Problem p02794 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;

#define REP(i, m, n) for (int i = (m); i < (int)(n); i++)
#define RREP(i, m, n) for (int i = (int)(n - 1); i >= m; i--)
#define rep(i, n) REP(i, 0, n)
#define rrep(i, n) RREP(i, 0, n)
#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define aut(r, v) __typeof(v) r = (v)
#define each(it, o) for (aut(it, (o).begin()); it != (o).end(); ++it)
#define reach(it, o) for (aut(it, (o).rbegin()); it != (o).rend(); ++it)
#define fi first
#define se second
#define debug(...)                                                             \
  {                                                                            \
    cerr << "[L" << __LINE__ << "] ";                                          \
    _debug(__VA_ARGS__);                                                       \
  }

template <typename T1, typename T2>
ostream &operator<<(ostream &o, const pair<T1, T2> &p) {
  return o << "(" << p.first << ", " << p.second << ")";
}
template <typename T> string join(const vector<T> &v, string del = ", ") {
  stringstream s;
  rep(i, v.size()) s << del << v[i];
  return s.str().substr(del.size());
}
template <typename T> ostream &operator<<(ostream &o, const vector<T> &v) {
  if (v.size())
    o << "[" << join(v) << "]";
  return o;
}
template <typename T>
ostream &operator<<(ostream &o, const vector<vector<T>> &vv) {
  int l = vv.size();
  if (l) {
    o << endl;
    rep(i, l) {
      o << (i == 0 ? "[ " : ",\n  ") << vv[i] << (i == l - 1 ? " ]" : "");
    }
  }
  return o;
}
template <typename T> ostream &operator<<(ostream &o, const set<T> &st) {
  vector<T> v(st.begin(), st.end());
  o << "{ " << join(v) << " }";
  return o;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &o, const map<T1, T2> &m) {
  each(p, m) {
    o << (p == m.begin() ? "{ " : ",\n  ") << *p
      << (p == --m.end() ? " }" : "");
  }
  return o;
}
inline void _debug() { cerr << endl; }
template <class First, class... Rest>
void _debug(const First &first, const Rest &...rest) {
  cerr << first << " ";
  _debug(rest...);
}

typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<ll> vl;
typedef vector<vl> vvl;

const double PI = (1 * acos(0.0));
const double EPS = 1e-9;
const ll INF = 0x3f3f3f3f;
const ll INFL = 0x3f3f3f3f3f3f3f3fLL;
const ll mod = 1e9 + 7;

inline void finput(string filename) { freopen(filename.c_str(), "r", stdin); }

struct Edge {
  int to, id;
  Edge(int to, int id) : to(to), id(id) {}
};

int N, M;
vector<Edge> G[100];
vector<int> es;

bool dfs(int v, int tv, int p = -1) {
  if (v == tv) {
    es = vector<int>();
    return true;
  }
  for (auto e : G[v]) {
    if (e.to == p)
      continue;
    if (dfs(e.to, tv, v)) {
      es.push_back(e.id);
      return true;
    }
  }
  return false;
}

int main() {
  ios_base::sync_with_stdio(0);
  finput("./input");

  cin >> N;
  rep(i, N - 1) {
    int a, b;
    cin >> a >> b;
    a--;
    b--;
    G[a].emplace_back(b, i);
    G[b].emplace_back(a, i);
  }

  cin >> M;
  vector<ll> eset(M);
  rep(i, M) {
    int u, v;
    cin >> u >> v;
    u--;
    v--;
    dfs(u, v);
    for (auto e : es) {
      eset[i] |= 1ll << e;
    }
  }
  ll ans = 0;
  rep(i, 1 << M) {
    ll mask = 0;
    rep(j, M) {
      if ((i >> j) & 1)
        mask |= eset[j];
    }
    ll k = __builtin_popcountll(mask);
    ll x = 1ll << (N - 1 - k);
    if (__builtin_popcountll(i) & 1)
      ans -= x;
    else
      ans += x;
  }
  cout << ans << endl;
  return 0;
}

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

#define REP(i, m, n) for (int i = (m); i < (int)(n); i++)
#define RREP(i, m, n) for (int i = (int)(n - 1); i >= m; i--)
#define rep(i, n) REP(i, 0, n)
#define rrep(i, n) RREP(i, 0, n)
#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define aut(r, v) __typeof(v) r = (v)
#define each(it, o) for (aut(it, (o).begin()); it != (o).end(); ++it)
#define reach(it, o) for (aut(it, (o).rbegin()); it != (o).rend(); ++it)
#define fi first
#define se second
#define debug(...)                                                             \
  {                                                                            \
    cerr << "[L" << __LINE__ << "] ";                                          \
    _debug(__VA_ARGS__);                                                       \
  }

template <typename T1, typename T2>
ostream &operator<<(ostream &o, const pair<T1, T2> &p) {
  return o << "(" << p.first << ", " << p.second << ")";
}
template <typename T> string join(const vector<T> &v, string del = ", ") {
  stringstream s;
  rep(i, v.size()) s << del << v[i];
  return s.str().substr(del.size());
}
template <typename T> ostream &operator<<(ostream &o, const vector<T> &v) {
  if (v.size())
    o << "[" << join(v) << "]";
  return o;
}
template <typename T>
ostream &operator<<(ostream &o, const vector<vector<T>> &vv) {
  int l = vv.size();
  if (l) {
    o << endl;
    rep(i, l) {
      o << (i == 0 ? "[ " : ",\n  ") << vv[i] << (i == l - 1 ? " ]" : "");
    }
  }
  return o;
}
template <typename T> ostream &operator<<(ostream &o, const set<T> &st) {
  vector<T> v(st.begin(), st.end());
  o << "{ " << join(v) << " }";
  return o;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &o, const map<T1, T2> &m) {
  each(p, m) {
    o << (p == m.begin() ? "{ " : ",\n  ") << *p
      << (p == --m.end() ? " }" : "");
  }
  return o;
}
inline void _debug() { cerr << endl; }
template <class First, class... Rest>
void _debug(const First &first, const Rest &...rest) {
  cerr << first << " ";
  _debug(rest...);
}

typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<ll> vl;
typedef vector<vl> vvl;

const double PI = (1 * acos(0.0));
const double EPS = 1e-9;
const ll INF = 0x3f3f3f3f;
const ll INFL = 0x3f3f3f3f3f3f3f3fLL;
const ll mod = 1e9 + 7;

inline void finput(string filename) { freopen(filename.c_str(), "r", stdin); }

struct Edge {
  int to, id;
  Edge(int to, int id) : to(to), id(id) {}
};

int N, M;
vector<Edge> G[100];
vector<int> es;

bool dfs(int v, int tv, int p = -1) {
  if (v == tv) {
    es = vector<int>();
    return true;
  }
  for (auto e : G[v]) {
    if (e.to == p)
      continue;
    if (dfs(e.to, tv, v)) {
      es.push_back(e.id);
      return true;
    }
  }
  return false;
}

int main() {
  ios_base::sync_with_stdio(0);
  // finput("./input");

  cin >> N;
  rep(i, N - 1) {
    int a, b;
    cin >> a >> b;
    a--;
    b--;
    G[a].emplace_back(b, i);
    G[b].emplace_back(a, i);
  }

  cin >> M;
  vector<ll> eset(M);
  rep(i, M) {
    int u, v;
    cin >> u >> v;
    u--;
    v--;
    dfs(u, v);
    for (auto e : es) {
      eset[i] |= 1ll << e;
    }
  }
  ll ans = 0;
  rep(i, 1 << M) {
    ll mask = 0;
    rep(j, M) {
      if ((i >> j) & 1)
        mask |= eset[j];
    }
    ll k = __builtin_popcountll(mask);
    ll x = 1ll << (N - 1 - k);
    if (__builtin_popcountll(i) & 1)
      ans -= x;
    else
      ans += x;
  }
  cout << ans << endl;
  return 0;
}