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>
#include <vector>
#include <stack>
#include <queue>

using namespace std;

static const long long FAR = (1LL << 21);
static const int N = 100;

int n, m[N][N];
bool visited[N];
int d[N], p[N];

int prim(){
	for (int i = 0; i < n; ++i) {
		d[i] = FAR;
		p[i] = -1;
		visited[i] = false;
	}

	d[0] = 0;
	while (true)
	{
		int mi = FAR;
		int u = -1;
		for (int i = 0; i < n; ++i)	{
			if (!visited[i] && d[i] < mi){
				u = i;
				mi = d[i];
			}
		}

		if (-1 == u) break;
		visited[u] = true;

		for (int v = 0; v < n; ++v) {
			if (!visited[v] && m[u][v] < d[v]){
				p[v] = u;
				d[v] = m[u][v];
			}
		}
	}

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

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) ? FAR : e);
		}
	}

	cout << prim() << endl;

	return 0;
}

C++: #include <iostream>
#include <tuple>
#include <vector>
#include <algorithm>

int P[10000];

void init(int N)
{
    for (int i = 0; i < N; i++)
    {
        P[i] = i;
    }
}

int root(int a)
{
    if (P[a] == a)
    {
        return a;
    }
    else
    {
        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, weight = 0;
    std::cin >> n;
    init(n);
    std::vector<std::tuple<int, int, int>> edge;
    for (int i = 0; i < n; i++)
    {
        for (int j = 0; j < n; j++)
        {
            int a;
            std::cin >> a;
            if (j >= i + 1 && a != -1)
            {
                edge.push_back(std::make_tuple(a, i + 1, j + 1));
            }
        }
    }
    sort(edge.begin(), edge.end());
    for (int i = 0; i < edge.size(); i++)
    {
        //        std::cout << std::get<0>(edge.at(i)) << " " << std::get<1>(edge.at(i)) << " " << std::get<2>(edge.at(i)) << std::endl;
        if (is_same_set(std::get<1>(edge.at(i)), std::get<2>(edge.at(i))) == 0)
        {
            weight += std::get<0>(edge.at(i));
            unite(std::get<1>(edge.at(i)), std::get<2>(edge.at(i)));
        }
    }
    std::cout << weight << std::endl;
    return 0;
}

C++: #include<cstdio>
static const int kMaxVertices = 100;
static const int kInfinity = 3000;
static int Adjacency_matrix[kMaxVertices][kMaxVertices];

void Prim(int n){
	bool has_not_been_discovered[kMaxVertices];
	int lowest_cost_edge[kMaxVertices];
	int parent[kMaxVertices];
	for (int i = 0; i < n; ++i)	{
		has_not_been_discovered[i] = true;
		lowest_cost_edge[i] = kInfinity;
	}
	lowest_cost_edge[0] = 0;
	parent[0] = -1;

	while(true) {
		int mincost = kInfinity;
		int vertex_to_add;
		for (int i = 0; i < n; ++i)	{
			if(has_not_been_discovered[i] && lowest_cost_edge[i] < mincost){
				mincost = lowest_cost_edge[i];
				vertex_to_add = i;
			}
		}

		if(mincost == kInfinity)
			break;

		has_not_been_discovered[vertex_to_add] = false;
		int vertex_added = vertex_to_add;

		for (int i = 0; i < n; ++i)	{
			if(has_not_been_discovered[i] && Adjacency_matrix[vertex_added][i] < lowest_cost_edge[i]) {
				lowest_cost_edge[i] = Adjacency_matrix[vertex_added][i];
				parent[i] = vertex_added;
			}
		}
	}
	int total_cost = 0;
	for (int i = 0; i < n; ++i)
		total_cost += lowest_cost_edge[i];
	printf("%d\n", total_cost);
}

int main() {
	int number_of_vertices;
	scanf("%d", &number_of_vertices);
	for (int vertex_from = 0; vertex_from < number_of_vertices; ++vertex_from) {
		for (int vertex_to = 0; vertex_to < number_of_vertices; ++vertex_to)	{
			int tmp;
			scanf("%d", &tmp);
			Adjacency_matrix[vertex_from][vertex_to] = (tmp < 0) ? kInfinity : tmp;
		}
	}
	Prim(number_of_vertices);
	return 0;
}

C++: #include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <iostream>
#include <algorithm>
#define N 110
#define INF 0x3f3f3f3f
using namespace std;
typedef struct {
    int w;
    int s;
    int e;
}Edge;
Edge edge[N*N];
int m[N][N];
int father[N];
int n;
int k = 0;  //edge的下标
int result = 0;
int findfather(int x){
    return x == father[x] ? x : findfather(father[x]);
}
int cmp(Edge a, Edge b){
    return a.w < b.w;
}
void Kruscal(){
    for(int u = 0; u < k; u++){
        int s = edge[u].s;
        int e = edge[u].e;
        if(findfather(s) != findfather(e)){
            father[findfather(e)] = s;
            result += edge[u].w;
        }
    }
    printf("%d\n", result);
}
int main(){

    scanf("%d", &n);
    for(int i = 1; i <= n; i++) father[i] = i;
    for(int i = 1; i <= n; i++){
        for(int j = 1; j <= n; j++){
            int b;
            scanf("%d", &b);
            m[i][j] = b;
        }
    }
    for(int i = 1; i <= n; i++){
        for(int j = i; j <= n ;j++){
            if(m[i][j] != -1) {
                edge[k].w = m[i][j];
                edge[k].s = i;
                edge[k].e = j;
                k++;
            }
        }
    }
    sort(edge, edge+k, cmp);
    Kruscal();
    return 0;
}

C++: #include <iostream>
#include <utility>
#include <algorithm>
using namespace std;
const int MAX = 10005;
int P[MAX];
pair<int, pair<int,int> > C[MAX]; //<重み,<始点,終点>>

void init()
{
	for (int i = 0; i < MAX; ++i)
	{
		P[i] = i;
	}
}

int root(int a)
{
	if (a == P[a])
	{
		return a;
	}
	else
	{
		root(P[a]);
	}
}

bool same(int a, int b)
{
	if (root(a) == root(b))
	{
		return true;
	}
	else
	{
		return false;
	}
}

void unite(int a, int b)
{
	P[root(b)] = root(a); 
}

int main()
{
	int n;
	cin >> n;
	int cnt = 0;
	int temp;
	for (int i = 0; i < n; ++i)
	{
		for (int j = 0; j < n; ++j)
		{
			cin >> temp;
			if (temp != -1)
			{
				C[cnt].first = temp;
				C[cnt].second.first = i;
				C[cnt].second.second = j;
				cnt++;
			}
		}
	}

	init();
	int ans = 0;
	sort(C,C+cnt);
	for (int i = 0; i < cnt; ++i)
	{
		if (same(C[i].second.first,C[i].second.second) == false)
		{
			ans += C[i].first;
			unite(C[i].second.first,C[i].second.second);
		}
	}

	cout << ans << endl;
	return 0;
}