Go: package main

import (
	"bufio"
	"fmt"
	"log"
	"math"
	"os"
	"strconv"
	"strings"
)

type color int

const (
	white color = iota // 未訪問
	mst                // MSTとして確定
)

// 頂点
type node struct {
	matrix   []int // 隣接行列
	minToMst int   // MSTへの重み最小値
	parent   int   // 最小値の相手
	color    color
}

func main() {
	scanner := bufio.NewScanner(os.Stdin)
	n := eGetInt(scanner)

	nodes := make([]node, n)
	for i := 0; i < n; i++ {
		nodes[i].matrix = eAtoi((strings.Fields(eGetline(scanner))))
		nodes[i].parent = -1
		nodes[i].minToMst = math.MaxInt16
		nodes[i].color = white
	}

	// 初回はnode[0]がMSTに入るように,minToMstを設定
	nodes[0].minToMst = 0
	for { // 全部チェックするまで

		// どの頂点を新たにMSTに取り込むか
		min := math.MaxInt16
		u := -1
		for i := 0; i < n; i++ {
			if nodes[i].color == mst {
				continue
			}
			if nodes[i].minToMst < min {
				u = i
				min = nodes[i].minToMst
			}
		}

		if u == -1 {
			break
		}
		// MSTに追加する頂点決定
		nodes[u].color = mst

		// MSTに属していない頂点それぞれについて,MSTとの最小の重みを更新
		for i := 0; i < n; i++ {
			if nodes[i].color == mst {
				continue
			}
			if nodes[i].matrix[u] == -1 {
				// 接してない
				continue
			}
			if nodes[i].matrix[u] < nodes[i].minToMst {
				// node[u]との重みが最小
				nodes[i].minToMst = nodes[i].matrix[u]
				nodes[i].parent = u
			}
		}
	}

	sum := 0
	for _, v := range nodes {
		// node[0]はparentが入っていない
		if v.parent != -1 {
			sum += v.matrix[v.parent]
		}
	}
	fmt.Println(sum)
}

func eGetInt(scanner *bufio.Scanner) int {
	t := eAtoi(strings.Fields(eGetline(scanner)))
	return t[0]
}

func eGetline(p *bufio.Scanner) string {
	if !p.Scan() {
		if err := p.Err(); err != nil {
			log.Fatal(err.Error())
		}
		return ""
	}
	return p.Text()
}

func eAtoi(s []string) []int {
	ints := make([]int, len(s))
	var err error
	for i, e := range s {
		ints[i], err = strconv.Atoi(e)
		if err != nil {
			log.Fatal(err.Error())
		}
	}
	return ints
}

C++: #include <iostream>
#include <utility>
#include <vector>
#include <algorithm>


using namespace std;

int P[110];

void init(int N){
    for(int i = 0; i < N; i++) P[i] = i;
}

int root(int a){
    if (P[a] == a) return a;
    return (P[a] = root(P[a]));
}

bool is_same_set(int a, int b){
    return root(a) == root(b);
}

void unite(int a, int b){
    P[root(a)] = root(b);
}

int main(){
    int n; 
    cin >> n; 
    int w;
    init(n);
    vector< pair<int, pair<int, int> > > edges;
    for (int i = 0; i < n; i++){
        for (int j = 0; j < n; j++){
            cin >> w;
            if (i > j && w != -1) edges.push_back(make_pair(w, make_pair(i, j)));
        }
    }
    sort(edges.begin(),edges.end());
    int sum = 0;
    while(!edges.empty()){
        pair<int, pair<int, int> > edge = edges[0];
        edges.erase(edges.begin());
        int cost = edge.first;
        int src = edge.second.first;
        int dst = edge.second.second;
        //cout << cost << " " << src << " " << dst << endl;
        if (!is_same_set(src, dst)){
            //cout << "unite " << src << " " <<  dst << " cost = " << cost << endl;
            unite(src, dst);
            sum += cost;
        }
    }
    cout << sum << endl;
}

C++: #include <iostream>
#include <vector>
#include <utility>
#include <algorithm>
using namespace std;

void init(int n, vector<int>& v){
  v.resize(n);
  for(int i = 0; i < n; i++){
    v[i] = i;
  }
}

int root(int a, vector<int>& v){
  if(v[a] == a) return a;
  return (v[a] = root(v[a], v));
}

bool same(int a, int b, vector<int>& v){
  return root(a,v) == root(b,v);
}

void unite(int a, int b, vector<int>& v){
  v[root(a,v)] = root(b,v);
}


int main(){
  int n;
  cin >> n;
  vector<vector<int> > a(n, vector<int>(n, -1));
  for(int i = 0; i < n; i++)
    for(int j = 0; j < n; j++)
      cin >> a[i][j];

  vector<int> cost, src, dst;
  for(int i = 0; i < n; i++){
    for(int j = i+1; j < n; j++){
      if(a[i][j] > -1){
        cost.push_back(a[i][j]);
        src.push_back(i);
        dst.push_back(j);
      }
    }
  }
  vector<pair<int, int> > edges(cost.size());

  for(int i = 0; i < cost.size(); i++){
    // cout << "cost" << cost[i] << endl;
    // cout << "i" << i << endl;
    edges[i] = make_pair(cost[i], i);
    // cout << edges[i].first << " : " << edges[i].second << endl;
  }

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

  // for(int i = 0; i < cost.size(); i++)
    // cout << edges[i].first << " : " << edges[i].second << endl;

  int sum = 0;
  vector<int> units;
  init(n, units);
  for(int i = 0; i < edges.size(); i++){
    int idx = edges[i].second;
    int s = src[idx], d = dst[idx];
    // cout << s << "," << d << endl;
    if(same(s, d, units))
      continue;
    unite(s, d, units);

    sum += cost[idx];
  }

  cout << sum << endl;
}

C++: #include <iostream>
#include <algorithm>
#include <string>
#include <stack>
#include <cstdlib>
#include <queue>
#include <vector>
#include <set>
#include <stdio.h>
#include <cstdio>
using namespace std;
int p[100001],s[100001],t[10001],cost[10001];
int root(int a) {
	if (p[a] == a) return a;
	return (p[a] = root(p[a]));
}
void unite(int a,int b){
	p[root(a)] = root(b);
}
bool same(int a ,int b){
	if (root(a) == root(b)) return 1;
	else return 0;
}
bool rcheck(int a){
	if (root(a) == a) return 1;
	else return 0;
}
pair <int,int> path[1001];
int main() {
	int n,coun = 0;
	cin >> n;
	for (int i = 0; i < n; i++){
		p[i] = i;
		for (int j = 0;j<n;j++) {
			s[n*i+j] = i; t[n*i+j]=j; 
			cin >> cost[n*i+j];
		}
	}
	for (int i = 0;i < n*n;i++) {
		path[i].first = cost[i];
		path[i].second = i;
	}
	sort(path,path+n*n);
	for (int i = 0; i < n*n;i++) {
		if (path[i].first > 0) {
			if(same(s[path[i].second],t[path[i].second]) == 0) {
				unite(s[path[i].second],t[path[i].second]);
				coun += path[i].first;
			}
		}
	}
	cout << coun << endl;
}

C++: #include <cstdio>
#include <iostream>
#include <vector>
#include <queue>
using namespace std;
int n;
int a[100][100];
int used[100];
int mincost[100];
const int INF=1000000000;
int prim(){
  for(int i=0;i<n;i++){
    used[i]=0;
    mincost[i]=INF;
  }
  mincost[0]=0;
  int res=0;
  while(1){
    int v=-1;
    for(int u=0;u<n;u++){
      if((v==-1 || mincost[v]>mincost[u]) && !used[u])v=u;
    }
    if(v==-1)break;
    used[v]=1;
    res+=mincost[v];
    for(int u=0;u<n;u++){
      mincost[u]=min(mincost[u],a[v][u]);
    }
  }
  return res;
}
int main(){
  scanf("%d",&n);
  for(int i=0;i<n;i++){
    for(int j=0;j<n;j++){
      scanf("%d",&a[i][j]);
      if(a[i][j]==-1)a[i][j]=INF;
    }
  }
  printf("%d\n",prim());
  return 0;
}

C++: #include <cstdio>
#include <iostream>
#include <algorithm>
#include <cstring>
#include <cstdlib>
#include <queue>
#include <stack>
using namespace std;
const int maxx=110;
const int infinity=(1<<30);

int A[maxx][maxx];
int d[maxx],p[maxx];//d[i]里面存的是V-T中的点，要加入到T中，选取的最小的权值；
					//d[i]=u;表示的是要把V-T中的点i加入到T中所需要的代价是u；
					//p[v]=u;表示的是V-T中的点v，要和T中的点u连接，即u是v的父节点；
bool flag[maxx];	//用flag来标记这个节点是不是在MST中，true是在T里，false是在V-T里；

int main (){
		int n;
		cin>>n;
		for(int i =1;i<=n;i++){
			for(int j=1;j<=n;j++){
				int e;cin>>e;
				A[i][j]=(e==-1)?infinity:e;
			}
		}

		for(int i=0;i<maxx;i++) {
			flag[i]=false;
			d[i]=infinity;
			p[i]=-1;
		}
		int sum=0,minv,minj;
		flag[1]=true;
		while(1){
			minv=infinity;
			minj=-1;
			for(int i=1;i<=n;i++){
				if(flag[i]==true){
					for(int j=1;j<=n;j++){
						if(A[i][j]!=-1 && A[i][j]<minv && flag[j]==false) {
							minv=A[i][j];
							minj=j;
						} 
					}
				}
			}
//			cout<<minv<<endl;
//			cout<<minj<<endl;
			if(minv==infinity || minj==-1) break;
			flag[minj]=true;
			sum+=minv;
		}
		
		cout<<sum<<endl;
		return 0;
		
}