Problem p01648 (C++)

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

#define rep(i, x, y) for (int i = (x); i < (y); ++i)
#define debug(x) #x << "=" << (x)

#ifdef DEBUG
#define _GLIBCXX_DEBUG
#define dump(x) std::cerr << debug(x) << " (L:" << __LINE__ << ")" << std::endl
#else
#define dump(x)
#endif

typedef long long int ll;
typedef pair<int, int> pii;
// template<typename T> using vec=std::vector<T>;

const int inf = 1 << 30;
const long long int infll = 1LL << 58;
const double eps = 1e-9;
const int dx[] = {1, 0, -1, 0}, dy[] = {0, 1, 0, -1};

template <typename T> ostream &operator<<(ostream &os, const vector<T> &vec) {
  os << "[";
  for (const auto &v : vec) {
    os << v << ",";
  }
  os << "]";
  return os;
}

class union_find {
private:
  vector<int> parent, rank;
  int size;

public:
  int cnt;

  union_find() = default;
  union_find(int n) { init(n); }

  void init(int n) {
    size = n;
    cnt = n;
    parent.resize(size);
    rank.resize(size);
    for (int i = 0; i < size; ++i) {
      parent[i] = i;
      rank[i] = 0;
    }
  }

  int find(int x) {
    if (parent[x] == x)
      return x;
    else
      return parent[x] = find(parent[x]);
  }

  void unite(int x, int y) {
    x = find(x);
    y = find(y);
    if (x == y)
      return;
    if (rank[x] < rank[y]) {
      parent[x] = y;
    } else {
      parent[y] = x;
      if (rank[x] == rank[y])
        ++rank[x];
    }
    --cnt;
  }

  bool is_same(int x, int y) { return find(x) == find(y); }
};

struct edge {
  int s, t, c;
  edge(int s, int t, int c) : s(s), t(t), c(c) {}
  bool operator<(const edge &other) const { return c < other.c; }
};

void solve() {
  while (true) {
    int n, m;
    cin >> n >> m;
    if (n == 0 and m == 0)
      break;
    vector<edge> edges;
    rep(i, 0, m) {
      int s, t, c;
      cin >> s >> t >> c;
      --s;
      --t;
      edges.emplace_back(edge(s, t, c));
    }
    sort(edges.begin(), edges.end());
    union_find uf(n);
    vector<int> costs;
    for (const auto &e : edges)
      if (!uf.is_same(e.s, e.t)) {
        uf.unite(e.s, e.t);
        costs.emplace_back(e.c);
      }
    cerr << costs << endl;
    cout << costs[(n - 2) / 2] << endl;
  }
}

int main() {
  std::ios::sync_with_stdio(false);
  std::cin.tie(0);
  cout << fixed << setprecision(8);
  solve();
  return 0;
}

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

#define rep(i, x, y) for (int i = (x); i < (y); ++i)
#define debug(x) #x << "=" << (x)

#ifdef DEBUG
#define _GLIBCXX_DEBUG
#define dump(x) std::cerr << debug(x) << " (L:" << __LINE__ << ")" << std::endl
#else
#define dump(x)
#endif

typedef long long int ll;
typedef pair<int, int> pii;
// template<typename T> using vec=std::vector<T>;

const int inf = 1 << 30;
const long long int infll = 1LL << 58;
const double eps = 1e-9;
const int dx[] = {1, 0, -1, 0}, dy[] = {0, 1, 0, -1};

template <typename T> ostream &operator<<(ostream &os, const vector<T> &vec) {
  os << "[";
  for (const auto &v : vec) {
    os << v << ",";
  }
  os << "]";
  return os;
}

class union_find {
private:
  vector<int> parent, rank;
  int size;

public:
  int cnt;

  union_find() = default;
  union_find(int n) { init(n); }

  void init(int n) {
    size = n;
    cnt = n;
    parent.resize(size);
    rank.resize(size);
    for (int i = 0; i < size; ++i) {
      parent[i] = i;
      rank[i] = 0;
    }
  }

  int find(int x) {
    if (parent[x] == x)
      return x;
    else
      return parent[x] = find(parent[x]);
  }

  void unite(int x, int y) {
    x = find(x);
    y = find(y);
    if (x == y)
      return;
    if (rank[x] < rank[y]) {
      parent[x] = y;
    } else {
      parent[y] = x;
      if (rank[x] == rank[y])
        ++rank[x];
    }
    --cnt;
  }

  bool is_same(int x, int y) { return find(x) == find(y); }
};

struct edge {
  int s, t, c;
  edge(int s, int t, int c) : s(s), t(t), c(c) {}
  bool operator<(const edge &other) const { return c < other.c; }
};

void solve() {
  while (true) {
    int n, m;
    cin >> n >> m;
    if (n == 0 and m == 0)
      break;
    vector<edge> edges;
    rep(i, 0, m) {
      int s, t, c;
      cin >> s >> t >> c;
      --s;
      --t;
      edges.emplace_back(edge(s, t, c));
    }
    sort(edges.begin(), edges.end());
    union_find uf(n);
    vector<int> costs;
    for (const auto &e : edges)
      if (!uf.is_same(e.s, e.t)) {
        uf.unite(e.s, e.t);
        costs.emplace_back(e.c);
      }
    cout << costs[max(0, (n - 2) / 2)] << endl;
  }
}

int main() {
  std::ios::sync_with_stdio(false);
  std::cin.tie(0);
  cout << fixed << setprecision(8);
  solve();
  return 0;
}