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 INF=(1<<21);
static const int APE=0;
static const int MID=1;
static const int BOT=2;

int n, M[MAX][MAX];

int prim(){
  int u, mv, i, v, sum=0;
  int d[MAX], p[MAX], c[MAX];
  for(i=0;i<n;i++){
    d[i]=INF;
    p[i]=-1;
    c[i]=APE;
  }
  d[0]=0;

  while(1){
    mv=INF;
    u=-1;
    for(i=0;i<n;i++){
      if(mv>d[i]&&c[i]!=BOT){
        u=i;
        mv=d[i];
      }
    }
    if(u==-1)break;
    c[u]=BOT;

    for(v=0;v<n;v++){
      if(c[v]!=BOT&&M[u][v]!=INF){
        if(d[v]>M[u][v]){
          d[v]=M[u][v];
          p[v]=u;
          c[v]=MID;
        }
      }
    }
  }
  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) ? INF : e;
    }
  }
  cout<<prim()<<endl;
  return 0;
}

C++: //MST
#if 0

#endif // 0
#include<iostream>
#include<vector>
#include<algorithm>
#define MAX 10010
using namespace std;
vector<int> s;
int root(int x)
{
    if(s[x]==x)return x;
    return root(s[x]);
}

int main()
{
    int cost[MAX],src[MAX],dst[MAX];
    pair<int,int> pos[MAX];
    int n,sum=0;
    cin>>n;
    s.resize(n);
    for(int i=0;i<n;i++)
    {
        s[i]=i;
    }
    int m=0;
    for(int i=0;i<n;i++)
    {
        for(int j=0;j<n;j++)
        {
            int a;
            cin>>a;
            if(a>=0&&j>i)
            {
                cost[m]=a;
                src[m]=i;
                dst[m]=j;
                pos[m].first=cost[m];
                pos[m].second=m;
                m++;
            }
        }
    }
    sort(pos,pos+m);
    int c=0;
    for(int i=0;i<m;i++)
    {
        int d=pos[i].second;
        if(root(src[d])!=root(dst[d]))
        {
            sum+=cost[d];
            s[root(src[d])]=root(dst[d]);
            c++;
        }
        if(c>=n-1)
            break;
    }

    cout<<sum<<endl;
    return 0;
}

C++: #include <iostream>
#include <vector>
#include <string>
#include <stack>
#include <queue>
#include <deque>
#include <set>
#include <map>
#include <algorithm>	// require sort next_permutation count __gcd reverse etc.
#include <cstdlib>	// require abs exit atof atoi 
#include <cstdio>		// require scanf printf
#include <functional>
#include <numeric>	// require accumulate
#include <cmath>		// require fabs
#include <climits>
#include <limits>
#include <cfloat>
#include <iomanip>	// require setw
#include <sstream>	// require stringstream 
#include <cstring>	// require memset
#include <cctype>		// require tolower, toupper
#include <fstream>	// require freopen
#include <ctime>		// require srand
#define rep(i,n) for(int i=0;i<(n);i++)
#define ALL(A) A.begin(), A.end()
#define INF 1<<20

using namespace std;

typedef long long ll;
typedef pair<int, int> P;

int cost[105][105];
int mincost[105];
bool used[105];

int prim (int n )
{
	memset (used, false, sizeof (used ) );
	rep (i, n ) mincost[i] = INF;

	mincost[0] = 0;
	int res = 0;

	while (true ){
		int v = -1;
		// X  に属さない頂点のうち X からの辺のコストが最小になる頂点を探す
		rep (u, n )
			if (!used[u] && (v == -1 || mincost[u] < mincost[v] ) ) v = u;

		if (v == -1 ) break;

		used[v] = true;
		res += mincost[v];

		rep (u, n ) mincost[u] = min (mincost[u], cost[v][u] );
	} // end while

	return res;
}

int main()
{
	memset (cost, -1, sizeof (cost ) );
	ios_base::sync_with_stdio(0);
	int n; cin >> n;
	rep (i, n ) rep (j, n ) cost[i][j] = INF;

	rep (i, n ) rep (j, n ){
		int in; cin >> in;
		if (in == -1 ) in = INF;
		cost[i][j] = in;
	} // end rep

	int res = prim(n );
	cout << res << endl;
		
	return 0;
}

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

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

int n;
vector<vector<int>> G;
vector<int> d;
vector<int> colors;

int mst(){
    int left = n;
    int res = 0;
    d[0] = 0;
    //int u = 0;
    while(left--){
        int min_id = -1;
        int cur_min = INT_MAX;
        for(int i = 0;i<n;i++){
            if(colors[i] !=BLACK && d[i] < cur_min){
                min_id = i;
                cur_min = d[i];
            }
        }
        if(min_id==-1) break;
        int u = min_id;
        //printf("%d\n",min_id);
        res += d[u];
        colors[u] = BLACK;
        for(int v =0;v<n;v++){
            if(colors[v]!=BLACK && G[u][v] < d[v]){
                d[v] = G[u][v];
            }
        }
    }
    return res;
}

int main()
{
    //freopen("data.txt","r",stdin);
    cin >> n;
    G.resize(n,vector<int>(n,INT_MAX));
    d.resize(n,INT_MAX);
    colors.resize(n,WHITE);
    int tmp;
    for(int i =0;i<n;i++){
        for(int j=0;j<n;j++){
            cin >> tmp;
            if(tmp!=-1) G[i][j] = tmp;
        }
    }
    cout << mst() <<endl;
    return 0;
}