Problem p02550 (C++)

Buggy submission (status: Runtime Error):
/*بِسْمِ اللَّهِ الرَّحْمَنِ الرَّحِيم*/

// #pragma GCC optimize("O3,unroll-loops")
// #pragma GCC target("avx,avx2,fma")

#include <bits/stdc++.h>
using namespace std;
#define FASTIO ios_base::sync_with_stdio(false), cin.tie(0), cout.tie(0);
typedef long long ll;
using pii = pair<int, int>;
const double PI = acos(-1.0);
const ll mod = 1e9 + 7;
// const ll mod = 998244353;

inline void normal(ll &a) {
  a %= mod;
  (a < 0) && (a += mod);
}
inline ll modMul(ll a, ll b) {
  a %= mod, b %= mod;
  normal(a), normal(b);
  return (a * b) % mod;
}
inline ll modAdd(ll a, ll b) {
  a %= mod, b %= mod;
  normal(a), normal(b);
  return (a + b) % mod;
}
inline ll modSub(ll a, ll b) {
  a %= mod, b %= mod;
  normal(a), normal(b);
  a -= b;
  normal(a);
  return a;
}
inline ll modPow(ll b, ll p) {
  ll r = 1;
  while (p) {
    if (p & 1)
      r = modMul(r, b);
    b = modMul(b, b);
    p >>= 1;
  }
  return r;
}
inline ll modInverse(ll a) { return modPow(a, mod - 2); }
inline ll modDiv(ll a, ll b) { return modMul(a, modInverse(b)); }

#define si(x) scanf("%d", &x)
#define sii(x, y) scanf("%d %d", &x, &y)
#define siii(x, y, z) scanf("%d %d %d", &x, &y, &z)
#define sl(x) scanf("%lld", &x)
#define sll(x, y) scanf("%lld %lld", &x, &y)
#define slll(x, y, z) scanf("%lld %lld %lld", &x, &y, &z)
#define ss(ch) scanf("%s", ch)
#define pi(x) printf("%d", x)
#define pii(x, y) printf("%d %d", x, y)
#define piii(x, y, z) printf("%d %d %d", x, y, z)
#define pl(x) printf("%lld", x)
#define pll(x, y) printf("%lld %lld", x, y)
#define plll(x, y, z) printf("%lld %lld %lld", x, y, z)
#define ps(ch) printf("%s", ch)
#define F(i, a, b) for (int i = a; i <= b; i++)
#define R(i, b, a) for (int i = b; i >= a; i--)
#define REP(i, n) for (int i = 0; i < (n); i++)

int dx[] = {1, -1, 0, 0};
int dy[] = {0, 0, 1, -1};
int dx8[] = {0, 0, 1, 1, 1, -1, -1, -1};
int dy8[] = {1, -1, -1, 0, 1, -1, 0, 1};
int kx8[] = {1, 1, 2, 2, -1, -1, -2, -2};
int ky8[] = {2, -2, 1, -1, 2, -2, 1, -1};
/* for Random Number generate
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
*/
///**
template <typename F, typename S>
ostream &operator<<(ostream &os, const pair<F, S> &p) {
  return os << "(" << p.first << ", " << p.second << ")";
}
template <typename T> ostream &operator<<(ostream &os, const vector<T> &v) {
  os << "{";
  for (auto it = v.begin(); it != v.end(); ++it) {
    if (it != v.begin())
      os << ", ";
    os << *it;
  }
  return os << "}";
}
template <typename T> ostream &operator<<(ostream &os, const set<T> &v) {
  os << "[";
  for (auto it = v.begin(); it != v.end(); ++it) {
    if (it != v.begin())
      os << ", ";
    os << *it;
  }
  return os << "]";
}
template <typename F, typename S>
ostream &operator<<(ostream &os, const map<F, S> &v) {
  os << "[";
  for (auto it = v.begin(); it != v.end(); ++it) {
    if (it != v.begin())
      os << ", ";
    os << it->first << " = " << it->second;
  }
  return os << "]";
}
#define dbg(args...)                                                           \
  do {                                                                         \
    cerr << #args << " : ";                                                    \
    faltu(args);                                                               \
  } while (0)
clock_t tStart = clock();
#define timeStamp                                                              \
  dbg("Execution Time: ", (double)(clock() - tStart) / CLOCKS_PER_SEC)
void faltu() { cerr << endl; }
template <typename T> void faltu(T a[], int n) {
  for (int i = 0; i < n; ++i)
    cerr << a[i] << ' ';
  cerr << endl;
}
template <typename T, typename... hello>
void faltu(T arg, const hello &...rest) {
  cerr << arg << ' ';
  faltu(rest...);
}

// Program showing a policy-based data structure.
#include <ext/pb_ds/assoc_container.hpp> // Common file
#include <ext/pb_ds/tree_policy.hpp>
#include <functional> // for less
using namespace __gnu_pbds;

// GNU link : https://goo.gl/WVDL6g
typedef tree<int, null_type, less_equal<int>, rb_tree_tag,
             tree_order_statistics_node_update>
    new_data_set;
// find_by_order(k) – ফাংশনটি kth ordered element এর একটা পয়েন্টার রিটার্ন করে।
// অর্থাৎ তুমি চাইলেই kth ইন্ডেক্সে কি আছে, সেটা জেনে ফেলতে পারছো! order_of_key(x) –
// ফাংশনটি x এলিমেন্টটা কোন পজিশনে আছে সেটা বলে দেয়।
//*//**___________________________________________________**/
const int N = 1000006;
ll cycle_len, cycle_sm;
vector<int> g[N];
bitset<N> vis;

int main() {
  FASTIO
  /*
#ifndef ONLINE_JUDGE
  freopen("in.txt", "r", stdin);
  freopen("out.txt", "w", stdout);
  freopen("error.txt", "w", stderr);
#endif
//*/
  ll n, m, x;
  cin >> n >> x >> m;
  // dbg(n,x,m);
  for (int i = 0; i < m; i++) {
    ll j = (i * i) % m;
    // dbg(i,j);
    g[i].push_back(j);
  }
  ll ans = 0;
  while (n > 0) {
    if (vis[x])
      break;
    ans += x;
    vis[x] = 1;
    x = g[x][0];
    n--;
  }
  // dbg(n);
  if (n > 0) {
    vis = 0;
    ll y = x;
    while (!vis[y]) {
      cycle_len++;
      cycle_sm += y;
      vis[y] = 1;
      y = g[y][0];
    }

    ll d = n / cycle_len, r = n % cycle_len;
    // dbg(r);
    ans += (d * cycle_sm);
    for (ll i = 0; i < r; i++) {
      ans += x;
      x = g[x][0];
    }
  }
  cout << ans << "\n";
  return 0;
}

Fixed submission:
/*بِسْمِ اللَّهِ الرَّحْمَنِ الرَّحِيم*/

// #pragma GCC optimize("O3,unroll-loops")
// #pragma GCC target("avx,avx2,fma")

#include <bits/stdc++.h>
using namespace std;
#define FASTIO ios_base::sync_with_stdio(false), cin.tie(0), cout.tie(0);
typedef long long ll;
using pii = pair<int, int>;
const double PI = acos(-1.0);
const ll mod = 1e9 + 7;
// const ll mod = 998244353;

inline void normal(ll &a) {
  a %= mod;
  (a < 0) && (a += mod);
}
inline ll modMul(ll a, ll b) {
  a %= mod, b %= mod;
  normal(a), normal(b);
  return (a * b) % mod;
}
inline ll modAdd(ll a, ll b) {
  a %= mod, b %= mod;
  normal(a), normal(b);
  return (a + b) % mod;
}
inline ll modSub(ll a, ll b) {
  a %= mod, b %= mod;
  normal(a), normal(b);
  a -= b;
  normal(a);
  return a;
}
inline ll modPow(ll b, ll p) {
  ll r = 1;
  while (p) {
    if (p & 1)
      r = modMul(r, b);
    b = modMul(b, b);
    p >>= 1;
  }
  return r;
}
inline ll modInverse(ll a) { return modPow(a, mod - 2); }
inline ll modDiv(ll a, ll b) { return modMul(a, modInverse(b)); }

#define si(x) scanf("%d", &x)
#define sii(x, y) scanf("%d %d", &x, &y)
#define siii(x, y, z) scanf("%d %d %d", &x, &y, &z)
#define sl(x) scanf("%lld", &x)
#define sll(x, y) scanf("%lld %lld", &x, &y)
#define slll(x, y, z) scanf("%lld %lld %lld", &x, &y, &z)
#define ss(ch) scanf("%s", ch)
#define pi(x) printf("%d", x)
#define pii(x, y) printf("%d %d", x, y)
#define piii(x, y, z) printf("%d %d %d", x, y, z)
#define pl(x) printf("%lld", x)
#define pll(x, y) printf("%lld %lld", x, y)
#define plll(x, y, z) printf("%lld %lld %lld", x, y, z)
#define ps(ch) printf("%s", ch)
#define F(i, a, b) for (int i = a; i <= b; i++)
#define R(i, b, a) for (int i = b; i >= a; i--)
#define REP(i, n) for (int i = 0; i < (n); i++)

int dx[] = {1, -1, 0, 0};
int dy[] = {0, 0, 1, -1};
int dx8[] = {0, 0, 1, 1, 1, -1, -1, -1};
int dy8[] = {1, -1, -1, 0, 1, -1, 0, 1};
int kx8[] = {1, 1, 2, 2, -1, -1, -2, -2};
int ky8[] = {2, -2, 1, -1, 2, -2, 1, -1};
/* for Random Number generate
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
*/
///**
template <typename F, typename S>
ostream &operator<<(ostream &os, const pair<F, S> &p) {
  return os << "(" << p.first << ", " << p.second << ")";
}
template <typename T> ostream &operator<<(ostream &os, const vector<T> &v) {
  os << "{";
  for (auto it = v.begin(); it != v.end(); ++it) {
    if (it != v.begin())
      os << ", ";
    os << *it;
  }
  return os << "}";
}
template <typename T> ostream &operator<<(ostream &os, const set<T> &v) {
  os << "[";
  for (auto it = v.begin(); it != v.end(); ++it) {
    if (it != v.begin())
      os << ", ";
    os << *it;
  }
  return os << "]";
}
template <typename F, typename S>
ostream &operator<<(ostream &os, const map<F, S> &v) {
  os << "[";
  for (auto it = v.begin(); it != v.end(); ++it) {
    if (it != v.begin())
      os << ", ";
    os << it->first << " = " << it->second;
  }
  return os << "]";
}
#define dbg(args...)                                                           \
  do {                                                                         \
    cerr << #args << " : ";                                                    \
    faltu(args);                                                               \
  } while (0)
clock_t tStart = clock();
#define timeStamp                                                              \
  dbg("Execution Time: ", (double)(clock() - tStart) / CLOCKS_PER_SEC)
void faltu() { cerr << endl; }
template <typename T> void faltu(T a[], int n) {
  for (int i = 0; i < n; ++i)
    cerr << a[i] << ' ';
  cerr << endl;
}
template <typename T, typename... hello>
void faltu(T arg, const hello &...rest) {
  cerr << arg << ' ';
  faltu(rest...);
}

// Program showing a policy-based data structure.
#include <ext/pb_ds/assoc_container.hpp> // Common file
#include <ext/pb_ds/tree_policy.hpp>
#include <functional> // for less
using namespace __gnu_pbds;

// GNU link : https://goo.gl/WVDL6g
typedef tree<int, null_type, less_equal<int>, rb_tree_tag,
             tree_order_statistics_node_update>
    new_data_set;
// find_by_order(k) – ফাংশনটি kth ordered element এর একটা পয়েন্টার রিটার্ন করে।
// অর্থাৎ তুমি চাইলেই kth ইন্ডেক্সে কি আছে, সেটা জেনে ফেলতে পারছো! order_of_key(x) –
// ফাংশনটি x এলিমেন্টটা কোন পজিশনে আছে সেটা বলে দেয়।
//*//**___________________________________________________**/
const int N = 1000006;
ll cycle_len, cycle_sm;
vector<int> g[N];
bitset<N> vis;

int main() {
  FASTIO
  /*
#ifndef ONLINE_JUDGE
  freopen("in.txt", "r", stdin);
  freopen("out.txt", "w", stdout);
  freopen("error.txt", "w", stderr);
#endif
//*/
  ll n, m, x;
  cin >> n >> x >> m;
  // dbg(n,x,m);
  for (ll i = 0; i < m; i++) { // int i lekhar jonno RTE khaisi
    ll j = (i * i) % m;
    // dbg(i,j);
    g[i].push_back(j);
  }
  ll ans = 0;
  while (n > 0) {
    if (vis[x])
      break;
    ans += x;
    vis[x] = 1;
    x = g[x][0];
    n--;
  }
  // dbg(n);
  if (n > 0) {
    vis = 0;
    ll y = x;
    while (!vis[y]) {
      cycle_len++;
      cycle_sm += y;
      vis[y] = 1;
      y = g[y][0];
    }

    ll d = n / cycle_len, r = n % cycle_len;
    // dbg(r);
    ans += (d * cycle_sm);
    for (ll i = 0; i < r; i++) {
      ans += x;
      x = g[x][0];
    }
  }
  cout << ans << "\n";
  return 0;
}