Problem p00997 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <cassert>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <vector>

using namespace std;

#define FOR(i, k, n) for (int i = (k); i < (int)(n); ++i)
#define REP(i, n) FOR(i, 0, n)
#define FORIT(i, c)                                                            \
  for (__typeof((c).begin()) i = (c).begin(); i != (c).end(); ++i)

template <class T> void debug(T begin, T end) {
  for (T i = begin; i != end; ++i)
    cerr << *i << " ";
  cerr << endl;
}
inline bool valid(int x, int y, int W, int H) {
  return (x >= 0 && y >= 0 && x < W && y < H);
}

typedef long long ll;
const int INF = 100000000;
const double EPS = 1e-8;
const int MOD = 1000000007;
int dx[8] = {1, 0, -1, 0, 1, -1, -1, 1};
int dy[8] = {0, 1, 0, -1, 1, 1, -1, -1};

struct UnionFind {
  vector<int> data;
  UnionFind(int N) : data(N, -1) {}
  bool uni(int x, int y) {
    x = root(x);
    y = root(y);
    if (x != y) {
      if (data[x] > data[y])
        swap(x, y);
      data[x] += data[y];
      data[y] = x;
    }
    return x != y;
  }
  bool find(int x, int y) { return root(x) == root(y); }
  int root(int x) { return data[x] < 0 ? x : data[x] = root(data[x]); }
  int size(int x) { return -data[root(x)]; }
};

struct edge {
  int src, dst, cost;
  edge(int s, int d, int c) : src(s), dst(d), cost(c) {}
  edge() {}
  bool operator<(const edge &e) const { return cost < e.cost; }
};

int main() {
  int N;
  cin >> N;
  vector<edge> v(N - 1);
  REP(i, N - 1) {
    int a, b, c;
    cin >> a >> b >> c;
    v[i] = edge(a, b, c);
  }
  UnionFind uf(N);
  sort(v.begin(), v.end());
  ll ans = 0;
  REP(i, N) {
    ans += (ll)uf.size(v[i].src) * uf.size(v[i].dst) * v[i].cost;
    uf.uni(v[i].src, v[i].dst);
  }
  cout << ans << endl;
  return 0;
}

Fixed submission:
#include <algorithm>
#include <cassert>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <vector>

using namespace std;

#define FOR(i, k, n) for (int i = (k); i < (int)(n); ++i)
#define REP(i, n) FOR(i, 0, n)
#define FORIT(i, c)                                                            \
  for (__typeof((c).begin()) i = (c).begin(); i != (c).end(); ++i)

template <class T> void debug(T begin, T end) {
  for (T i = begin; i != end; ++i)
    cerr << *i << " ";
  cerr << endl;
}
inline bool valid(int x, int y, int W, int H) {
  return (x >= 0 && y >= 0 && x < W && y < H);
}

typedef long long ll;
const int INF = 100000000;
const double EPS = 1e-8;
const int MOD = 1000000007;
int dx[8] = {1, 0, -1, 0, 1, -1, -1, 1};
int dy[8] = {0, 1, 0, -1, 1, 1, -1, -1};

struct UnionFind {
  vector<int> data;
  UnionFind(int N) : data(N, -1) {}
  bool uni(int x, int y) {
    x = root(x);
    y = root(y);
    if (x != y) {
      if (data[x] > data[y])
        swap(x, y);
      data[x] += data[y];
      data[y] = x;
    }
    return x != y;
  }
  bool find(int x, int y) { return root(x) == root(y); }
  int root(int x) { return data[x] < 0 ? x : data[x] = root(data[x]); }
  int size(int x) { return -data[root(x)]; }
};

struct edge {
  int src, dst, cost;
  edge(int s, int d, int c) : src(s), dst(d), cost(c) {}
  edge() {}
  bool operator<(const edge &e) const { return cost < e.cost; }
};

int main() {
  int N;
  cin >> N;
  vector<edge> v(N - 1);
  REP(i, N - 1) {
    int a, b, c;
    cin >> a >> b >> c;
    v[i] = edge(a, b, c);
  }
  UnionFind uf(N);
  sort(v.begin(), v.end());
  ll ans = 0;
  REP(i, N - 1) {
    ans += (ll)uf.size(v[i].src) * uf.size(v[i].dst) * v[i].cost;
    uf.uni(v[i].src, v[i].dst);
  }
  cout << ans << endl;
  return 0;
}