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>
using namespace std;
#define MAX 100
#define INF (1<<21)
#define WHITE 0
#define GRAY 1
#define BLACK 2

int n,M[MAX][MAX];

int prim(){
  int u,minv;
  int d[MAX],p[MAX],color[MAX];

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

  d[0] = 0;

  while(1){
    minv = INF;
    u=-1;
    for(int i=0;i<n;i++){
      if(minv > d[i] && color[i] != BLACK){
	u=i;
	minv = d[i];
      }
    }
    if(u==-1) break;
    color[u] = BLACK;
    for(int v=0;v<n;v++){
      if(color[v] != BLACK && M[u][v] != INF){
	if(d[v] > M[u][v]){
	  d[v] = M[u][v];
	  p[v] = u;
	  color[v] =GRAY;
	}
      }
    }
  }
  int sum = 0;
  for(int i = 0;i<n;i++){
    if(p[i] != -1)sum += M[i][p[i]];
  }
  return sum;
}

int main(){
  cin >> n;

  for(int i=0;i<n;i++){
    for(int j = 0;j<n;j++){
      int e;
      cin >> e;
      M[i][j] = (e == -1) ? INF : e;
    }
  }
  cout << prim() << endl;
  
  return 0;
}

C++: #include<cstdio>
#include<queue>
using namespace std;

static const int MAX = 100;
static const int INFTY = (1 << 30);
static const int WHITE = 0;
static const int GRAY = 1;
static const int BLACK = 2;

int color[MAX];
int M[MAX][MAX];
int d[MAX];
int p[MAX];
int n;
int prim()
{
	int u, minv;
	for(int i=0; i<n; i++)
	{
		d[i] = INFTY;
		p[i] = -1;
		color[i] = WHITE;
	}

	d[0] = 0;

	while(1)
	{
		minv = INFTY;
		u = -1;
		for(int i=0; i<n; i++)
		{
			if(minv > d[i] && color[i] != BLACK)
			{
				u = i;
				minv = d[i];
			}
		}

		if(u == -1) break;
		color[u] = BLACK;
		for(int v=0; v<n; v++)
		{
			if(color[v] != BLACK && M[u][v] != INFTY)
			{
				if(d[v] > M[u][v]){
					d[v] = M[u][v];
					p[v] = u;
					color[v] = GRAY;
				}
			}
		}
	}

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

int main()
{
	scanf("%d", &n);

	for(int i=0; i<n; i++)
	{
		for(int j=0; j<n; j++)
		{
			int e;
			scanf("%d", &e);
			M[i][j] = (e == -1) ? INFTY:e;
		}
	}

	printf("%d\n", prim());

	return 0;
}

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

typedef long long ll;

#define MAX_V 100
#define INF int(1e9)

// g : ??°??????, V : g???????????°, s : ????????????????????????
int Prim (int g[MAX_V + 1][MAX_V + 1], int V, int s) {
    ll ret = 0, min_, nxtv, v = s, size_;
    // bool used[V] = {false}; // c++11??????
    bool used[V]; memset(used, 0, sizeof(used)); // c++11??\???
    vector<int> usedv;
    usedv.push_back(s);
    used[s] = true;

    while (usedv.size() < V) {
        min_ = INF;
        size_ = usedv.size();

        // ?????¨???????????????????????°?????????????????\?¶????????????????????°????????????¢???
        for (int i = 0; i < size_; ++i) {
            for (int j = 0; j < V; ++j) {
                if (usedv[i] == j) continue;
                if (!used[j] && min_ > g[usedv[i]][j]) {
                    min_ = g[usedv[i]][j];
                    v = j;
                }
            }
        }

        ret += min_;
        usedv.push_back(v);
        used[v] = true;
    }

    return ret;
}

int n, g[MAX_V + 1][MAX_V + 1];

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

    cin >> n;
    for (int i = 0; i < n; ++i) {
        for (int j = 0; j < n; ++j) {
            cin >> g[i][j];
            if (g[i][j] == -1) g[i][j] = INF;
        }
    }

    cout << Prim(g, n, 0) << endl;


    return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;
static const int MAX=100;
static const int INF=(1<<21);
static const int WHI=0;
static const int GRA=1;
static const int BLA=2;
int n,M[MAX][MAX];
int prim(){
  int u,minv;
  int d[MAX],p[MAX],color[MAX];
  for(int i=0;i<n;i++){
    d[i]=INF;
    p[i]=-1;
    color[i]=WHI;
  }
  d[0]=0;
  while(1){
    minv=INF;
    u=-1;
    for(int i=0;i<n;i++){
      if(minv>d[i]&&color[i]!=BLA){
	u=i;
	minv=d[i];
      }
    }
    if(u==-1) break;
    color[u]=BLA; //ここで訪問に記録
    for(int v=0;v<n;v++){
      if(color[v]!=BLA&&M[u][v]!=INF){
	if(M[u][v]<d[v]){
	  d[v]=M[u][v];
	  p[v]=u;
	  color[v]=GRA;
	}
      }
    }
  }
  int sum=0;
  for(int i=0;i<n;i++){
    if(p[i]!=-1) sum+=M[i][p[i]];
  }
  return sum;
}
int main(){
  cin>>n;
  for(int i=0;i<n;i++){
    for(int j=0;j<n;j++){
      int e;
      cin>>e;
      if(e==-1){
	M[i][j]=INF;
      }else{
	M[i][j]=e;
      }
    }
  }
  cout<<prim()<<endl;
  return(0);
}

C++: #include <bits/stdc++.h>
using namespace std;
#define N 100
int G[N][N];
int n;

int prim()
{
    int visited[n];
    int d[n];
    int p[n];

    for (int i; i < n; i++)
    {
        visited[i] = 0;
        d[i] = 114514;
    }

    d[0] = 0;
    p[0] = -1;

    while (true)
    {
        int mincost = 114514;
        int u;
        for (int i = 0; i < n; i++)
        {
            if (!visited[i] && d[i] < mincost)
            {
                mincost = d[i];
                u = i;
            }
        }

        if (mincost == 114514)
            break;

        visited[u] = 1;

        for (int i = 0; i < n; i++)
        {
            if (!visited[i] && G[u][i] != -1)
            {
                if (G[u][i] < d[i])
                {
                    d[i] = G[u][i];
                    p[i] = u;
                }
            }
        }
    }

    int sum = 0;
    for (int i = 1; i < n; i++)
        sum += d[i];
    return sum;
}

int main()
{
    cin >> n;
    int a;
    for (int i = 0; i < n; i++)
    {
        for (int j = 0; j < n; j++)
        {
            cin >> a;
            if (a == -1)
                G[i][j] = 114514;
            else
                G[i][j] = a;
        }
    }
    cout << prim() << endl;
}