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

#include <bits/stdc++.h>
using namespace std;

#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;

template <typename T>
using oset =
    tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;

// macros
#define int long long
#define ll long long
#define ld long double
#define TIME clock() * 1.0 / CLOCKS_PER_SEC
#define sz(x) ((int)((x).size()))

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(1, s)); }

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

string to_string(bool b) { return (b ? "1" : "0"); }

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 += "}";
  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 += "}";
  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)) + ")";
}

void debug_out() { cerr << endl; }

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

#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)

template <typename T1, typename T2> inline auto mini(T1 a, T2 b) {
  return (a < b ? a : b);
}
template <typename T, typename... Args> inline auto mini(T a, Args... args) {
  return mini(a, mini(args...));
}
template <typename T1, typename T2> inline auto maxi(T1 a, T2 b) {
  return (a > b ? a : b);
}
template <typename T, typename... Args> inline auto maxi(T a, Args... args) {
  return maxi(a, maxi(args...));
}
template <typename T> T gcd(T a, T b) {
  if (a == 0 or b == 0)
    return a + b;
  return gcd(b, a % b);
}
template <typename T> T lcm(T a, T b) {
  if (a == 0 or b == 0)
    return 0;
  return a / gcd(a, b) * b;
}

// random number generator
mt19937 rng(chrono::steady_clock::now()
                .time_since_epoch()
                .count()); // use mt19937_64 for 64 bit

// constants
const long double eps = LDBL_EPSILON;
const int inf = 1e15;
const int modn = 1e9 + 7;
const int maxn = 2e5 + 3;

int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  // Your code here
  int n, x, m;
  cin >> n >> x >> m;
  if (x == 0) {
    cout << "0\\n";
    return 0;
  }
  if (x == 1) {
    cout << n << "\\n";
    return 0;
  }
  vector<int> timv(m, -1);
  x %= m;
  int orx = x;
  timv[x] = 0;
  int tim = 1;
  int t1 = 0, t2 = 0;
  bool flag = 0;
  for (int i = 0; i < n; i++) {
    x = (x * x) % m;
    if (timv[x] == -1) {
      timv[x] = tim++;
    } else {
      t2 = tim - timv[x];
      t1 = timv[x];
      flag = 1;
      break;
    }
  }
  if (flag == 0) {
    int ans = 0;
    for (int i = 0; i < n; i++) {
      ans += orx;
      orx = (orx * orx) % m;
    }
    cout << ans << "\\n";
    return 0;
  }
  debug(t1, t2);
  int ans = 0;
  int curx = orx;
  for (int i = 0; i < t1; i++) {
    ans += curx;
    curx = (curx * curx) % m;
  }
  int cur2x = curx;
  n -= t1;
  int no = n / t2;
  for (int i = 0; i < t2; i++) {
    ans += curx * no;
    curx = (curx * curx) % m;
  }
  int rem = n % t2;
  for (int i = 0; i < rem; i++) {
    ans += cur2x;
    cur2x = (cur2x * cur2x) % m;
  }
  cout << ans << "\\n";
  return 0;
}

Error: 0
[t1, t2]: 2 4


Fixed submission:

#include <bits/stdc++.h>
using namespace std;

#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;

template <typename T>
using oset =
    tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;

// macros
#define int long long
#define ll long long
#define ld long double
#define TIME clock() * 1.0 / CLOCKS_PER_SEC
#define sz(x) ((int)((x).size()))

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(1, s)); }

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

string to_string(bool b) { return (b ? "1" : "0"); }

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 += "}";
  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 += "}";
  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)) + ")";
}

void debug_out() { cerr << endl; }

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

#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)

template <typename T1, typename T2> inline auto mini(T1 a, T2 b) {
  return (a < b ? a : b);
}
template <typename T, typename... Args> inline auto mini(T a, Args... args) {
  return mini(a, mini(args...));
}
template <typename T1, typename T2> inline auto maxi(T1 a, T2 b) {
  return (a > b ? a : b);
}
template <typename T, typename... Args> inline auto maxi(T a, Args... args) {
  return maxi(a, maxi(args...));
}
template <typename T> T gcd(T a, T b) {
  if (a == 0 or b == 0)
    return a + b;
  return gcd(b, a % b);
}
template <typename T> T lcm(T a, T b) {
  if (a == 0 or b == 0)
    return 0;
  return a / gcd(a, b) * b;
}

// random number generator
mt19937 rng(chrono::steady_clock::now()
                .time_since_epoch()
                .count()); // use mt19937_64 for 64 bit

// constants
const long double eps = LDBL_EPSILON;
const int inf = 1e15;
const int modn = 1e9 + 7;
const int maxn = 2e5 + 3;

int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  // Your code here
  int n, x, m;
  cin >> n >> x >> m;
  if (x == 0) {
    cout << "0\\n";
    return 0;
  }
  if (x == 1) {
    cout << n << "\\n";
    return 0;
  }
  vector<int> timv(m, -1);
  x %= m;
  int orx = x;
  timv[x] = 0;
  int tim = 1;
  int t1 = 0, t2 = 0;
  for (int i = 0; i < n; i++) {
    x = (x * x) % m;
    if (timv[x] == -1) {
      timv[x] = tim++;
    } else {
      t2 = tim - timv[x];
      t1 = timv[x];
      break;
    }
  }
  debug(t1, t2);
  int ans = 0;
  int curx = orx;
  for (int i = 0; i < t1; i++) {
    ans += curx;
    curx = (curx * curx) % m;
  }
  int cur2x = curx;
  n -= t1;
  int no = n / t2;
  for (int i = 0; i < t2; i++) {
    ans += curx * no;
    curx = (curx * curx) % m;
  }
  int rem = n % t2;
  for (int i = 0; i < rem; i++) {
    ans += cur2x;
    cur2x = (cur2x * cur2x) % m;
  }
  cout << ans << "\\n";
  return 0;
}