Problem p02591 (C++) — buggy submission, verdict: Time Limit Exceeded

// And you curse yourself for things you never done

#include <bits/stdc++.h>

#define F first
#define S second
#define PB push_back
#define sz(s) int((s).size())
#define bit(n, k) (((n) >> (k)) & 1)

using namespace std;

typedef long long ll;
typedef pair<int, int> pii;

const int maxn = 3e5 + 10, mod = 1e9 + 7, inf = 1e9 + 10;

vector<pii> v[maxn];
int cof[maxn], iv[maxn], p[maxn];

int Pow(int a, int b) {
  int ans = 1;
  for (; b; b >>= 1, a = 1ll * a * a % mod)
    if (b & 1)
      ans = 1ll * ans * a % mod;
  return ans;
}

int cnt[maxn];

int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie();

  iv[0] = iv[1] = 1;
  cof[1] = 1;
  for (int i = 2; i < maxn; i++) {
    cof[i] = 1ll * cof[i >> 1] * i % mod;
    iv[i] = Pow(cof[i], mod - 2);
  }

  int n;
  cin >> n;
  --n;
  int L = 1 << n;
  for (int i = 0; i < L; i++) {
    cin >> p[i];
    --p[i];
  }
  int ans = 0;
  for (int a = 0; a < n; a++) {
    for (int b = 0; b < n; b++) {
      for (int i = 0; i < L; i++) {
        int A = L + i, B = L + p[i];
        int num = 1ll * cof[A] * cof[B] % mod;
        v[A >> a].PB({B >> b, num});
      }
    }
  }
  for (int A = 1; A < maxn - 5; A++) {
    if (v[A].empty() || v[A ^ 1].empty())
      continue;
    for (auto [B, x] : v[A]) {
      cnt[B] = (cnt[B] + x) % mod;
    }
    for (auto [B, x] : v[A ^ 1]) {
      B ^= 1;
      int lc1 = A >> 1, lc2 = B >> 1;
      ans = (1ll * ans + 1ll * cnt[B] * x % mod * iv[lc1] % mod * iv[lc2] %
                             mod * iv[lc1 >> 1] % mod * iv[lc2 >> 1] % mod) %
            mod;
    }
    for (auto [B, x] : v[A]) {
      cnt[B] = 0;
    }
  }
  ans = 1ll * ans * ((mod + 1) / 2) % mod;
  return cout << ans << endl, 0;
}


Error: TLE

Fixed submission:

// And you curse yourself for things you never done

#include <bits/stdc++.h>

#define F first
#define S second
#define PB push_back
#define sz(s) int((s).size())
#define bit(n, k) (((n) >> (k)) & 1)

using namespace std;

typedef long long ll;
typedef pair<int, int> pii;

const int maxn = 3e5 + 10, mod = 1e9 + 7, inf = 1e9 + 10;

struct Hash {
  int operator()(const pii &p) const {
    return hash<ll>()(((1ll * p.F) << 32) | p.S);
  }
};

unordered_map<pii, int, Hash> mp;
int cof[maxn], iv[maxn], p[maxn];

int Pow(int a, int b) {
  int ans = 1;
  for (; b; b >>= 1, a = 1ll * a * a % mod)
    if (b & 1)
      ans = 1ll * ans * a % mod;
  return ans;
}

int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie();

  mp.reserve(1 << 17);
  mp.max_load_factor(0.25);

  iv[0] = iv[1] = 1;
  cof[1] = 1;
  for (int i = 2; i < maxn; i++) {
    cof[i] = 1ll * cof[i >> 1] * i % mod;
    iv[i] = Pow(cof[i], mod - 2);
  }

  int n;
  cin >> n;
  --n;
  int L = 1 << n;
  for (int i = 0; i < L; i++) {
    cin >> p[i];
    --p[i];
  }
  int ans = 0;
  for (int a = 0; a < n; a++) {
    for (int b = 0; b < n; b++) {
      mp.clear();
      for (int i = 0; i < L; i++) {
        int A = L + i, B = L + p[i];
        int num = 1ll * cof[A] * cof[B] % mod;
        int &x = mp[{A >> a, B >> b}];
        x = (x + num) % mod;
      }
      for (auto it : mp) {
        if (mp.count({it.F.F ^ 1, it.F.S ^ 1})) {
          int lc1 = it.F.F >> 1, lc2 = it.F.S >> 1;
          ans = (1ll * ans + 1ll * it.S * mp[{it.F.F ^ 1, it.F.S ^ 1}] % mod *
                                 iv[lc1] % mod * iv[lc2] % mod * iv[lc1 >> 1] %
                                 mod * iv[lc2 >> 1] % mod) %
                mod;
        }
      }
    }
  }
  ans = 1ll * ans * ((mod + 1) / 2) % mod;
  return cout << ans << endl, 0;
}