Problem p01648 (C++) — buggy submission, verdict: Runtime Error

#define __USE_MINGW_ANSI_STDIO 0
#include <bits/stdc++.h>

using namespace std;
typedef long long ll;
#define int ll
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<ll> VL;
typedef vector<VL> VVL;
typedef pair<int, int> PII;

#define FOR(i, a, n) for (ll i = (ll)a; i < (ll)n; ++i)
#define REP(i, n) FOR(i, 0, n)
#define ALL(x) x.begin(), x.end()
#define IN(a, b, x) (a <= x && x < b)
#define MP make_pair
#define PB push_back
#ifdef int
const ll INF = (1LL << 60);
#else
const int INF = (1LL << 30);
#endif
const double PI = 3.14159265359;
const double EPS = 1e-12;
const int MOD = 1000000007;

template <typename T> T &chmin(T &a, const T &b) { return a = min(a, b); }
template <typename T> T &chmax(T &a, const T &b) { return a = max(a, b); }
template <class S, class T>
ostream &operator<<(ostream &out, const pair<S, T> &a) {
  out << '(' << a.first << ',' << a.second << ')';
  return out;
}

int dx[] = {0, 1, 0, -1}, dy[] = {1, 0, -1, 0};

class UnionFind {
public:
  const static int MAX_N = 100010;
  int par[MAX_N];
  int rank[MAX_N];
  int s[MAX_N];
  bool used[MAX_N];
  UnionFind() { init(); }
  UnionFind(int n) { init(n); }
  void init() {
    for (int i = 0; i < MAX_N; ++i)
      par[i] = i, rank[i] = 0, s[i] = 1;
  }
  void init(int n) {
    for (int i = 0; i < n; ++i)
      par[i] = i, rank[i] = 0, s[i] = 1;
  }
  int find(int x) {
    if (par[x] == x)
      return x;
    return par[x] = find(par[x]);
  }
  void unite(int x, int y) {
    x = find(x);
    y = find(y);
    if (x == y)
      return;
    if (rank[x] < rank[y]) {
      par[x] = y;
      s[y] = s[x] + s[y];
    } else {
      par[y] = x;
      s[x] = s[x] + s[y];
      if (rank[x] == rank[y])
        rank[x]++;
    }
  }
  int size(int x) { return s[find(x)]; }
  bool same(int x, int y) { return find(x) == find(y); }
  int group(int n) {
    REP(i, n) used[find(i)] = true;
    int ret = 0;
    REP(i, n) if (used[i]) ret++;
    return ret;
  }
};
UnionFind uf;

VI a[10010];
signed main(void) {
  while (true) {
    int n, m;
    cin >> n >> m;
    if (!n)
      break;
    REP(i, m) {
      int x, y, z;
      cin >> x >> y >> z;
      x--, y--;
      a[i] = {z, x, y};
    }
    sort(a, a + m);
    if (n == 2) {
      cout << a[0][0] << endl;
      continue;
    }

    uf.init(n);
    VI v;
    REP(i, m) {
      if (!uf.same(a[i][1], a[i][2])) {
        v.PB(a[i][0]);
        uf.unite(a[i][1], a[i][2]);
      }
    }
    sort(ALL(v));
    cout << v[n / 2 - 1] << endl;
  }

  return 0;
}

Error: 0

Fixed submission:

#define __USE_MINGW_ANSI_STDIO 0
#include <bits/stdc++.h>

using namespace std;
typedef long long ll;
#define int ll
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<ll> VL;
typedef vector<VL> VVL;
typedef pair<int, int> PII;

#define FOR(i, a, n) for (ll i = (ll)a; i < (ll)n; ++i)
#define REP(i, n) FOR(i, 0, n)
#define ALL(x) x.begin(), x.end()
#define IN(a, b, x) (a <= x && x < b)
#define MP make_pair
#define PB push_back
#ifdef int
const ll INF = (1LL << 60);
#else
const int INF = (1LL << 30);
#endif
const double PI = 3.14159265359;
const double EPS = 1e-12;
const int MOD = 1000000007;

template <typename T> T &chmin(T &a, const T &b) { return a = min(a, b); }
template <typename T> T &chmax(T &a, const T &b) { return a = max(a, b); }
template <class S, class T>
ostream &operator<<(ostream &out, const pair<S, T> &a) {
  out << '(' << a.first << ',' << a.second << ')';
  return out;
}

int dx[] = {0, 1, 0, -1}, dy[] = {1, 0, -1, 0};

VI a[1010];
bool used[1010];
signed main(void) {
  while (true) {
    int n, m;
    cin >> n >> m;
    if (!n)
      break;
    REP(i, m) {
      int x, y, z;
      cin >> x >> y >> z;
      x--, y--;
      a[i] = {z, x, y};
    }
    sort(a, a + m);
    if (n == 2) {
      cout << a[0][0] << endl;
      continue;
    }

    int ans;
    memset(used, false, sizeof(used));
    REP(i, n / 2) used[a[i][0]] = used[a[i][1]] = true;
    FOR(i, n / 2, m) {
      used[a[i][0]] = true;
      used[a[i][1]] = true;
      int cnt = 0;
      REP(i, n) if (used[i]) cnt++;
      // cout << i << " " << cnt << endl;
      if (cnt >= n / 2) {
        ans = a[i - 1][0];
        break;
      }
    }
    cout << ans << endl;
  }

  return 0;
}