Problem p03298 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
typedef pair<ll, ll> P;
typedef pair<int, int> Pi;
#define rep(i, n) for (ll i = 0; i < n; i++)
#define FOR(i, a, b) for (ll i = a; i < b; i++)
#define fi first
#define se second
#define endl "\n"

template <typename T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> ostream &operator<<(ostream &s, const complex<T> &d) {
  return s << "(" << d.real() << ", " << d.imag() << ")";
}
template <typename T1, typename T2>
ostream &operator<<(ostream &s, const pair<T1, T2> &d) {
  return s << "(" << d.first << ", " << d.second << ")";
}
template <typename T> ostream &operator<<(ostream &s, const vector<T> &d) {
  int len = d.size();
  rep(i, len) {
    s << d[i];
    if (i < len - 1)
      s << " ";
  }
  return s;
}
template <typename T>
ostream &operator<<(ostream &s, const vector<vector<T>> &d) {
  int len = d.size();
  rep(i, len) { s << d[i] << endl; }
  return s;
}
template <typename T> ostream &operator<<(ostream &s, const set<T> &v) {
  s << "{ ";
  for (auto itr = v.begin(); itr != v.end(); ++itr) {
    if (itr != v.begin()) {
      s << ", ";
    }
    s << (*itr);
  }
  s << " }";
  return s;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &s, const map<T1, T2> &m) {
  s << "{" << endl;
  for (auto itr = m.begin(); itr != m.end(); ++itr) {
    s << " " << (*itr).first << " : " << (*itr).second << endl;
  }
  s << "}" << endl;
  return s;
}

const ll mod = 1e9 + 7;
const ll inf = 1e17;
const int INF = 1e9;
const double PI = acos(-1);
const double EPS = 1e-10;

typedef pair<string, string> Ps;

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  int n;
  cin >> n;
  string s, a, b;
  cin >> s;
  a = s.substr(0, n), b = s.substr(n, n);
  reverse(begin(b), end(b));
  map<Ps, ll> cnt1, cnt2;
  rep(i, 1 << n) {
    string k, l;
    rep(j, n) {
      if (i & 1 << j) {
        k.push_back(a[j]);
      } else {
        l.push_back(a[j]);
      }
    }
    cnt1[Ps(k, l)]++;
  }
  rep(i, 1 << n) {
    string k, l;
    rep(j, n) {
      if (i & 1 << j) {
        k.push_back(b[j]);
      } else {
        l.push_back(b[j]);
      }
    }
    cnt2[Ps(k, l)]++;
  }
  ll ans = 0;
  for (auto itr1 : cnt1) {
    for (auto itr2 : cnt2) {
      if (itr1.fi.fi == itr2.fi.fi && itr1.fi.se == itr2.fi.se) {
        ans += itr1.se * itr2.se;
      }
    }
  }
  cout << ans << endl;
}

Fixed submission:
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
typedef pair<ll, ll> P;
typedef pair<int, int> Pi;
#define rep(i, n) for (ll i = 0; i < n; i++)
#define FOR(i, a, b) for (ll i = a; i < b; i++)
#define fi first
#define se second
#define endl "\n"

template <typename T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> ostream &operator<<(ostream &s, const complex<T> &d) {
  return s << "(" << d.real() << ", " << d.imag() << ")";
}
template <typename T1, typename T2>
ostream &operator<<(ostream &s, const pair<T1, T2> &d) {
  return s << "(" << d.first << ", " << d.second << ")";
}
template <typename T> ostream &operator<<(ostream &s, const vector<T> &d) {
  int len = d.size();
  rep(i, len) {
    s << d[i];
    if (i < len - 1)
      s << " ";
  }
  return s;
}
template <typename T>
ostream &operator<<(ostream &s, const vector<vector<T>> &d) {
  int len = d.size();
  rep(i, len) { s << d[i] << endl; }
  return s;
}
template <typename T> ostream &operator<<(ostream &s, const set<T> &v) {
  s << "{ ";
  for (auto itr = v.begin(); itr != v.end(); ++itr) {
    if (itr != v.begin()) {
      s << ", ";
    }
    s << (*itr);
  }
  s << " }";
  return s;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &s, const map<T1, T2> &m) {
  s << "{" << endl;
  for (auto itr = m.begin(); itr != m.end(); ++itr) {
    s << " " << (*itr).first << " : " << (*itr).second << endl;
  }
  s << "}" << endl;
  return s;
}

const ll mod = 1e9 + 7;
const ll inf = 1e17;
const int INF = 1e9;
const double PI = acos(-1);
const double EPS = 1e-10;

typedef pair<string, string> Ps;

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  int n;
  cin >> n;
  string s, a, b;
  cin >> s;
  a = s.substr(0, n), b = s.substr(n, n);
  reverse(begin(b), end(b));
  map<Ps, ll> cnt1, cnt2;
  rep(i, 1 << n) {
    string k, l;
    rep(j, n) {
      if (i & 1 << j) {
        k.push_back(a[j]);
      } else {
        l.push_back(a[j]);
      }
    }
    cnt1[Ps(k, l)]++;
  }
  rep(i, 1 << n) {
    string k, l;
    rep(j, n) {
      if (i & 1 << j) {
        k.push_back(b[j]);
      } else {
        l.push_back(b[j]);
      }
    }
    cnt2[Ps(k, l)]++;
  }
  ll ans = 0;
  for (auto itr : cnt1) {
    ll num = cnt2[itr.fi];
    ans += num * itr.se;
  }
  cout << ans << endl;
}