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<iostream>

using namespace std;

static const int MAX = 100;
static const int INFTY = (1 << 21);
static const int WHITE = 0;
static const int GREY = 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 <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>
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 (true) {
		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<bits/stdc++.h>
using namespace std;

int n,m[100][100];

int prim();

int main(){
  cin>>n;
  
  for(int i=0;i<n;i++){
    for(int j=0;j<n;j++){
      cin>>m[i][j];
      if(m[i][j]==-1)m[i][j]=200002;
    }
  }

  cout<<prim()<<endl;

}

int prim(){
  int u,mn;
  int d[100],p[100],ck[100];
  
  for(int i=0;i<n;i++){
    d[i]=200002;
    p[i]=-1;
    ck[i]=0;
  }

  d[0]=0;

  while(1){
    mn=200002;
    u=-1;
    for(int i=0;i<n;i++){
      if(mn>d[i] && ck[i]!=2){
	u=i;
	mn=d[i];
      }
    }
    if(u==-1)break;
    ck[u]=2;
    for(int i=0;i<n;i++){
      if(ck[i]!=2 && m[u][i]!=200002){
	if(d[i]>m[u][i]){
	  d[i]=m[u][i];
	  p[i]=u;
	  ck[i]=1;
	}
      }
    }
  }
  int sum=0;
  for(int i=0;i<n;i++){
    if(p[i]!=-1)sum+=m[i][p[i]];
  }
  return(sum);
}

C++: #include<bits/stdc++.h>
#define wgt(x) (x.first)
#define s(x) (x.second.first)
#define t(x) (x.second.second)
#define mwe(w,s,t) make_pair(w,make_pair(s,t))

using namespace std;

typedef pair<int,int> edge;
typedef pair<int,edge> we;

int main()
{
	int n,G[100][100];
	bool used[100];
	scanf("%d",&n);
	for(int i=0;i<n;i++)
		for(int j=0;j<n;j++)
			scanf("%d",&G[i][j]);
	memset(used,false,sizeof(used));
	priority_queue<we,vector<we>,greater<we> > pq;
	int tot=0;
	used[0]=true;
	for(int i=0;i<n;i++)
		if(G[0][i]>=0)
			pq.push(mwe(G[0][i],0,i));
	while(!pq.empty())
	{
		we t=pq.top();pq.pop();
		if(used[t(t)]) continue;
		tot+=wgt(t);
		used[t(t)]=true;
		for(int i=0;i<n;i++)
			if(!used[i]&&G[t(t)][i]>=0)
				pq.push(mwe(G[t(t)][i],t(t),i));
	}
	printf("%d\n",tot);
	return 0;
}