Problem p03575 (C++)

Buggy submission (status: Runtime Error):
#include "bits/stdc++.h"

// Begin Header {{{
using namespace std;

#ifndef DEBUG
#define dump(...)
#endif

#define all(x) x.begin(), x.end()
#define rep(i, b, e) for (intmax_t i = (b), i##_limit = (e); i < i##_limit; ++i)
#define reps(i, b, e)                                                          \
  for (intmax_t i = (b), i##_limit = (e); i <= i##_limit; ++i)
#define repr(i, b, e)                                                          \
  for (intmax_t i = (b), i##_limit = (e); i >= i##_limit; --i)
#define var(Type, ...)                                                         \
  Type __VA_ARGS__;                                                            \
  input(__VA_ARGS__)

constexpr size_t operator""_zu(unsigned long long value) { return value; };
constexpr intmax_t operator""_jd(unsigned long long value) { return value; };
constexpr uintmax_t operator""_ju(unsigned long long value) { return value; };

constexpr int INF = 0x3f3f3f3f;
constexpr intmax_t LINF = 0x3f3f3f3f3f3f3f3f_jd;

template <class T, class Compare = less<>>
using MaxHeap = priority_queue<T, vector<T>, Compare>;
template <class T, class Compare = greater<>>
using MinHeap = priority_queue<T, vector<T>, Compare>;

inline void input() {}
template <class Head, class... Tail>
inline void input(Head &&head, Tail &&...tail) {
  cin >> head;
  input(forward<Tail>(tail)...);
}

template <class T> inline istream &operator>>(istream &is, vector<T> &vec) {
  for (auto &e : vec) {
    is >> e;
  }
  return is;
}

inline void output() { cout << "\n"; }
template <class Head, class... Tail>
inline void output(Head &&head, Tail &&...tail) {
  cout << head;
  if (sizeof...(tail)) {
    cout << " ";
  }
  output(forward<Tail>(tail)...);
}

template <class T>
inline ostream &operator<<(ostream &os, const vector<T> &vec) {
  static constexpr const char *delim[] = {" ", ""};
  for (const auto &e : vec) {
    os << e << delim[&e == &vec.back()];
  }
  return os;
}

template <class T> inline vector<T> makeVector(const T &initValue, size_t sz) {
  return vector<T>(sz, initValue);
}

template <class T, class... Args>
inline auto makeVector(const T &initValue, size_t sz, Args... args) {
  return vector<decltype(makeVector<T>(initValue, args...))>(
      sz, makeVector<T>(initValue, args...));
}

template <class Func> class FixPoint : Func {
public:
  explicit constexpr FixPoint(Func &&f) noexcept : Func(forward<Func>(f)) {}

  template <class... Args>
  constexpr decltype(auto) operator()(Args &&...args) const {
    return Func::operator()(*this, std::forward<Args>(args)...);
  }
};

template <class Func>
static inline constexpr decltype(auto) makeFixPoint(Func &&f) noexcept {
  return FixPoint<Func>{forward<Func>(f)};
}

template <class Container> struct reverse_t {
  Container &c;
  reverse_t(Container &c) : c(c) {}
  auto begin() { return c.rbegin(); }
  auto end() { return c.rend(); }
};

template <class Container> auto reversed(Container &c) {
  return reverse_t<Container>(c);
}

template <class T> inline bool chmax(T &a, const T &b) noexcept {
  return b > a && (a = b, true);
}

template <class T> inline bool chmin(T &a, const T &b) noexcept {
  return b < a && (a = b, true);
}

template <class T> inline T diff(const T &a, const T &b) noexcept {
  return a < b ? b - a : a - b;
}
// End Header }}}

// Edge {{{
template <class Weight> struct Edge {
  size_t from, to;
  Weight weight;

  Edge() {}
  Edge(size_t from, size_t to, Weight weight = 1)
      : from(from), to(to), weight(weight) {}

  bool operator<(const Edge &rhs) const { return weight < rhs.weight; }

  bool operator>(const Edge &rhs) const { return weight > rhs.weight; }

  operator size_t() const { return to; }
};
// }}}

// Graph {{{
template <class Weight> class Graph : public vector<vector<Edge<Weight>>> {
  using graph = vector<vector<Edge<Weight>>>;

public:
  Graph() {}
  Graph(const size_t V) : graph(V) {}

  void connect(size_t from, size_t to, Weight weight = 1) {
    (*this)[from].emplace_back(from, to, weight);
  }

  friend ostream &operator<<(ostream &strm, const Graph &G) {
    for (size_t v = 0; v < G.size(); ++v) {
      strm << '[' << setw(2) << v << ']';
      for (const auto &e : G[v]) {
        strm << ' ' << setw(2) << e.to;
      }
      strm << '\n';
    }
    return strm;
  }
};
// }}}

// UnionFind {{{
struct UnionFind {
  vector<int> tree;
  explicit UnionFind(const size_t n) : tree(n + 1, -1) {}

  size_t size(size_t i) { return -tree[root(i)]; }

  size_t root(size_t x) { return (tree[x] < 0) ? x : tree[x] = root(tree[x]); }

  bool connected(size_t x, size_t y) { return root(x) == root(y); }

  bool unite(size_t x, size_t y) {
    x = root(x);
    y = root(y);
    if (x == y) {
      return false;
    } else {
      tree[x] += tree[y];
      tree[y] = x;
      return true;
    }
  }
};
// }}}

signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.setf(ios_base::fixed);
  cout.precision(10);
  var(size_t, N, M);
  vector<Edge<size_t>> es(M);
  rep(i, 0, M) {
    var(size_t, a, b);
    a--, b--;
    es[i] = {a, b};
  }
  size_t res = 0;
  rep(re, 0, M) {
    UnionFind uf(N);
    size_t v = 0;
    rep(i, 0, M) {
      if (i == re)
        continue;
      uf.unite(es[i].from, es[i].to);
      v = i;
    }
    res += (uf.size(v) != N);
  }
  output(res);
  return 0;
}


Fixed submission:
#include "bits/stdc++.h"

// Begin Header {{{
using namespace std;

#ifndef DEBUG
#define dump(...)
#endif

#define all(x) x.begin(), x.end()
#define rep(i, b, e) for (intmax_t i = (b), i##_limit = (e); i < i##_limit; ++i)
#define reps(i, b, e)                                                          \
  for (intmax_t i = (b), i##_limit = (e); i <= i##_limit; ++i)
#define repr(i, b, e)                                                          \
  for (intmax_t i = (b), i##_limit = (e); i >= i##_limit; --i)
#define var(Type, ...)                                                         \
  Type __VA_ARGS__;                                                            \
  input(__VA_ARGS__)

constexpr size_t operator""_zu(unsigned long long value) { return value; };
constexpr intmax_t operator""_jd(unsigned long long value) { return value; };
constexpr uintmax_t operator""_ju(unsigned long long value) { return value; };

constexpr int INF = 0x3f3f3f3f;
constexpr intmax_t LINF = 0x3f3f3f3f3f3f3f3f_jd;

template <class T, class Compare = less<>>
using MaxHeap = priority_queue<T, vector<T>, Compare>;
template <class T, class Compare = greater<>>
using MinHeap = priority_queue<T, vector<T>, Compare>;

inline void input() {}
template <class Head, class... Tail>
inline void input(Head &&head, Tail &&...tail) {
  cin >> head;
  input(forward<Tail>(tail)...);
}

template <class T> inline istream &operator>>(istream &is, vector<T> &vec) {
  for (auto &e : vec) {
    is >> e;
  }
  return is;
}

inline void output() { cout << "\n"; }
template <class Head, class... Tail>
inline void output(Head &&head, Tail &&...tail) {
  cout << head;
  if (sizeof...(tail)) {
    cout << " ";
  }
  output(forward<Tail>(tail)...);
}

template <class T>
inline ostream &operator<<(ostream &os, const vector<T> &vec) {
  static constexpr const char *delim[] = {" ", ""};
  for (const auto &e : vec) {
    os << e << delim[&e == &vec.back()];
  }
  return os;
}

template <class T> inline vector<T> makeVector(const T &initValue, size_t sz) {
  return vector<T>(sz, initValue);
}

template <class T, class... Args>
inline auto makeVector(const T &initValue, size_t sz, Args... args) {
  return vector<decltype(makeVector<T>(initValue, args...))>(
      sz, makeVector<T>(initValue, args...));
}

template <class Func> class FixPoint : Func {
public:
  explicit constexpr FixPoint(Func &&f) noexcept : Func(forward<Func>(f)) {}

  template <class... Args>
  constexpr decltype(auto) operator()(Args &&...args) const {
    return Func::operator()(*this, std::forward<Args>(args)...);
  }
};

template <class Func>
static inline constexpr decltype(auto) makeFixPoint(Func &&f) noexcept {
  return FixPoint<Func>{forward<Func>(f)};
}

template <class Container> struct reverse_t {
  Container &c;
  reverse_t(Container &c) : c(c) {}
  auto begin() { return c.rbegin(); }
  auto end() { return c.rend(); }
};

template <class Container> auto reversed(Container &c) {
  return reverse_t<Container>(c);
}

template <class T> inline bool chmax(T &a, const T &b) noexcept {
  return b > a && (a = b, true);
}

template <class T> inline bool chmin(T &a, const T &b) noexcept {
  return b < a && (a = b, true);
}

template <class T> inline T diff(const T &a, const T &b) noexcept {
  return a < b ? b - a : a - b;
}
// End Header }}}

// Edge {{{
template <class Weight> struct Edge {
  size_t from, to;
  Weight weight;

  Edge() {}
  Edge(size_t from, size_t to, Weight weight = 1)
      : from(from), to(to), weight(weight) {}

  bool operator<(const Edge &rhs) const { return weight < rhs.weight; }

  bool operator>(const Edge &rhs) const { return weight > rhs.weight; }

  operator size_t() const { return to; }
};
// }}}

// Graph {{{
template <class Weight> class Graph : public vector<vector<Edge<Weight>>> {
  using graph = vector<vector<Edge<Weight>>>;

public:
  Graph() {}
  Graph(const size_t V) : graph(V) {}

  void connect(size_t from, size_t to, Weight weight = 1) {
    (*this)[from].emplace_back(from, to, weight);
  }

  friend ostream &operator<<(ostream &strm, const Graph &G) {
    for (size_t v = 0; v < G.size(); ++v) {
      strm << '[' << setw(2) << v << ']';
      for (const auto &e : G[v]) {
        strm << ' ' << setw(2) << e.to;
      }
      strm << '\n';
    }
    return strm;
  }
};
// }}}

// UnionFind {{{
struct UnionFind {
  vector<int> tree;
  explicit UnionFind(const size_t n) : tree(n + 1, -1) {}

  size_t size(size_t i) { return -tree[root(i)]; }

  size_t root(size_t x) { return (tree[x] < 0) ? x : tree[x] = root(tree[x]); }

  bool connected(size_t x, size_t y) { return root(x) == root(y); }

  bool unite(size_t x, size_t y) {
    x = root(x);
    y = root(y);
    if (x == y) {
      return false;
    } else {
      tree[x] += tree[y];
      tree[y] = x;
      return true;
    }
  }
};
// }}}

signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.setf(ios_base::fixed);
  cout.precision(10);
  var(size_t, N, M);
  vector<Edge<size_t>> es(M);
  rep(i, 0, M) {
    var(size_t, a, b);
    a--, b--;
    es[i] = {a, b};
  }
  size_t res = 0;
  rep(re, 0, M) {
    UnionFind uf(N);
    size_t v = 0;
    rep(i, 0, M) {
      if (i == re)
        continue;
      uf.unite(es[i].from, es[i].to);
      v = es[i].from;
    }
    res += (uf.size(v) != N);
  }
  output(res);
  return 0;
}