - 1];
  cout << ans;

  printclock;
  return 0;
}