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<string.h>
#include<stdlib.h>
#include<math.h>
#include<stack>
#include<queue>
#include<vector>
#include<algorithm>
#include<string>
#include<iostream>
using namespace std;
typedef long long ll;
int mod(ll a){
    int c=a%1000000007;
    if(c>=0)return c;
    else return c+1000000007;
}
typedef pair<int,int> i_i;

#define inf 1<<21
/*
int map[5][5];
int p[5];
int d[5];
bool color[5];

*/
int main(){
    int n;cin>>n;
    int map[n][n];int p[n];int d[n];bool color[n];
    for(int i=0;i<=n-1;i++)for(int j=0;j<=n-1;j++){
        int k;cin>>k;
        map[i][j]=k==-1?inf:k;
    }
    for(int i=0;i<=n-1;i++)p[i]=-1;
    for(int i=0;i<=n-1;i++)d[i]=inf;
    for(int i=0;i<=n-1;i++)color[i]=false;
    
    /*color[0]=true;*/
    d[0]=0;
    
    while(1){
        int mincost=inf;
        int u=-1;
        for(int i=0;i<=n-1;i++){
            if(color[i]==false&&mincost>d[i]){
                mincost=d[i];
                u=i;
            }
        }
        if(u==-1)break;
        
        color[u]=true;
        for(int i=0;i<=n-1;i++){
            if(color[i]==false&&d[i]>map[u][i]){
                d[i]=map[u][i];
                p[i]=u;
            }
        }
        
    }
    ll sum=0;
    for(int i=1;i<=n-1;i++){
        sum+=map[i][p[i]];
    }
    cout<<sum<<endl;
    return 0;
}

C++: #include <stdio.h>

#define MAX_N (105)
#define INF (10000)
int G[MAX_N][MAX_N];
int N;

int dist[MAX_N];
bool arrived[MAX_N];

int prim()
{
    int ret = 0;
    for ( int i = 0; i < N; i++ ) {
	arrived[i] = false;
	dist[i] = INF;
    }

    dist[0] = 0;

    while ( true ) {
	int min_dist = INF;
	int u = 0;
	for ( int i = 0; i < N; i++ ) {
	    if ( !arrived[i] && dist[i] < min_dist ) {
		min_dist = dist[i];
		u = i;
	    }
	}
	
	if ( min_dist == INF ) break;

	ret += dist[u];
	arrived[u] = true;

	for ( int i = 0; i < N; i++ ) {
	    if ( !arrived[i] && (G[u][i] != -1) && (dist[i] > G[u][i]) ) {
		dist[i] = G[u][i];
	    }
	}
    }

    return ret;
}

int main()
{
    int ans = 0;
    scanf("%d", &N);

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

    ans = prim();

    printf("%d\n", ans);

    return 0;
}

C++: #include<iostream>
#include<cstdio>
#define N 101
using namespace std;

int A[N][N];
int n;
int father[N];

int getparent(int i){
    while(father[i]!=i){
        i=father[i];
    }
    return i;
}
void mergeGroup(int a,int b){
    int fa=getparent(a);
    int fb=getparent(b);
    father[fb]=fa;
}

int prim(){
    int ans=0;
    int c=0;
    int minval=4000;
    int ab,ae;
    while(c<n-1){
        int b,e;
        minval=4000;
        for(b=0;b<n;b++){
            for(e=b+1;e<n;e++){
                if(A[b][e]!=-1 && A[b][e]<minval &&getparent(b)!=getparent(e)){
                    minval=A[b][e];
                    ab=b;
                    ae=e;
                    //printf("b= %d  e= %d \n",b,e);
                }
            }
        }
        ans+=A[ab][ae];
        //printf("ab= %d  ae= %d  mincost=%d \n ",ab,ae,minval);
        c++;
        mergeGroup(ab,ae);
    }
    return ans;
}

int main(){

    int k,i,j;
    cin>>n;
    for(int i=0;i<n;i++){
        father[i]=i;
        for(int j=0;j<n;j++){
            cin>>A[i][j];
        }
    }
    cout<<prim()<<endl;
    return 0;
}

C++: #include<iostream>

using namespace std;
#define MAX 100
#define INF 10000
int M[MAX][MAX];
int n;
int d[MAX];
int color[MAX];
int p[MAX];



int main() {
  cin >> n;
  for (int i = 0; i < n; i++) {
    for (int j = 0; j < n; j++) {
      cin >> M[i][j];
    }
  }

  for (int i = 0; i < n; i++) {
    d[i] = INF;
    color[i] = 0;
  }

  int w = 0;
  d[0] = 0;
  while(true) {

    int com = INF;
    int u;
    for (int i = 0; i < n; i++) {
      if (color[i] != 2 && com > d[i] && d[i] != -1){
        com = d[i];
        u = i;
      }
    }

    if (com == INF) {
      break;
    }

    w += com;
    color[u] = 2;

    for (int j = 0; j < n; j++) {
      if (M[u][j] != -1 && M[u][j] < d[j] && color[j] != 2) {
        d[j] = M[u][j];
        p[j] = u;
      }
    }
  }

  cout << w << endl;
  return 0;
}

C++: #include <cstdio>
#include <iostream>
#include <queue>
#include <utility>

using namespace std;

const int NIL = 2001;
const int MAX = 100;
int G[MAX][MAX];
int color[MAX];
int W[MAX];
int n;

const int WHITE = 0;
const int GRAY = 1;
const int BLACK = 2;

void MST() {
	priority_queue<pair<int, int>> PQ;
	PQ.push(make_pair(0, 0));
	W[0] = 0;
	color[0] = BLACK;
	while (!PQ.empty()) {
		int u = PQ.top().second;
		PQ.pop();
		color[u] = BLACK;
		for (int i = 0; i < n; ++i) {
			if (G[u][i] != -1 && G[u][i] < W[i] && color[i] != BLACK) {
				PQ.push(make_pair(G[u][i] * -1, i));
				W[i] = G[u][i];
//				cout << u << ":" << i << ":" << G[u][i] << endl;
			}
		}
	}
}

int main() {

	scanf("%d", &n);
	for (int i = 0; i < n; ++i) {
		W[i] = NIL;
		color[i] = WHITE;
	}

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

	MST();

	cout << endl;

	int sum = 0;
	for (int i = 0; i < n; ++i) {
//		cout << i << ":" << W[i] << endl;
		sum += W[i];
	}
	cout << sum << endl;

	return 0;
}