Go: package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
)

// I/O
type Scanner struct {
	sc *bufio.Scanner
}

func NewScanner() *Scanner {
	sc := bufio.NewScanner(os.Stdin)
	sc.Split(bufio.ScanWords)
	sc.Buffer(make([]byte, 1024), int(1e+9))
	return &Scanner{sc}
}

func (s *Scanner) nextStr() string {
	s.sc.Scan()
	return s.sc.Text()
}

func (s *Scanner) nextInt() int {
	i, e := strconv.Atoi(s.nextStr())
	if e != nil {
		panic(e)
	}
	return i
}

func (s *Scanner) nextFloat() float64 {
	f, e := strconv.ParseFloat(s.nextStr(), 64)
	if e != nil {
		panic(e)
	}
	return f
}

func (s *Scanner) nextRuneSlice() []rune {
	return []rune(s.nextStr())
}

func (s *Scanner) nextIntSlice(n int) []int {
	res := make([]int, n)
	for i := 0; i < n; i++ {
		res[i] = s.nextInt()
	}
	return res
}

func (s *Scanner) nextFloatSlice(n int) []float64 {
	res := make([]float64, n)
	for i := 0; i < n; i++ {
		res[i] = s.nextFloat()
	}
	return res
}

// Arithmetic
func max(nums ...int) int {
	m := nums[0]
	for _, i := range nums {
		if m < i {
			m = i
		}
	}
	return m
}

func min(nums ...int) int {
	m := nums[0]
	for _, i := range nums {
		if m > i {
			m = i
		}
	}
	return m
}

func abs(x int) int {
	if x > 0 {
		return x
	}
	return -x
}

func pow(x, y int) int {
	res := 1
	for i := 0; i < y; i++ {
		res *= x
	}
	return res
}

func ceil(a, b int) int {
	if a%b == 0 {
		return a / b
	} else {
		return a/b + 1
	}
}

// Sort
type RuneSlice []rune

func (a RuneSlice) Len() int           { return len(a) }
func (a RuneSlice) Less(i, j int) bool { return a[i] < a[j] }
func (a RuneSlice) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }

const INF = 1 << 60

// Main
func main() {
	sc := NewScanner()
	wtr := bufio.NewWriter(os.Stdout)
	N := sc.nextInt()
	G := make([][]int, N)
	for i := 0; i < N; i++ {
		G[i] = make([]int, N)
		for j := 0; j < N; j++ {
			G[i][j] = sc.nextInt()
		}
	}
	d := make([]int, N)
	for i := 0; i < N; i++ {
		d[i] = INF
	}
	seen := make([]bool, N)
	d[0] = 0
	ans := 0
	for {
		cur := 0
		minCost := INF
		for i := 0; i < N; i++ {
			if !seen[i] && d[i] < minCost {
				minCost = d[i]
				cur = i
			}
		}
		if minCost == INF {
			break
		}
		seen[cur] = true
		ans += d[cur]
		for i := 0; i < N; i++ {
			cost := G[cur][i]
			if cost != -1 && cost < d[i] {
				d[i] = cost
			}
		}
	}

	fmt.Fprintln(wtr, ans)
	wtr.Flush()
}

C++: #include <iostream>

using namespace std;
static const int inf = 10000;
static const int emp = -1;
static const int black = 1;
static const int white = 0;

int   graph[124][124] = {{}};;
int d[124] = {};
int color[124] = {};
int p[124] = {};
int n;

int  prim(){

  for(int i = 0;i < n;i++){
    d[i] = inf;
    color[i] = white;
    p[i] = -1;
  }

  d[0] = 0;
  int u ,v;
 

  while(1){

    v = inf;
    u = -1;

    for(int i = 0;i < n;i++){
      if(d[i] < v && color[i] != black){
	v= d[i];
	u = i;
      }
    }
    //    cout << "u = " << u << endl; 

    if(u == -1) break;
    color[u] = black;

    // cout << 'a' << endl;
    for(int i = 0;i < n;i++){
      if(graph[u][i] != -1 && color[i] != black){
	if(graph[u][i] < d[i]){
	  d[i] = graph[u][i];
	  p[i]= u;
	  //  cout << "d = " << d[i] <<endl;
	  // cout << "i = " << i << endl;
	}
      }
    }
  }

  int sum = 0;
  for(int i = 0;i < n;i++){
    if(p[i] != -1) sum += graph[i][p[i]];
  }
  return sum;
}



int main(){

  cin >> n;

  for(int i = 0;i < n;i++){
    for(int j = 0;j < n;j++){
      cin >> graph[i][j];
    }
  }



  cout <<  prim() << endl;

  return 0;
}

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

struct UnionFind{
  int n;
  vector<int> r, p;
  UnionFind() {}
  UnionFind(int sz): n(sz), r(sz, 1), p(sz, 0){ iota(p.begin(), p.end(), 0); }
  int find(int x) {
    return (x == p[x] ? x : p[x] = find(p[x]));
  }
  bool same(int x, int y) {
    return find(x) == find(y);
  }
  void unite(int x, int y){
    x = find(x); y = find(y);
    if ( x == y ) return;
    if ( r[x] < r[y] ) swap(x, y);
    r[x] += r[y];
    p[y] = x;
  }
};

struct Edge {
  int cost, to, from;
  bool operator<(const Edge& e) const {
    return cost < e.cost;    
  }  
};

int main() {
  int n;
  cin >> n;

  vector<Edge> G;
  for ( int i = 0; i < n; i++ ) {
    for ( int j = 0; j < n; j++ ) {
      int cost;
      cin >> cost;
      if ( j > i && cost >= 0 ) G.push_back({cost, i, j});      
    }
  }

  sort(G.begin(), G.end());

  UnionFind uf(n);
  int cnt = 0;
  int sum = 0;
  for ( Edge e: G ) {
    int cost = e.cost, to = e.to, from = e.from;
    if ( uf.same(to, from) ) continue;
    sum += cost;
    if ( ++cnt >= n-1 ) break;
    uf.unite(to, from);    
  }

  cout << sum << endl;
  
  return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;
#define int long long

struct edge{ int u,v,cost; };
bool comp(const edge& e1,const edge& e2){
  return e1.cost < e2.cost;
}

vector<edge> es;
int V,E; //頂点数、辺数
const int N = 1e5+10;

struct UF{
  const int N = 1e5+10;
  vector<int> par; //親
  vector<int> depth; //木の深さ

  void init(){
    par.resize(N);
    depth.resize(N);
    for(int i = 0; i < N; i++){
      par[i] = i;
      depth[i] = 0;
    }
  }

  //木の根を求める
  int root(int x){
    if(par[x] == x){
      return x;
    }
    else {
      return par[x] = root(par[x]);
    }
  }

  //xとyの属する集合を併合
  void unite(int x,int y){
    x = root(x);
    y = root(y);
    if(x == y) return;

    if(depth[x] < depth[y]) par[x] = y;
    else {
      par[y] = x;
      if(depth[x] == depth[y]) depth[x]++;
    }
  }
}uf;

int kruskal(){
  sort(es.begin(),es.end(),comp);
  uf.init();
  int min_cost = 0;
  for(int i = 0; i < es.size(); i++){
    edge e = es[i];
    if(e.cost == -1) continue;
    if(uf.root(e.u) != uf.root(e.v)){
      min_cost += e.cost;
      uf.unite(e.u,e.v);
    }
  }
  return min_cost;
}

signed main(){
  cin.tie(0);
  ios::sync_with_stdio(false);

  cin >> V;
  for(int i = 0; i < V; i++){
    for(int j = 0; j < V; j++){
      int cost; cin >> cost;
      es.push_back({i,j,cost});
    }
  }
  cout << kruskal() << endl;

  return 0;
}

C++: #include <stdio.h>
#include <iostream>
#include <utility>
#include <algorithm>
#include <math.h>
#include <vector>
using namespace std;

class edge
{
  public:
    int i, j, weigt;
    bool operator()(edge e1, edge e2)
    {
        return e1.weigt < e2.weigt;
    }
};
edge e[10000];
int main()
{
    int n;
    cin >> n;
    int b[n][n] = {};
    int k = 0;
    int tmp = 0;
    int sum = 0;
    for (int i = 0; i < n; i++)
    {
        for (int j = 0; j < n; j++)
        {
            cin >> tmp;
            if (j < i + 1 || tmp < 0)
                continue;
            e[k].i = i;
            e[k].j = j;
            e[k].weigt = tmp;
            k++;
        }
    }
    sort(e, e + k, edge());
    for (int i = 0; i < k; i++)
    {
        if (b[e[i].i][e[i].j] == 0)
        {
            sum += e[i].weigt;
            b[e[i].i][e[i].j] = 1;
            b[e[i].j][e[i].i] = 1;
            for (int j = 0; j < n; j++)
            {
                if (b[e[i].i][j] || b[e[i].j][j])
                    b[e[i].i][j] = b[e[i].j][j] = b[j][e[i].i] = b[j][e[i].j] = 1;
            }
        }
    }
    cout << sum << "\n";
    return 0;
}