Language: C++

#include <bits/stdc++.h>

//{{{ graph.hpp
#ifndef INCLUDE_GRAPH_HPP
#define INCLUDE_GRAPH_HPP
#include <vector>
namespace orislib {
struct Edge {
  typedef int weight_t;
  int from, to;
  weight_t w;
  Edge(int from, int to, weight_t w) : from(from), to(to), w(w) {}
  bool operator<(const Edge& e) const {
    return w > e.w;
  }
  static const weight_t INF;
};
const int Edge::INF=1<<29;
typedef std::vector<Edge> Edges;
typedef std::vector<Edges> Graph;
}
#endif
//}}}
//{{{ uf.hpp
#ifndef INCLUDE_UF_HPP
#define INCLUDE_UF_HPP
#include <vector>
#include <algorithm>
namespace orislib {
namespace ds {
struct UF {
  std::vector<int> data;
  int cnt;
  UF(int n) : data(n, -1), cnt(n) {}
  bool merge(int a, int b){
    a = root(a);
    b = root(b);
    if (a != b){
      cnt--;
      if (data[b] < data[a]) std::swap(a, b);
      data[a] += data[b];
      data[b] = a;
    }
    return a != b;
  }
  bool same(int a, int b){ return root(a) == root(b); }
  int root(int a){ return data[a] >= 0 ? data[a] = root(data[a]) : a; }
  int size(int a){ return -data[root(a)]; }
  int count(){ return cnt; }
};
struct UF2d {
  UF uf;
  const int W, H;
  UF2d(int w, int h) : uf(w * h), W(w), H(h) {}
  bool merge(int ax, int ay, int bx, int by){ return uf.merge(ay * W + ax, by * W + bx); }
  bool same(int ax, int ay, int bx, int by){ return uf.same(ay * W + ax, by * W + bx); }
  int size(int x, int y){ return uf.size(y * W + x); }
  int count(){ return uf.count(); }
};
}
}
#endif
//}}}

using namespace std;
using namespace orislib;

typedef long long ll;
typedef long double ld;
typedef tuple<int, int> duo;
//{{{ templates
#define TMPINL_(...) template<__VA_ARGS__>inline
#define TT_ TMPINL_(typename T)
#define TTF_ TMPINL_(typename T,typename F)
#define TTTS_ TMPINL_(typename T,typename...Ts)
#define TITS_ TMPINL_(size_t I=0,typename...Ts)
TT_ T sq(T x){return x*x;}
TT_ T In(){T x;cin>>x;return x;}
TT_ void Out(T&x){cout<<x;}
TT_ void sort(T&v){sort(begin(v),end(v));}
TT_ void revs(T&v){reverse(begin(v),end(v));}
TT_ void uniq(T&v){sort(v);v.erase(unique(begin(v),end(v)),end(v));}
TT_ int ubnd(T&v,typename T::value_type&x){return upper_bound(begin(v),end(v),x)-begin(v);}
TT_ int lbnd(T&v,typename T::value_type&x){return lower_bound(begin(v),end(v),x)-begin(v);}
TTF_ void inpt(T&v,int n,F f){for(v.reserve(n);n--;v.emplace_back(f()));}
TTF_ void show(T&v,F f,string d=" ",string e="\n"){int i=0;for(auto&x:v)i++&&(cout<<d),f(x);cout<<e;}
TITS_ typename enable_if<I==tuple_size<tuple<Ts...>>::value-1,string>::type join(string s,tuple<Ts...>t){return to_string(get<I>(t));}
TITS_ typename enable_if<I<tuple_size<tuple<Ts...>>::value-1,string>::type join(string s,tuple<Ts...>t){return to_string(get<I>(t))+s+join<I+1>(s,t);}
TT_ string join(string s,T t){return to_string(t);}
TTTS_ string join(string s,T t,Ts...ts){return join(s,t)+s+join(s,ts...);}

inline void fast_io(){ios::sync_with_stdio(0);cin.tie(0);}
inline int in(){int x;scanf("%d",&x);return x;}
inline ll pow_mod(ll a,ll k,ll m){ll r=1;for(;k>0;a=a*a%m,k>>=1)if(k&1)r=r*a%m;return r;}
inline ll mod_inv(ll a,ll p){return pow_mod(a,p-2,p);}
inline int puts(const string&s){ return puts(s.c_str()); }
//}}} priority_queue queue deque front stringstream max_element min_element insert count make_tuple
const int dx[] = {0, 0, 1, -1, 1, 1, -1, -1};
const int dy[] = {1, -1, 0, 0, 1, -1, 1, -1};
const int Mod = 1000000000 + 0;


//{{{ Edges kruskal(g)
Edges kruskal(const Graph& g){
  const int V = g.size();
  int E = 0;
  for (const auto& v : g) E += v.size();
  Edges es;
  es.reserve(E);
  for (const auto& v : g){
    for (const auto& e : v){
      es.emplace_back(e);
    }
  }
  sort(es);
  UF uf(V);
  Edges result;
  for (const auto& e : es){
    if (uf.same(e.from, e.to)) continue;
    uf.merge(e.from, e.to);
    result.emplace_back(e);
  }
  return result;
}
//}}}
//{{{ Edges prim(g, s)
Edges prim(const Graph& g, int s){
  const int V = g.size();
  vector<bool> visited(V);
  priority_queue<Edge> pq;
  Edges result;
  for (pq.push(Edge(-2, s, 0)); !pq.empty(); pq.pop()){
    Edge c = pq.top();
    if (visited[c.to]) continue;
    visited[c.to] = true;
    result.emplace_back(c);
    for (const auto& e : g[c.to]){
      if (!visited[e.to]) pq.push(e);
    }
  }
  return result;
}
//}}}

int main()
{
  int V, E;
  V = in(), E = in();
  Graph g(V);
  for (int i = 0; i < E; i++){
    int s, t, d;
    s = in(), t = in(), d = in();
    g[s].emplace_back(s, t, d);
    g[t].emplace_back(t, s, d);
  }
  auto es = kruskal(g);
  int tot = 0;
  for (const auto& e : es) tot += e.w;
  cout << tot << endl;
  return 0;
}

Compiler output:
a.cc: In function 'orislib::Edges kruskal(const orislib::Graph&)':
a.cc:116:3: error: 'UF' was not declared in this scope; did you mean 'orislib::ds::UF'?
  116 |   UF uf(V);
      |   ^~
      |   orislib::ds::UF
a.cc:31:8: note: 'orislib::ds::UF' declared here
   31 | struct UF {
      |        ^~
a.cc:119:9: error: 'uf' was not declared in this scope
  119 |     if (uf.same(e.from, e.to)) continue;
      |         ^~
a.cc:120:5: error: 'uf' was not declared in this scope
  120 |     uf.merge(e.from, e.to);
      |     ^~