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<bits/stdc++.h>
using namespace std;
#define rep(i, n) for(int i = 0; i < (int)(n); i++)

static const int MAX = 100;
static const int INFTY = (1 << 21);
static const int WHITE = 0;
static const int GRAY = 1;
static const int 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] = INFTY;
        p[i] = -1;
        color[i] = WHITE;
    }

    d[0] = 0;

    while(1){
        u = -1;
        minv = INFTY;

        //到達可能で最も近いものを探索
        for(int i = 0; i < n; i++){
            if(color[i] != BLACK && d[i] < minv){
                u = i;
                minv = d[i];
            }
        }
        
        //全て到達したらbreak
        if(u == -1) break;

        //1接点を確定
        color[u] = BLACK;

        //到達可能な接点との距離を更新
        for(int v = 0; v < n; v++){
            if(color[v] != BLACK && M[u][v] != INFTY){
                if(M[u][v] < d[v]){
                    d[v] = M[u][v];
                    color[v] = GRAY; //この行はなくても機能しそう
                    p[v] = u;
                }
            }
        }

    }
    //MSTのコストを計算
    int sum = 0; 
    for(int v = 0; v < n; v++){
        if(p[v] != -1) sum += M[p[v]][v];
    }
    return sum;
}

int main(){
    cin >> n;

    rep(i,n)rep(j,n){
        int e;
        cin >> e;
        M[i][j] = (e == -1) ? INFTY : e;
    }

    cout << prim() << endl;
    
    return 0;
}

C++: #include <iostream>
using namespace std;
static const int MAX = 100;
static const int INFTY = (1<<21);

enum Color {
  WHITE, GRAY, BLACK
};

int M[MAX][MAX];
int d[MAX];
int p[MAX];
Color color[MAX];
int n;

void init() {
  for (int i = 0; i < n; i++) {
    color[i] = WHITE;
    d[i] = INFTY;
  }
}

void prim() {
  init();
  d[0] = 0;
  p[0] = -1;

  while (true) {
    int mincost = INFTY;
    int u;
    for (int i = 0; i < n; i++) {
      if (color[i] != BLACK && d[i] < mincost) {
        mincost = d[i];
        u = i;
      }
    }
    if (mincost == INFTY) break;

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

int main() {
  cin >> n;
  for (int i = 0; i < n; i++) {
    for (int j = 0; j < n; j++) {
      cin >> M[i][j];
    }
  }
  prim();
  int result = 0;
  for (int i = 0; i < n; i++) {
    result += d[i];
  }
  cout << result << endl;
  return 0;
}

C++: #include <iostream>
using namespace std;
#define N 100
static const int white=0;
static const int gray=1;
static const int black=2;
static const int infty=(1<<21);

int n,M[N][N];

int prim(){
    int u,minv;
    int d[N],p[N],color[N];
    
    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(){
    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)?infty:e;
        }
    }
    cout<<prim()<<endl;
    
    return 0;
}

C++: #include<iostream>
#include<cstdio>
#include<string>
#include<algorithm>
#include<cstring>
using namespace std;
#define int(x) int x; scanf("%d",&x)
#define input(x) scanf("%d",&x)
#define rep(x,n) for(int x = 0; x < n; x++)
#define _br printf("\n")
#define INF 2000000000

bool complete[100];
int d[100];

int rec_msp(int n, int Path[][100]){
    int moP = 0;
    rep(i,n){
        complete[i] = false;
        d[i] = INF;
    }
    d[0] = 0;
    int num = 0;
    
    
    while (num != n) {
    
        int mind = INF;
        int mindi = 0;
        rep(i,n){
            if (mind > d[i] && !complete[i]) {
                mind = d[i];
                mindi = i;
            }
        }
        
        complete[mindi] = true;
        num++;
        
        //update d[] with mindi.
        
        rep(i,n){
            if (Path[mindi][i] != -1 && d[i] > Path[mindi][i] && !complete[i]) {
                d[i] = Path[mindi][i];
            }
        }
        
    }
    
    int sum = 0;
    rep(i,n) sum += d[i];
    
    return sum;
}

int main(){
    int(n);
    int Path[100][100];
    rep(r,n){
        rep(c,n) input(Path[r][c]);
    }
    
    printf("%d\n",rec_msp(n,Path));
    
    return 0;
}

C++: #include<iostream>
using namespace std;
#define N 105
#define INF 1<<21
int n;
int a[N][N];
int d[N],p[N],color[N];              //0.未访问的节点   1.正在访问的节点  2.已经访问过的节点 
int prim(){
	int u ,minv;
	for (int i = 0;i < n;i++){
		d[i] = INF;
		p[i] = -1;
		color[i] = 0; 
	}
	d[0] = 0;
	while (1){
		minv = INF;
		u = -1;
		for (int i = 0;i < n;i++){
			if (minv > d[i] && color[i] != 2){
				u = i;
				minv = d[i]; 
			}
		}
		if ( u == -1) break;
		color[u] = 2;
		for (int v = 0;v < n;v++){
			if (color[v] != 2 && a[u][v] != INF){
				if (d[v] > a[u][v]){
					d[v] = a[u][v];
					p[v] = u;
					color[v] = 1;
				}
			}
		}
	}
	int sum = 0;
	for (int i = 0;i < n;i ++){
		if (p[i] != -1){
			sum = sum + a[i][p[i]];
		}
	}
	return sum;
}

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