Problem p02550 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>

using namespace std;

typedef long long int li;
typedef long double ld;
typedef vector<li> vi;
typedef pair<li, li> pi;
#define PB push_back
#define MP make_pair
#define F first
#define S second
#define B begin()
#define E end()
#define f(i, l, r) for (li i = l; i <= r; ++i)
#define fr(i, l, r) for (li i = l; i >= r; --i)
#define u_map unordered_map
#define endl "\n"
#define debug(x) cerr << #x << " = " << x << endl

li fastpow(li base, li exp, li M) {
  li res = 1;
  while (exp > 0) {
    if (exp & 1)
      res = (res * base) % M;

    base = (base * base) % M;
    exp >>= 1;
  }
  return res;
}

void solve() {
  li n, x, M;

  cin >> n >> x >> M;

  if (x == 1) {
    cout << n;
    return;
  }

  if (n == 0) {
    cout << 0;
    return;
  }

  vi v(2e5);

  li ans = 0;

  while (n > 0) {
    ans += x;

    x *= x;

    if (x >= M) {
      n--;
      break;
    }
    n--;
  }

  if (n <= 0) {
    cout << ans;
    return;
  }

  vi freq(2e5, -1);

  li pos = 0;

  while (n > 0) {
    n--;
    x %= M;

    if (freq[x] == -1) {
      ans += x;
      debug(ans);
      freq[x] = pos++;
      v[pos - 1] = x;
    } else {
      n++;
      break;
    }

    x *= x;
    // n--;
  }

  if (n <= 0) {
    cout << ans;
    return;
  }

  debug(ans);

  li sz = pos;
  pos = freq[x];

  li sum = 0, deno = 0;

  f(i, pos, sz - 1) {
    sum += v[i];
    deno++;
  }

  li tp = n / deno;
  li rem = n % deno;

  ans += tp * (sum);

  f(i, pos, pos + rem - 1) { ans += v[i]; }

  // ans+=pre[rem-1];

  cout << ans;
}

int main() {
  // remove this in kickstart

#ifndef ONLINE_JUDGE
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
  freopen("error.txt", "w", stderr);
#endif

  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);

  li t = 1;

  // cin>>t;

  f(i, 1, t) {
    // cout<<"Case #"<<i<<": ";

    solve();
  }

  return 0;
}


Fixed submission:
#include <bits/stdc++.h>

using namespace std;

typedef long long int li;
typedef long double ld;
typedef vector<li> vi;
typedef pair<li, li> pi;
#define PB push_back
#define MP make_pair
#define F first
#define S second
#define B begin()
#define E end()
#define f(i, l, r) for (li i = l; i <= r; ++i)
#define fr(i, l, r) for (li i = l; i >= r; --i)
#define u_map unordered_map
#define endl "\n"
#define debug(x) cerr << #x << " = " << x << endl

li fastpow(li base, li exp, li M) {
  li res = 1;
  while (exp > 0) {
    if (exp & 1)
      res = (res * base) % M;

    base = (base * base) % M;
    exp >>= 1;
  }
  return res;
}

void solve() {
  li n, x, M;

  cin >> n >> x >> M;

  if (x == 1) {
    cout << n;
    return;
  }

  if (x == 0) {
    cout << 0;
    return;
  }

  vi v(2e5);

  li ans = 0;

  while (n > 0) {
    ans += x;

    x *= x;

    if (x >= M) {
      n--;
      break;
    }
    n--;
  }

  if (n <= 0) {
    cout << ans;
    return;
  }

  vi freq(2e5, -1);

  li pos = 0;

  while (n > 0) {
    n--;
    x %= M;

    if (freq[x] == -1) {
      ans += x;
      debug(ans);
      freq[x] = pos++;
      v[pos - 1] = x;
    } else {
      n++;
      break;
    }

    x *= x;
    // n--;
  }

  if (n <= 0) {
    cout << ans;
    return;
  }

  debug(ans);

  li sz = pos;
  pos = freq[x];

  li sum = 0, deno = 0;

  f(i, pos, sz - 1) {
    sum += v[i];
    deno++;
  }

  li tp = n / deno;
  li rem = n % deno;

  ans += tp * (sum);

  f(i, pos, pos + rem - 1) { ans += v[i]; }

  // ans+=pre[rem-1];

  cout << ans;
}

int main() {
  // remove this in kickstart

#ifndef ONLINE_JUDGE
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
  freopen("error.txt", "w", stderr);
#endif

  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);

  li t = 1;

  // cin>>t;

  f(i, 1, t) {
    // cout<<"Case #"<<i<<": ";

    solve();
  }

  return 0;
}