Go: package main

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

// Define disjoint-set forests

type vertexNo int

type vertex struct {
	number vertexNo
	rank int
	parent *vertex
}

func makeSet(number vertexNo) vertex {
	return vertex {
		number: number,
		rank: 0,
		parent: nil,
	}
}

func union(x, y *vertex) *vertex {
	return link(findSet(x), findSet(y))
}

func link(x, y *vertex) *vertex {
	if x.rank > y.rank {
		y.parent = x
		return x
	} else {
		x.parent = y
		if x.rank == y.rank {
			y.rank += 1
		}
		return y
	}
}

func findSet(v *vertex) *vertex {
	if v.parent == nil {
		return v
	}
	v.parent = findSet(v.parent)
	return v.parent
}

// Function to calculate minimum spanning tree

type matrix [][]int

type edge struct {
	one vertexNo
	two vertexNo
	weight int
}

const(
	MAX_VERTICES = 100
)

func newEdgesFromMatrix(matrix matrix) []edge {
	edges := make([]edge, 0, MAX_VERTICES * MAX_VERTICES)

	for i := 0; i < len(matrix); i++ {
		row := matrix[i]
		for j := i; j < len(row); j++ {
			if row[j] >= 0 {
				edge := edge{one: vertexNo(i+1), two: vertexNo(j+1), weight: row[j]}
				edges = append(edges, edge)
			}
		}
	}

	return edges
}

func solve(matrix matrix) int {
	nVertices := len(matrix)

	// initialize vertices
	vertices := make([]vertex, nVertices)
	for i := 0; i < nVertices; i++ {
		vertices[i] = makeSet(vertexNo(i+1))
	}

	// prepare edges sorted in the ascending order of weight
	edges := newEdgesFromMatrix(matrix)
	sort.Slice(edges, func(i, j int) bool {
		return edges[i].weight < edges[j].weight
	})

	unionCount := 0
	weightSum := 0
	for unionCount < nVertices-1 {
		for i := 0; i < len(edges); i++ {
			edge := edges[i]
			v1 := &vertices[edge.one-1]
			v2 := &vertices[edge.two-1]
			rep1 := findSet(v1)
			rep2 := findSet(v2)
			if rep1.number == rep2.number {
				continue
			}
			union(rep1, rep2)
			edges = append(edges[0:i], edges[i+1:]...)
			weightSum += edge.weight
			unionCount++
			break
		}
	}

	return weightSum
}

// main

func main() {
	sc := bufio.NewScanner(os.Stdin)
	sc.Split(bufio.ScanWords)

	n, err := nextInt(sc)
	if err != nil {
		panic(err)
	}

	matrix := parseMatrix(n, sc)

	edges := newEdgesFromMatrix(matrix)
	sort.Slice(edges, func(i, j int) bool {
		return edges[i].weight < edges[j].weight
	})

	fmt.Println(solve(matrix))
}

func parseMatrix(n int, sc *bufio.Scanner) [][]int {
	matrix := make([][]int, n)
	for i := 0; i < n; i++ {
		matrix[i] = make([]int, n)
		for j := 0; j < n; j++ {
			x, err := nextInt(sc)
			if err != nil {
				panic(err)
			}
			matrix[i][j] = x
		}
	}
	return matrix
}

func nextText(sc *bufio.Scanner) string {
	sc.Scan()
	return sc.Text()
}

func nextInt(sc *bufio.Scanner) (int, error) {
	return strconv.Atoi(nextText(sc))
}

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;
}