Go: package main

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

func getScanner(fp *os.File) *bufio.Scanner {
	scanner := bufio.NewScanner(fp)
	scanner.Split(bufio.ScanWords)
	scanner.Buffer(make([]byte, 500001), 500000)
	return scanner
}

func getNextString(scanner *bufio.Scanner) string {
	scanner.Scan()
	return scanner.Text()
}

func getNextInt(scanner *bufio.Scanner) int {
	i, _ := strconv.Atoi(getNextString(scanner))
	return i
}

func main() {
	fp := os.Stdin
	if len(os.Args) > 1 {
		fp, _ = os.Open(os.Args[1])
	}

	scanner := getScanner(fp)
	writer := bufio.NewWriter(os.Stdout)

	n := getNextInt(scanner)

	g := make([][]int, n)
	ans := make([][]int, n)

	inf := 1 << 30
	for i := 0; i < n; i++ {
		g[i] = make([]int, n)
		ans[i] = make([]int, n)
		for ii := 0; ii < n; ii++ {
			g[i][ii] = getNextInt(scanner)
			if g[i][ii] == -1 {
				g[i][ii] = inf
			}
			ans[i][ii] = -1
		}
	}

	im := make(IntMap, 1)

	im.increment(1)

	for len(im) < n {
		min := inf
		u := -1
		v := -1
		for i := range im {
			for ii := 0; ii < n; ii++ {
				if im.get(ii) != 0 {
					continue
				}
				if min > g[i][ii] {
					min = g[i][ii]
					u = i
					v = ii
				}
			}
		}
		im.increment(v)
		ans[u][v] = min
		ans[v][u] = min
	}

	sum := 0
	for i := 0; i < n-1; i++ {
		for ii := i + 1; ii < n; ii++ {
			if ans[i][ii] != -1 {
				sum += ans[i][ii]
			}
		}
	}

	fmt.Fprintln(writer, sum)
	writer.Flush()
}

// IntMap ...
type IntMap map[int]int

func (m IntMap) get(key int) int {
	if _, ok := m[key]; ok {
		return m[key]
	}
	return 0
}
func (m IntMap) increment(key int) {
	if _, ok := m[key]; ok == false {
		m[key] = 0
	}
	m[key]++
}

C++: 
#include<bits/stdc++.h>

using namespace std;

const int maxn=1e2+5;
const int inf=(1<<21);
const int white=0;
const int gray=1;
const int black=2;

int n,m[maxn][maxn];

int prim()
{
    int u,minv,sum=0;
    int d[maxn],p[maxn],color[maxn];
    for(int i=0;i<n;i++){
        d[i]=inf;
        color[maxn]=white;
        p[i]=-1;
    }
    d[0]=0;
    while(1){
        minv=inf;
        u=-1;
        for(int i=0;i<n;i++){
            if(minv<d[i]&&color[i]==white){
                minv=d[i];
                u=i;
            }
        }
        if(u==-1){
            break;
        }
        color[u]=black;
        sum+=d[u];
        for(int i=0;i<n;i++){
            if(color[i]!=black&&d[i]<m[u][i]){
                d[i]=m[u][i];
                p[i]=u;
                color[i]=gray;
            }
        }
    }
}

int main()
{
    scanf("%d",&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;
}
/*
5
-1 2 3 1 -1
2 -1 -1 4 -1
3 -1 -1 1 1
1 4 1 -1 3
-1 -1 1 3 -1
*/

C++: #include <iostream>
#include <vector>
#include <algorithm>
#include <utility>
using namespace std;
int P[1010];
int cost[5000];
int src[5000];	
int dst[5000];
int A[100 + 10][100 + 10];
int M = 0;
vector<pair<int,int>> edges;

void init(int N)
{
    for(int i = 1; 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 = 0;
	cin >> n; 
	for(int r = 0; r < n; ++r)
	{
		for(int c = 0; c < n; ++c)
		{
			cin >> A[r][c];
		}
	}
	for (int r = 0; r < n; ++r)
	{
		for(int c = r + 1; c < n; ++c)
		{
			if(A[r][c] > -1)
			{
				cost[M] = A[r][c];
				src[M] = r;
				dst[M] = c;
				edges.push_back(make_pair(cost[M],M));
				M += 1;
			}
		}
	}
	sort(edges.begin(),edges.end());
	int sum = 0;
	init(n);
	for(int i = 0; i < M; ++i)
	{
		if(is_same_set(src[edges[i].second], dst[edges[i].second]) == 1) continue;
			unite(dst[edges[i].second],src[edges[i].second]);
			sum += edges[i].first;
	}
	cout << sum << endl;
}

C++: #include <iostream>
#include <queue>

using ll = long long;
using namespace std;
int n, p, A[100][100];
int d[100], P[100], c[100];
/* initialize */

int bfs(){
    int u, minv;
    int sum;
    for (int i = 0; i < n; ++i) {
        d[i] = 1<<30;
        P[i] = -1;
        c[i] = 0;
    }
    d[0] = 0;
    
    while(true) {
        minv = 1 << 30;
        u = -1;
        for (int i = 0; i < n; ++i) {
            if (minv > d[i] && c[i] != 2) {
                u = i;
                minv = d[i];
            }
        }
        if (u == -1)break;
        c[u] = 2;
        for (int j = 0; j < n; ++j) {
            if (c[j] != 2 && A[u][j] != 1 << 30) {
                if (d[j] > A[u][j]) {
                    d[j] = A[u][j];
                    P[j] = u;
                    c[j] = 1;
                }
            }
        }
    }
        sum = 0;
        for (int i = 0; i < n; ++i) {
            if (P[i] != -1) sum += A[i][P[i]];
        }
    return sum;
}
/* main */
int main() {
    cin >> n;
    for (int i = 0; i < n; ++i) {
        for (int j = 0; j < n; ++j) {
            cin >> p;
            if (p == -1) A[i][j] = 1<<30;
            else A[i][j] = p;
        }
    }
    cout << bfs() << "\n";
}

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(M[u][v]<d[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 b;cin>>b;
            M[i][j]=(b==-1)? INFTY:b;
        }
    }
    
    cout << prim() << endl;
    
    return 0;
}

C++: #include <cstdio>
using namespace std;
#define MAXN 100
#define INFTY (1 << 21)
#define WHITE 0
#define GRAY 1
#define BLACK 2

int n, M[MAXN][MAXN];

int prim()
{
	int d[MAXN], p[MAXN], color[MAXN];
	int u, minv;
	for(int i = 0; i < n; i++)
	{
		d[i] = INFTY;
		p[i] = -1;
		color[i] = WHITE;
		//printf("===\n");
	}
	d[0] = 0;
	while(true)
	{
		u = -1;
		minv = INFTY;
		for(int i = 0; i < n; i++)
		{
			if(color[i] != BLACK && d[i] < minv)
			{
				u = i;
				minv = d[i];
				//printf("%d \n", u);
			}
		}
		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;
}