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 <string>
#include <algorithm>
#include <functional>
#include <vector>
#include <utility>
#include <cstring>
#include <iomanip>
#include <numeric>
#include <cmath>
#include <queue>
#define MAX_N 110
using namespace std;
typedef long long ll;
const int INF = 1<<30; 
const int MOD = 1e9 + 7;
const int dx[] = {1, 0, -1, 0};
const int dy[] = {0, 1, 0, -1};
int cost[MAX_N][MAX_N];
int mincost[MAX_N];
bool used[MAX_N];
int n;
int prim()
{
    int res = 0;
    mincost[0] = 0;
    while(true)
    {
        int v = -1;
        for(int u = 0; u < n; u++)
        {
            if(!used[u] && (v == -1 || mincost[u] < mincost[v])) v = u;
        }
        if(v == -1) break;
        used[v] = true;
        res += mincost[v];
        for(int u = 0; u < n; u++) mincost[u] = min(mincost[u], cost[v][u]);
    }
    return res;
}
int main()
{
    cin.tie(0);
    ios::sync_with_stdio(false);
    cin >> n;
    memset(used, false, sizeof(used));
    for(int i = 0; i < n; i++)
    {
        for(int j = 0; j < n; j++)
        {
            cin >> cost[i][j];
            if(cost[i][j] == -1) cost[i][j] = INF;
        }
        mincost[i] = INF;
    }
    int ans = prim();
    cout << ans << endl;
    return 0;
}

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


using namespace std;


using ll = long long;


const ll MAX = 100;


ll adj[MAX][MAX];
ll d[MAX] = {};
bool finished[MAX] = {};
ll n;


void msp()
{
    priority_queue<pair<ll, ll>, vector<pair<ll, ll>>, greater<pair<ll, ll>>> pq;
    pq.push(make_pair(0, 0));

    while (!pq.empty()) {
        auto p = pq.top(); pq.pop();
        if (finished[p.second])
            continue;

        for (ll j = 0; j < n; j++) {
            if (adj[p.second][j] == -1 || finished[j])
                continue;
            pq.push(make_pair(adj[p.second][j], j));
        }
        // cout << p.second << ": " << p.first << endl;
        d[p.second] = p.first;
        finished[p.second] = true;
    }
}


int main()
{
    cin >> n;
    for (ll i = 0; i < n; i++) {
        for (ll j = 0;j < n; j++) {
            cin >> adj[i][j];
        }
    }

    msp();

    ll acc = 0;
    for (ll i = 0; i < n; i++)
        acc += d[i];
    cout << acc << endl;
}

C++: #include <iostream>
#include <functional>
using namespace std;

const int inf = 100000;
const int NIL = -1;

int G[100][100], d[100];
bool color[100];

int main(){
    int n, minc, tag, ans = 0;
    cin >> n;
    for(int i = 0; i < n; i++){
        d[i] = inf;
        color[i] = false;
        for(int j = 0; j < n; j++) cin >> G[i][j];
    }
    d[0] = 0;
    while(true){
        minc = inf;
        for(int i = 0; i < n; i++){
            if(color[i] == false && d[i] < minc){
                minc = d[i];
                tag = i;
            }
        }
        if(minc == inf) break;
        ans += minc;
        color[tag] = true;
        for(int v = 0; v < n; v++){
            if(G[tag][v] != -1){
                if(color[v] == false && G[tag][v] < d[v]){
                    d[v] = G[tag][v];
                }
            }
        }
    }
    cout << ans << endl;
    return 0;
}

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

#define NIL -2
#define INF 1000000001

using namespace std;

void prim(const vector<vector<int> > &link_weight,vector<int> &color, vector<int> &mincost_array,vector<int> &parents){
    int n = mincost_array.size();
    int mincost;
    int u;
    while(true){
        mincost = INF;
        for(int i = 0;i < n;i++){
            if(color[i] != 1 && mincost_array[i] < mincost){
                mincost = mincost_array[i];
                u = i;      
            }
        }
        if(mincost == INF) break;
        color[u] = 1;

        for(int v = 0;v < n;v++){
            if(color[v] != 1 && link_weight[u][v] < INF){
                if(link_weight[u][v] < mincost_array[v]){
                    mincost_array[v] = link_weight[u][v];
                    parents[v] = u;
                    color[v] = 0;
                }
            }
        }
    }
}

int main(){
    int mat_size;
    int temp;
    int cost = 0;

    scanf("%d",&mat_size);
    vector<vector<int> > link_weight(mat_size,vector<int>(mat_size,INF));
    vector<int> color(mat_size,-1);
    vector<int> mincost_array(mat_size,INF);
    vector<int> parents(mat_size,-1);

    for(int i = 0;i < mat_size;i++){
        for(int j = 0;j < mat_size;j++){
            scanf("%d",&temp);
            if(temp != -1) link_weight[i][j] = temp;
        }
    }
    mincost_array[0] = 0;
    prim(link_weight,color,mincost_array,parents);
    for(int i = 0;i < mat_size;i++) cost += mincost_array[i];
    printf("%d\n",cost);

    return 0;   
}