Problem p02798 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
using namespace std;
using lint = long long;
const lint mod = 1e9 + 7;
#define all(x) (x).begin(), (x).end()
#define bitcount(n) __builtin_popcountl((lint)(n))
#define fcout cout << fixed << setprecision(15)
#define highest(x) (63 - __builtin_clzl(x))
#define rep(i, n) for (int i = 0; i < n; i++)
const int inf9 = 1e9;
const lint inf18 = 1e18;
template <class T> inline void YES(T condition) {
  if (condition)
    cout << "YES" << endl;
  else
    cout << "NO" << endl;
}
template <class T> inline void Yes(T condition) {
  if (condition)
    cout << "Yes" << endl;
  else
    cout << "No" << endl;
}
template <class T = string, class U = char>
int character_count(T text, U character) {
  int ans = 0;
  for (U i : text) {
    ans += (i == character);
  }
  return ans;
}
lint power(lint base, lint exponent, lint module) {
  if (exponent % 2) {
    return power(base, exponent - 1, module) * base % module;
  } else if (exponent) {
    lint root_ans = power(base, exponent / 2, module);
    return root_ans * root_ans % module;
  } else {
    return 1;
  }
}
struct position {
  int y, x;
};
position mv[4] = {
    {0, -1},
    {1, 0},
    {0, 1},
    {-1, 0}}; // double euclidean(position first, position second){ return
              // sqrt((second.x - first.x) * (second.x - first.x) + (second.y -
              // first.y) * (second.y - first.y)); }
template <class T, class U> string to_string(pair<T, U> x) {
  return to_string(x.first) + "," + to_string(x.second);
}
string to_string(string x) { return x; }
template <class itr> void array_output(itr start, itr goal) {
  string ans;
  for (auto i = start; i != goal; i++)
    ans += to_string(*i) + " ";
  if (!ans.empty())
    ans.pop_back();
  cout << ans << endl;
}
template <class itr> void cins(itr first, itr last) {
  for (auto i = first; i != last; i++) {
    cin >> (*i);
  }
}
template <class T> T gcd(T a, T b) {
  if (a && b) {
    return gcd(min(a, b), max(a, b) % min(a, b));
  } else {
    return a;
  }
}
template <class T> T lcm(T a, T b) { return a / gcd(a, b) * b; }
struct combination {
  vector<lint> fact, inv;
  combination(int sz) : fact(sz + 1), inv(sz + 1) {
    fact[0] = 1;
    for (int i = 1; i <= sz; i++) {
      fact[i] = fact[i - 1] * i % mod;
    }
    inv[sz] = power(fact[sz], mod - 2, mod);
    for (int i = sz - 1; i >= 0; i--) {
      inv[i] = inv[i + 1] * (i + 1) % mod;
    }
  }
  lint P(int n, int r) {
    if (r < 0 || n < r)
      return 0;
    return (fact[n] * inv[n - r] % mod);
  }
  lint C(int p, int q) {
    if (q < 0 || p < q)
      return 0;
    return (fact[p] * inv[q] % mod * inv[p - q] % mod);
  }
};
template <class itr> bool next_sequence(itr first, itr last, int max_bound) {
  itr now = last;
  while (now != first) {
    now--;
    (*now)++;
    if ((*now) == max_bound) {
      (*now) = 0;
    } else {
      return true;
    }
  }
  return false;
}
template <class itr, class itr2>
bool next_sequence2(itr first, itr last, itr2 first2, itr2 last2) {
  itr now = last;
  itr2 now2 = last2;
  while (now != first) {
    now--, now2--;
    (*now)++;
    if ((*now) == (*now2)) {
      (*now) = 0;
    } else {
      return true;
    }
  }
  return false;
}
inline int at(lint i, int j) { return (i >> j) & 1; }

int N;
vector<int> num[2];

int dp[1 << 18][20];

int card(int already_bit, int now, int back) {
  if (now == N) {
    return 0;
  } else if (dp[already_bit][back] != -1) {
    return dp[already_bit][back];
  }
  int ans = inf9, cnt = 0;
  for (int i = N - 1; i >= 0; i--) {
    if (at(already_bit, i)) {
      cnt++;
      continue;
    }
    // cout << i << endl;
    if (back < 18) {
      int back_num = num[abs(back - now + 1) % 2][back];
      int this_num = num[abs(i - now) % 2][i];
      if (back_num > this_num) {
        continue;
      }
    }
    ans = min(ans, card(already_bit | (1 << i), now + 1, i) + cnt);
  }
  return ans;
}

int main() {
  cin >> N;
  num[0].resize(N);
  num[1].resize(N);
  cins(all(num[0]));
  cins(all(num[1]));
  memset(dp, -1, sizeof(dp));
  int ans = card(0, 0, 18);
  if (ans >= inf9) {
    cout << -1 << endl;
  } else {
    cout << ans << endl;
  }
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;
using lint = long long;
const lint mod = 1e9 + 7;
#define all(x) (x).begin(), (x).end()
#define bitcount(n) __builtin_popcountl((lint)(n))
#define fcout cout << fixed << setprecision(15)
#define highest(x) (63 - __builtin_clzl(x))
#define rep(i, n) for (int i = 0; i < n; i++)
const int inf9 = 1e9;
const lint inf18 = 1e18;
template <class T> inline void YES(T condition) {
  if (condition)
    cout << "YES" << endl;
  else
    cout << "NO" << endl;
}
template <class T> inline void Yes(T condition) {
  if (condition)
    cout << "Yes" << endl;
  else
    cout << "No" << endl;
}
template <class T = string, class U = char>
int character_count(T text, U character) {
  int ans = 0;
  for (U i : text) {
    ans += (i == character);
  }
  return ans;
}
lint power(lint base, lint exponent, lint module) {
  if (exponent % 2) {
    return power(base, exponent - 1, module) * base % module;
  } else if (exponent) {
    lint root_ans = power(base, exponent / 2, module);
    return root_ans * root_ans % module;
  } else {
    return 1;
  }
}
struct position {
  int y, x;
};
position mv[4] = {
    {0, -1},
    {1, 0},
    {0, 1},
    {-1, 0}}; // double euclidean(position first, position second){ return
              // sqrt((second.x - first.x) * (second.x - first.x) + (second.y -
              // first.y) * (second.y - first.y)); }
template <class T, class U> string to_string(pair<T, U> x) {
  return to_string(x.first) + "," + to_string(x.second);
}
string to_string(string x) { return x; }
template <class itr> void array_output(itr start, itr goal) {
  string ans;
  for (auto i = start; i != goal; i++)
    ans += to_string(*i) + " ";
  if (!ans.empty())
    ans.pop_back();
  cout << ans << endl;
}
template <class itr> void cins(itr first, itr last) {
  for (auto i = first; i != last; i++) {
    cin >> (*i);
  }
}
template <class T> T gcd(T a, T b) {
  if (a && b) {
    return gcd(min(a, b), max(a, b) % min(a, b));
  } else {
    return a;
  }
}
template <class T> T lcm(T a, T b) { return a / gcd(a, b) * b; }
struct combination {
  vector<lint> fact, inv;
  combination(int sz) : fact(sz + 1), inv(sz + 1) {
    fact[0] = 1;
    for (int i = 1; i <= sz; i++) {
      fact[i] = fact[i - 1] * i % mod;
    }
    inv[sz] = power(fact[sz], mod - 2, mod);
    for (int i = sz - 1; i >= 0; i--) {
      inv[i] = inv[i + 1] * (i + 1) % mod;
    }
  }
  lint P(int n, int r) {
    if (r < 0 || n < r)
      return 0;
    return (fact[n] * inv[n - r] % mod);
  }
  lint C(int p, int q) {
    if (q < 0 || p < q)
      return 0;
    return (fact[p] * inv[q] % mod * inv[p - q] % mod);
  }
};
template <class itr> bool next_sequence(itr first, itr last, int max_bound) {
  itr now = last;
  while (now != first) {
    now--;
    (*now)++;
    if ((*now) == max_bound) {
      (*now) = 0;
    } else {
      return true;
    }
  }
  return false;
}
template <class itr, class itr2>
bool next_sequence2(itr first, itr last, itr2 first2, itr2 last2) {
  itr now = last;
  itr2 now2 = last2;
  while (now != first) {
    now--, now2--;
    (*now)++;
    if ((*now) == (*now2)) {
      (*now) = 0;
    } else {
      return true;
    }
  }
  return false;
}
inline int at(lint i, int j) { return (i >> j) & 1; }

int N;
vector<int> num[2];

int dp[1 << 18][20];

int card(int already_bit, int now, int back) {
  if (now == N) {
    return 0;
  } else if (dp[already_bit][back] != -1) {
    return dp[already_bit][back];
  }
  int ans = inf9, cnt = 0;
  for (int i = N - 1; i >= 0; i--) {
    if (at(already_bit, i)) {
      cnt++;
      continue;
    }
    // cout << i << endl;
    if (back < 18) {
      int back_num = num[abs(back - now + 1) % 2][back];
      int this_num = num[abs(i - now) % 2][i];
      if (back_num > this_num) {
        continue;
      }
    }
    ans = min(ans, card(already_bit | (1 << i), now + 1, i) + cnt);
  }
  return dp[already_bit][back] = ans;
}

int main() {
  cin >> N;
  num[0].resize(N);
  num[1].resize(N);
  cins(all(num[0]));
  cins(all(num[1]));
  memset(dp, -1, sizeof(dp));
  int ans = card(0, 0, 18);
  if (ans >= inf9) {
    cout << -1 << endl;
  } else {
    cout << ans << endl;
  }
}