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;

struct Edge{
  int to,cost;
  bool operator<(const Edge&a)const{
    return cost>a.cost;
  }
};
struct Graph{
  Graph(int n):E(n),T(n),N(n){}
  vector<vector<Edge>>E,T;
  int N;
  void add_edge(int a,int b,int c){
    E[a].push_back({b,c});
    E[b].push_back({a,c});
  }
  void prim(int s){
    bool visited[N];
    priority_queue<pair<Edge,int>>Q;
    memset(visited,0,N);
    Q.push({{s,0},-1});
    while(!Q.empty()){
      Edge e=Q.top().first;
      int from=Q.top().second;Q.pop();
      if(visited[e.to])continue;
      visited[e.to]=1;
      if(from+1)T[from].push_back(e),T[e.to].push_back({from,e.cost});
      for(auto&a:E[e.to])
      if(!visited[a.to])Q.push({a,e.to});
    }
  }
  int get_cost(){
    int ret=0;
    for(auto&a:T)for(auto&b:a)ret+=b.cost;
    return ret/2;
  }
  vector<Edge>* get_MST(){
    return T.data();
  }
};

main(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    int N;
    cin>>N;
    Graph G(N);
    for(int i=0;i<N;++i)
        for(int j=0;j<N;++j){
            int a;
            cin>>a;
            if(~a) G.add_edge(i,j,a);
        }
    G.prim(0);
    cout<<G.get_cost()<<endl;
}

C++: #include <iostream>
#define MAX 100
#define INF (1<<21)
#define WHITE 0
#define GRAY 1
#define BLACK 2
using namespace std;

int M[MAX][MAX],n;
int prim(){
	int u,minv;
	int d[MAX],p[MAX],color[MAX];
	for(int i = 0;i < n;i++){
		d[i] = INF;
		p[i] = -1;
		color[i] = WHITE;
	}

	d[0] = 0;

	while(1){
		minv = INF;
		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] != INF){
				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(){
	int e;
	cin >> n;
	for(int i = 0;i < n;i++){
		for(int j = 0;j < n;j++){
			cin >> e;
			if(e == -1)M[i][j] = INF;
			else M[i][j] = e;
		}
	}

	cout << prim() <<endl;
	return 0;
}

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(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] != 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]]; //辺の重みの総和を計算
    }

    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++: import java.util.Scanner;

class Main
{
    static int[][]M = new int[100][100];

    static int prim(int n)
    {
	int u, minv;
	int sum = 0;
	int[]d = new int[100];
	int[]f = new int[100];
	int[]cl = new int[100];

	for(int i = 0; i < n; i++)
	    {
		d[i] = 200000;
		f[i] = -1;
		cl[i] = 0;
	    }

	d[0] = 0;

	while(true)
	    {
		minv = 200000;
		u = -1;

		for(int i = 0; i < n; i++)
		    {
			if(minv > d[i] && cl[i] != 2)
			    {
				u = i;
				minv = d[i];
			    }
		    }

		if(u == -1) break;
		cl[u] = 2;

		for(int j = 0; j < n; j++)
		    {
			if(cl[j] != 2 && M[u][j] != 200000)
			    {
				if(d[j] > M[u][j])
				   {
				       d[j] = M[u][j];
				       f[j] = u;
				       cl[j] = 1;
				   }
			    }
		    }
	    }
		for(int i = 0; i < n; i++)
		    {
			if(f[i] != -1) sum += M[i][f[i]];
		    }
	    
	return sum;
    }

    public static void main(String[] args)
    {
	Scanner In = new Scanner(System.in);

	int n = In.nextInt();

	for(int i = 0; i < n; i++)
	    {
		for(int j = 0; j < n; j++)
		    {
			int e = In.nextInt();
			M[i][j] = ((e == -1) ? 200000 : e);
		    }
	    }

	System.out.println(prim(n));

    }
}

C++: #include <bits/stdc++.h>
#define P pair<int,int>
using namespace std;

int a[100][100];
bool used[100];
signed main(){
	int b; cin >> b;
	for (int c = 0; c < b; c++) {
		for (int d = 0; d < b; d++) {
			cin >> a[c][d];
			if (a[c][d] == -1) {
				a[c][d] = INT_MAX / 3;
			}
		}
	}
	int ans = 0;
	priority_queue<P,vector<P>, greater<P>>Q;
	Q.push({ 0,0 });
	while (Q.size()) {
		P t = Q.top(); Q.pop();
		if (!used[t.second]) {
			ans += t.first;
			used[t.second] = true;
			for (int i = 0; i < b; i++) {
				Q.push({ a[t.second][i],i });
			}
		}
	}
	cout << ans << endl;
}