Go: package main

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

//util
var sc = bufio.NewScanner(os.Stdin)

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

func nextInt() int {
	sc.Scan()
	i, e := strconv.Atoi(sc.Text())
	if e != nil {
		panic(e)
	}
	return i
}

func printList(list []int) {
	n := len(list)
	buf := bufio.NewWriter(os.Stdout)
	for _, v := range list[0 : n-1] {
		buf.Write([]byte(fmt.Sprintf("%d ", v)))
	}
	buf.Write([]byte(fmt.Sprintf("%d\n", list[n-1])))
	buf.Flush()
}

func max(a, b int) int {
	if a > b {
		return a
	} else {
		return b
	}
}

func min(a, b int) int {
	if a > b {
		return b
	} else {
		return a
	}
}

//solve
const (
	MAX   = 100
	INFTY = (1 << 21)
	WHITE = 0
	GRAY  = 1
	BLACK = 2
)

//V: グラフの頂点全体の集合
//T: MSTに属する頂点の集合
var (
	n     int
	M     [MAX][MAX]int //重みを記録する隣接行列
	color [MAX]int
	p     [MAX]int //MSTにおける頂点の親を記録
	d     [MAX]int //Tに属する頂点と、V−Tに属する頂点を繋ぐ辺の中で、重みが最小のものを記録
)

func prim() int {
	var u, minv int
	//初期化
	for i := 0; i < n; i++ {
		d[i] = INFTY
		p[i] = -1
		color[i] = WHITE
	}

	d[0] = 0

	for {
		//頂点uを決めていく
		//つまり、Tに属する頂点と、V−Tに属する頂点を繋ぐ辺の中で、重みが最小のものを決める
		minv = INFTY
		u = -1
		for i := 0; i < n; i++ {
			//未訪問かつ、辺の重みが最小となるuを選ぶ
			if minv > d[i] && color[i] != BLACK {
				u = i
				minv = d[i]
			}
		}

		if u == -1 {
			break
		}

		color[u] = BLACK
		for v := 0; v < n; v++ {
			//未訪問かつ、辺が存在するとき
			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
				}
			}
		}
	}

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

func main() {
	sc.Split(bufio.ScanWords)
	n = nextInt()

	for i := 0; i < n; i++ {

		for j := 0; j < n; j++ {
			tmp := nextInt()
			if tmp == -1 {
				tmp = INFTY
			}
			M[i][j] = tmp
		}

	}

	fmt.Println(prim())

}

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

struct edge
{
    int u, v, cost;
};

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

int n, E, A[100][100];
int p[10010];
vector<edge> es(0);

void init(int n)
{
    rep(i, n) 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 kruskal()
{
    int sum = 0;
    E = 0;
    rep(i, n) rep(j, i)
    {
        if (A[i][j] != -1)
        {
            E++;
            edge e;
            e.u = i;
            e.v = j;
            e.cost = A[i][j];
            es.push_back(e);
        }
    }
    sort(es.begin(), es.end(), comp);
    rep(i, E)
    {
        edge e = es[i];
        if (!is_same_set(e.u, e.v))
        {
            unite(e.u, e.v);
            sum += e.cost;
        }
    }
    return sum;
}

int main()
{
    cin >> n;
    rep(i, n) rep(j, n) cin >> A[i][j];
    init(n);
    int ans = kruskal();
    cout << ans << endl;
    return 0;
}

C++: #include<iostream>
using namespace std;
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){
    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 <vector>
#include <string>
#include <algorithm>
#include <utility>
#include <numeric>
#include <array>
#include <cmath>
#include <memory>
using namespace std;
//変数デバッグ
#define DEB(variable) cout << #variable << '=' << variable << endl

//for簡易表記(引数ミス防止)
#define FOR(LoopVariable,numberOFbegin,numberOFend) for (int LoopVariable = (numberOFbegin); (LoopVariable) < (numberOFend); (LoopVariable)++)
#define DEFOR(LoopVariable,numberOFbegin,numberOFend) for (int LoopVariable = (numberOFbegin)-1; (LoopVariable) >= (numberOFend); (LoopVariable)--)
#define REP(LoopVariable,numberOFend) for(int LoopVariable = 0;(LoopVariable)<(numberOFend);LoopVariable++)

constexpr long long int linf = numeric_limits<long long int>::max()-5;
constexpr int inf= numeric_limits<int>::max()-5;
constexpr long long int mod = 1000000007;



template<typename T>
class Wgraph{
public:
    //隣接行列の入力を受ける
    Wgraph(int _n){
        n = _n;
        mat.resize(n, vector<int>(n));
        colors.resize(n);
        min_weis.resize(n);
        parents.resize(n);
        REP(i,n){
            REP(j,n){
                scanf("%d", &mat[i][j]);
                if(mat[i][j]==-1){
                    mat[i][j] = inf;
                }
            }
        }

    }

    long long int prim(){
        REP(i,n){
            colors[i] = 0;
            parents[i] = -1;
            min_weis[i] = inf;
        }
        int u,mincost;
        min_weis[0] = 0;
    

        while(1){
            mincost = inf;
            u = -1;
            REP(i,n){
                if(colors[i]!=2&&min_weis[i]<mincost){
                    mincost = min_weis[i];
                    u = i;
                }
            }

            if(u==-1){
                break;
            }

            colors[u] = 2;
            REP(i,n){
                if(colors[i]!=2){
                    if(mat[u][i]<min_weis[i]){
                        min_weis[i] = mat[u][i];
                        parents[i] = u;
                        colors[i] = 1;
                    }
                }
            }
        }
        int sum = 0;
        REP(i,n){
            if(parents[i]!=-1){
                sum += mat[i][parents[i]];
            }
        }
        return sum;
    }

private:
    vector<vector<int>> mat;
    int n;

    vector<int> colors,min_weis, parents;
};


int main(){

    int n;
    cin>>n;
    unique_ptr<Wgraph<int>> wg(new Wgraph<int>(n));

    cout << wg->prim() << endl;
    return 0;
}

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

constexpr int MAX = 110;
constexpr int INF = 1e9;

int N;
int G[MAX][MAX];
bool seen[MAX];
int par[MAX];
int d[MAX];

int main(){
  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;
    }
  }

  for( int i = 0; i < MAX; i++ ){
    par[i] = -1;
    d[i] = INF;
  }
  d[0] = 0;

  while( 1 ){
    int mincost = INF;
    int u;
    for( int i = 0; i < N; i++ ){
      if( d[i] < mincost && !seen[i] ){
	mincost = d[i];
	u = i;
      }
    }

    if( mincost == INF ) break;
    seen[u] = true;
    
    for( int i = 0; i < N; i++ ){
      if( G[u][i] != INF && !seen[i] ){
	if( G[u][i] < d[i] ){
	  d[i] = G[u][i];
	  par[i] = u;
	}
      }
    }
  }

  int ans = 0;
  for( int i = 1; i < N; i++ ){
    ans += G[par[i]][i];
  }

  cout << ans << endl;
}