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<algorithm>
using namespace std;

#define INF 2000

int n;
int cost[110][110], mincost[110];
bool used[110];

int prim(){

  //initialization
  for(int i = 0;i < n;i++){
    mincost[i] = INF;
    used[i] = false;
  }

  //cost of starting position is 0
  mincost[0] = 0;
  int ans = 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; //finish
    used[v] = true; //use v
    ans += mincost[v];

    //update mincost
    for(int u = 0;u < n;u++){
      mincost[u] = min(mincost[u], cost[v][u]);
    }
  }
  
  return ans;
}

int main(){

  //input
  cin >> n;
  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;
    }
  }

  //output
  cout << prim() << endl;
  
  return 0;
}

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

const int MAX = 100;

struct Edge {
	Edge(int v1, int v2, int cost) :v1(v1), v2(v2), cost(cost) {};
	bool operator < (const Edge& e) const {
		return cost > e.cost;
	}
	int v1;
	int v2;
	int cost;
};

int parent(int uft[], int c) {
	if (uft[c] == -1)return c;
	return uft[c] = parent(uft, uft[c]);
}

int main() {
	int n;
	int uft[MAX];
	int sum = 0;
	priority_queue<Edge> q;
	
	cin >> n;

	for (int i = 0; i < n; i++) {
		uft[i] = -1;
	}

	for (int i = 0; i < n; i++) {
		for (int j = 0; j < n; j++) {
			int temp;
			cin >> temp;
			if (temp != -1){
				Edge e(i, j, temp);
				q.push(e);
			}
		}
	}

	while (!q.empty()) {
		Edge e = q.top(); q.pop();
		int p1 = parent(uft, e.v1);
		int p2 = parent(uft, e.v2);

		if (p1 != p2) {
			uft[p1] = p2;
			sum += e.cost;
		}
	}

	cout << sum << endl;
}

C++: #include<iostream>
#define MAX 101
#define INF (2<<29)
using namespace std;
int main(){
  int n;
  cin>>n;
  int graph[MAX][MAX];
  
  for(int i=1;i<n+1;i++){
    for(int j=1;j<n+1;j++){
      cin>>graph[i][j];
      if(graph[i][j]==-1)graph[i][j]=INF;
    }
  }
  
  int visited[MAX];
  int min[MAX];
  for(int i=1;i<n+1;i++){
    visited[i]=0;
    min[i]=INF;
  }
  
  min[1]=0;
  int ans=0;
  while(true){
    int a=-1;
    for(int i=1;i<n+1;i++){
      if(visited[i]==0&&(a==-1||min[i]<min[a]))a=i;
    }

    if(a==-1)break;
    visited[a]=1;
    ans +=min[a];

    for(int i=1;i<n+1;i++){
      min[i]=min[i]<graph[a][i] ? min[i]:graph[a][i];
    }
  }
  cout<<ans<<endl;
  return 0;
}

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

struct edge {
	int x;
	int y;
	int cost;
	edge(int x, int y, int cost) { this->x = x; this->y = y; this->cost = cost; }
};

bool operator<(const edge& e, const edge& f) { return e.cost < f.cost; }
bool operator<=(const edge& e, const edge& f) { return e.cost <= f.cost; }
bool operator>(const edge& e, const edge& f) { return e.cost > f.cost; }
bool operator>=(const edge& e, const edge& f) { return e.cost >= f.cost; }
bool operator==(const edge& e, const edge& f) { return e.cost == f.cost; }
bool operator!=(const edge& e, const edge& f) { return e.cost != f.cost; }

int root(int* par, int x)
{
	if (par[x] == x) return x;
	else return root(par, par[x]);
}
	
bool isSame(int* par, int x, int y)
{
	return root(par, x) == root(par, y);
}

void unite(int* par, int x, int y)
{
	if (!isSame(par, x, y)) par[root(par, x)] = root(par, y);
}

int main()
{
	int n; std::cin >> n;
	
	int *par = new int[n];
	for (int i=0; i<n; i++) par[i] = i;
	
	std::vector<edge> edge_list;
	
	for (int i=0; i<n; i++) {
		for (int j=0; j<n; j++) {
			int cost;
			std::cin >> cost;
			if (i < j && cost >= 0) edge_list.push_back( edge(i, j, cost) );
		}
	}
	
	std::sort(edge_list.begin(), edge_list.end());
	
	int totalcost = 0;
	int num_of_edges = 0;
	for (std::vector<edge>::iterator it = edge_list.begin(); it != edge_list.end(); ++it) {
		if (isSame(par, (*it).x, (*it).y)) continue;
		unite(par, (*it).x, (*it).y);
		totalcost += (*it).cost;
		num_of_edges++;
		// std::cout << (*it).x << "," << (*it).y << " " << (*it).cost << std::endl;
		if (num_of_edges == n-1) break;
	}
	
	std::cout << totalcost << std::endl;

	delete[] par;
	
	return 0;
}

C++: #include<stdio.h>
static const int INFTY = (1<<21);
#define MAX 100
#define WHITE 0
#define GRAY 1
#define BLACK 2


int n,v,i,M[MAX][MAX];
int prim(void);

int main(){
  int j,e;
  scanf("%d",&n);

  for(i=0;i<n;i++){
    for(j=0;j<n;j++){
      scanf("%d",&e);
      M[i][j]=(e==-1)?INFTY:e;
    }
  }
  printf("%d",prim());
  return 0;
}

int prim(){
  int u,minv,sum=0;
  int d[MAX],p[MAX],color[MAX];

  for(i=0;i<n;i++){
    d[i]=INFTY;
    p[i]=-1;
    color[i]=WHITE;
  }

  d[0]=0;

  while(1){
    minv=INFTY;
    u=-1;
    for(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(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;
        }
      }
    }
  }
  for(i=0;i<n;i++){
    if(p[i]!=-1)sum+=M[i][p[i]];
  }
  return sum;
}