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<bits/stdc++.h>
using namespace std;
#define int long long

struct edge{ int to,cost; };
const int N = 1e5+10;
vector< edge > G[N];

int prim(){
  typedef pair<int, int> P;
  const int INF = 1 << 30;

  priority_queue<P,vector<P>,greater<P>> que;
  vector<bool> visited(N,false);

  que.push({0,0}); //{距離,頂点}
  int total = 0;

  while( !que.empty() ){
    P p = que.top(); que.pop();
    int v = p.second;
    if(visited[v]) continue;
    visited[v] = true;
    total += p.first;
    for(auto e : G[v]){
      if(!visited[e.to]){
        que.push( {e.cost,e.to} );
      }
    }
  }
  return total;
}

signed main(){
  cin.tie(0);
  ios::sync_with_stdio(false);

  int V; cin >> V;
  for(int i = 0; i < V; i++){
    for(int j = 0; j < V; j++){
      int cost; cin >> cost;
      if(cost == -1) continue;
      G[i].push_back({j,cost});
    }
  }
  cout << prim() << endl;

  return 0;
}

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

#define NUM 100
#define BLACK 0
#define GREY 1
#define WHITE 2
#define NIL -1
#define INF 1000000000
int A[NUM][NUM];

int V[NUM];
int color[NUM];

void init(){
    for(int i = 0; i < NUM; i++){
        color[i] = WHITE;
        V[i] = INF;
    }

}

int main(void){
    init();

    int n;
    scanf("%d", &n);

    for(int i = 0; i < n; i++){
        for(int j = 0; j < n; j++){
            scanf("%d", &A[i][j]);
        }
    }

    int next_node = 0;
    int sum = 0;
    while(1){
        color[next_node] = BLACK;
        for(int i = 0; i < n; i++){
            if(A[next_node][i] == NIL){
                continue;
            }
            V[i] = min(V[i], A[next_node][i]);
        }

        next_node = NIL;
        int min_dist = INF;
        for(int i = 0; i < n; i++){
            if(color[i] == BLACK){
                continue;
            }
            if(V[i] < min_dist){
                next_node = i;
                min_dist = V[i];
            }
        }
        if(min_dist == INF){
            break;
        }else{
            sum += min_dist;
        }
    }

    printf("%d\n", sum);
}

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 u, 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 (true) {
		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] != BLACK&&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>
#include <set>

#define rep(i, n) for (int i = 0; i < (int)(n); ++i)

const int INF = 1 << 21;
const int SIZE = 101;
int a[SIZE][SIZE];

int prim(int n) {
  int d[n], p[n], state[n];

  rep(i, n) {
    d[i] = INF;
    p[i] = -1;
    state[i] = 0;
  }

  d[0] = 0;

  while (1) {
    int minv = INF, u = -1;

    rep(i, n) {
      if (d[i] < minv && state[i] != 1) {
        minv = d[i];
        u = i;
      }
    }

    if (u == -1) break;
    state[u] = 1;

    rep(i, n) {
      if (a[u][i] != INF && state[i] != 1) {
        if (d[i] > a[u][i]) {
          d[i] = a[u][i];
          p[i] = u;
        }
      }
    }
  }

  int sum = 0;
  rep(i, n) {
    if (p[i] != -1) { sum += a[i][p[i]]; }
  }

  return sum;
}

int main() {
  int n;
  std::cin >> n;

  rep(i, n) rep(j, n) {
    int m;
    std::cin >> m;
    if (m == -1) m = INF;
    a[i][j] = m;
  }

  std::cout << prim(n) << '\n';

  return 0;
}