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>
using namespace std;
static const int MAX=100;
static const int INFTY=(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 n,minv;
    int d[MAX],p[MAX],color[MAX];
    for(int i=0;i<n;i++){
	d[i]=INFTY;
	p[i]=-1;
	color[i]=white;
    }
    d[0]=0;
    while(1){
	minv=INFTY;
	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]!=balck&&M[u][v]!=INFTY){
		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(){
    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)?INFTY:e;
	}
    }
    cout<<prim()<<endl;
    return 0;
}

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

#define INFTY 2001
#define WHITE 0
#define GRAY 1
#define BLACK 2
#define N 100

// 最小全域木を構成し、重みの総和を返す関数
int prim(int n, int A[N][N]) {
  int i, vertex, min_cost, sum_weight, cost[N], color[N];
  // 初期化
  for (i = 0; i < n; i++) {
    cost[i] = INFTY;
    color[i] = WHITE;
  }
  cost[0] = 0;
  color[0] = GRAY;
  sum_weight = 0;
  
  while (1) {
    // Tに隣接する頂点の中で最小のコストを持つものを選ぶ
    vertex = -1;
    min_cost = INFTY;
    for (i = 0; i < n; i++) {
      if (color[i] == GRAY && cost[i] < min_cost) {
        min_cost = cost[i];
        vertex = i;
      }      
    }
    // T = V ならば終了
    if (vertex == -1) break;

    color[vertex] = BLACK;  // vertexをTに加える
    sum_weight += cost[vertex];

    // vertexに隣接し、Tに属していない頂点のコストを更新
    for (i = 0; i < n; i++) {
      if (color[i] != BLACK && A[vertex][i] < INFTY) {
        color[i] = GRAY;
        if (cost[i] > A[vertex][i]) cost[i] = A[vertex][i];
      }
    }
  }

  return sum_weight;
}


// 最小全域木の辺の重みの和を計算する関数
int sum_weight(int n, int parent[], int A[N][N]) {
  int sum = 0;
  
  for (int i = 1; i < n; i++) sum += A[i][parent[i]];
  
  return sum;
}

int main() {
  int i, j, n, w, A[N][N];

  cin >> n;
  // 隣接行列が与えられる
  for (i = 0; i < n; i++) {
    for (j = 0; j < n; j++) {
      cin >> w;
      if (w == -1) A[i][j] = INFTY;
      else A[i][j] = w;
    }
  }

  cout << prim(n, A) << endl;

  return 0;
}

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

using namespace std;

#define MAX_V 100
#define MAX_E 5050

int par[MAX_V],rank[MAX_V];
int V,E;

struct edge{ int u,v,cost; };
edge es[MAX_E];

bool comp(const edge &e1, const edge &e2){
  return e1.cost < e2.cost;
}

void init(int n){
  for(int i = 0 ; i < n ; i++){
    par[i] = i;
    rank[i] = 0;
  }
}

int find(int x){
  if(par[x] == x){
    return x;
  }else{
    return par[x] = find(par[x]);
  }
}

void unite(int x,int y){
  x = find(x);
  y = find(y);

  if(x == y){
    return;
  }

  if(rank[x] < rank[y]){
    par[x] = y;
  }else{
    par[y] = x;
    if(rank[x] == rank[y]){
      rank[x]++;
    } 
  }
}

bool same(int x,int y){
  return find(x) == find(y);
}

int kruskal(){
  sort(es, es + E, comp);

  init(V);
  int res = 0;

  for(int i = 0 ; i < E ; i++){
    edge e = es[i];

    if(!same(e.u,e.v)){
      unite(e.u,e.v);
      res += e.cost;
    }
  }
  return res;
}

int main(){
  int c;

  cin >> V;
  for(int i = 0 ; i < V ; i++){
    for(int j = 0 ; j < V ; j++){
      cin >> c;

      if(c == -1){
	continue;
      }

      edge e = {i,j,c};
      es[E++] = e;
    }
  }

  cout << kruskal() << endl; 

  return 0;
}