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 <stdio.h>
#include <iostream>
#include <utility>
#include <algorithm>
#include <math.h>
#include <vector>
#define N 10000
using namespace std;

class edge
{
  public:
    int i, j, weigt;
    bool operator()(edge e1, edge e2)
    {
        return e1.weigt < e2.weigt;
    }
};
edge e[N];
vector<int> adj[N];
int n;
int c = 0;
int f[N] = {};
int g[N] = {};

void setGraphNum(int v, int i)
{
    g[v] = i;
    f[v]++;
    vector<int>::iterator itr = adj[v].begin();
    while (itr != adj[v].end())
    {
        if (f[*itr] !=c)
            setGraphNum(*itr, i);
        itr++;
    }
}
void CalcGraphNum()
{
    int k = 1;
    for (int i = 0; i < n; i++)
    {
        if (f[i]!=c)
            setGraphNum(i, k++);
    }
}
int main()
{
    cin >> n;
    int k = 0, tmp = 0, sum = 0;
    for (int i = 0; i < n; i++)
    {
        for (int j = 0; j < n; j++)
        {
            cin >> tmp;
            if (j < i + 1 || tmp < 0)
                continue;
            e[k].i = i;
            e[k].j = j;
            e[k].weigt = tmp;
            k++;
        }
    }
    sort(e, e + k, edge());

    for (int i = 0; i < k; i++)
    {
        if (find(adj[e[i].i].begin(), adj[e[i].i].end(), e[i].j) != adj[e[i].i].end())
            continue;
        adj[e[i].i].push_back(e[i].j);
        adj[e[i].j].push_back(e[i].i);
        sum += e[i].weigt;
        if(++c==n-1)
            break;
        CalcGraphNum();
    }

    cout << sum << "\n";
    return 0;
}

C++: #include <iostream>
using namespace std;
#define INF 1000000

int main(){
  long n;
  long G[100][100],d[100];
  long minc,mi,i;
  bool flg[i];
  long sum=0;
  
  cin >> n;
  for (i=0;i<n;i++){
    for (int j=0;j<n;j++){
      cin >> G[i][j];
      if (G[i][j]==-1) G[i][j]=INF;
    }
    d[i]=INF;
    flg[i]=false;
  }

  d[0]=0;
  while (true){
    minc=INF;
    for (i=0;i<n;i++){
      if (!flg[i]&&d[i]<minc){
	minc=d[i];
	mi=i;
      }
    }
    if (minc==INF) break;
    flg[mi]=true;
    for (i=0;i<n;i++){
      if (G[mi][i]==INF)continue;
      if (!flg[i]&&d[i]>G[mi][i]){
	d[i]=G[mi][i];
	//cout << d[i];
      }
    }
  }
  
  for (i=0;i<n;i++){
    sum+=d[i];
  }
  cout << sum<<endl;
  
  return 0;
}

C++: 
#include<iostream>

using namespace std;


static const int MAX = 100;
static const int inf = (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] = inf;
		p[i] = -1;
		color[i] = white;
	}

	d[0] = 0;

	while (1)
	{
		minv = inf;
		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] != inf) {
				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() {


	int s;

	cin >> n;

	for (int i = 0;i < n;i++) {
		for (int k = 0; k < n; k++)
		{
			cin >> s;
			m[i][k] = (s == -1) ? inf : s;
		}
	}


	cout << prim() << endl;

	return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;
#define INF 10000000

int prim(int v, vector<vector<pair<int, int> > > adjlist){

    priority_queue<pair<int, int> >  wait;    // first := 候補辺のコスト   second　:= 行先ノード
    vector<int> result(v, INF);               // 頂点iと木の間の最小コスト
    int ans = 0, size = 0;                    // 木のコスト　頂点数

    //初期条件
    result[0] = 0;
    wait.push(make_pair(0, 0));

    while(!wait.empty() && size < v){

        //候補辺のうち最小のものを取り出す
        int newpoint = wait.top().second;
        int newcost = -wait.top().first;
        wait.pop();

        if(result[newpoint] < newcost) continue;

        //出てきた頂点を木に追加
        ans += newcost;
        result[newpoint] = -INF;
        size++;

        // 新しい点から出ている辺をすべてなめる
        for(int i = 0; i < adjlist[newpoint].size(); i++){
            int nextpoint = adjlist[newpoint][i].second;
            int nextcost = adjlist[newpoint][i].first;
            //より木に近くなった頂点があれば,　そこへの枝をqueueに追加
            if(result[nextpoint] > nextcost){
                result[nextpoint] = nextcost;
                wait.push(make_pair(-nextcost, nextpoint));
            }
        }
    }

    return ans;
}


int main(){

    int V; cin >> V;     // V:= 頂点の数
    vector<vector<pair<int, int> > > adjlist(V);      //first := コスト　second := 行先

    //入力
    for(int i = 0; i < V; i++){
        for(int j = 0; j < V; j++){
            int a; cin >> a;
            if(a == -1) continue;
            else{
                adjlist[i].push_back(make_pair(a, j));
            }
        }  
    }

    int ans = prim(V, adjlist);

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