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>
#define inf 10000000
using namespace std;
int main(){
  int n,d[101][101],a[101],ans=0,c[101]={};
  cin>>n;
  for(int i=0;i<n;i++)a[i]=inf;
  for(int i=0;i<n;i++)
    for(int j=0;j<n;j++){
      cin>>d[i][j];
      if(d[i][j]==-1)d[i][j]=inf;    
    }
  a[0]=0;
  int s=0;
  while(1){
    c[s]=1;
    ans+=a[s];
    int mn=inf,ns=-1;
    for(int i=0;i<n;i++)
      a[i]=min(a[i],d[s][i]);

    for(int i=0;i<n;i++)
      if(!c[i]&&a[i]<mn)ns=i,mn=a[i];

    if(ns==-1)break;
    s=ns;
  }
  cout<<ans<<endl;
  return 0;
}

C++: #include <stdio.h>
#define rep(i, n) for(int i = 0; i < n; ++i)

int main(void) {
  int n, i, j, ans = 0;
  scanf("%d", &n);
  int a[n][n];
  char f[n];
  rep(i, n) rep(j, n) scanf("%d", &a[i][j]);
  rep(i, n) f[i] = 0;
  f[0] = 1;
  rep(i, n - 1) {
    int min = 10000, minx, miny;
    rep(j, n) {
      rep(k, n) {
        if(f[j] && !f[k] && a[j][k] => 0 && a[j][k] < min) min = a[j][k], minx = j, miny = k;
      }
    }
    ans += min;
    f[miny] = 1;
  }
  printf("%d\n", ans);
  return 0;
}

C++: #include<iostream>

using namespace std;

int n, M[100][100], W = 0, G = 1, B = 2;

int prim () {

    int u, minv, i, v, d[100], p[100], color[100], sum;

    for (i = 0; i < n; i++) {
        d[i] = (1 << 21);
        p[i] = -1;
        color[i] = W;
    }

    d[0] = 0;

    while (1) {

        minv = (1 << 21);
        u = -1;
        for (i = 0; i < n; i++) {
            if (minv > d[i] && color[i] != B) {
                u = i;
                minv = d[i];
            }
        }
        if (u == -1) {
            break;
        }
        color[u] = B;
        for (v = 0; v < n; v++) {
            if (color[v] != B && M[u][v] != (1 << 21)) {
                if (d[v] > M[u][v]) {
                    d[v] = M[u][v];
                    p[v] = u;
                    color[v] = G;
                }
            }
        }

    }

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

    return sum;

}

int main () {

    int i, j, e;

    cin >> n;

    for (i = 0; i < n; i++) {
        for (j = 0; j < n; j++) {
            cin >> e;
            M[i][j] = (e == -1) ? (1 << 21) : e;
        }
    }

    cout << prim () << endl;

    return 0;

}

C++: #define _GLIBCXX_DEBUG
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define ALL(v) v.begin(), v.end()
typedef long long ll;

template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }

#include <bits/stdc++.h>
using namespace std;

#define WHITE 0
#define GRAY 1
#define BLACK 2
#define INF 1000000010

  int n;
  int M[100][100];
  int color[100];
  int d[100];
  int p[100];
  int mincost;

void prim(){
  rep(u,n) color[u]=WHITE;
  rep(u,n) d[u]=INF;
  d[0]=0;
  p[0]=-1;
  
  int u;
  while(1){
    mincost=INF; 
    rep(i,n){
      if(color[i] != BLACK && d[i]<mincost){
        mincost=d[i];
        u=i;
      }
    }
    
    if(mincost==INF) break;
    
    color[u]=BLACK;
    
    rep(v,n){
      if(color[v] != BLACK && M[u][v] != -1){
        if(M[u][v]<d[v]){
          d[v]=M[u][v];
          p[v]=u;
          color[v]=GRAY;
        }
      }
    }
  }
}

int main(){
  cin>>n;
  
  rep(i,n){
    rep(j,n) cin>>M[i][j];
  }
  
  prim();
  int sum=0;
  rep(i,n) sum+=d[i];
  
  cout<<sum<<endl;
  
  return 0;
}

C++: #include<iostream>

using namespace std;

int n, M[100][100], W = 0, G = 1, B = 2;

int prim () {

    int u, minv, i, v, d[100], p[100], color[100], sum;

    for (i = 0; i < n; i++) {
        d[i] = (1 << 21);
        p[i] = -1;
        color[i] = W;
    }

    d[0] = 0;

    while (1) {

        minv = (1 << 21);
        u = -1;
        for (i = 0; i < n; i++) {
            if (minv > d[i] && color[i] != B) {
                u = i;
                minv = d[i];
            }
        }
        if (u == -1) {
            break;
        }
        color[u] = B;
        for (v = 0; v < n; v++) {
            if (color[v] != B && M[u][v] != (1 << 21)) {
                if (d[v] > M[u][v]) {
                    d[v] = M[u][v];
                    p[v] = u;
                    color[v] = G;
                }
            }
        }

    }

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

    return sum;

}

int main () {

    int i, j, e;

    cin >> n;

    for (i = 0; i < n; i++) {
        for (j = 0; j < n; j++) {
            cin >> e;
            M[i][j] = (e == -1) ? (1 << 21) : e;
        }
    }

    cout << prim () << endl;

    return 0;

}