Go: package main

import (
	"bufio"
	"fmt"
	"os"
	"sort"
	"strconv"
)

var scanner = bufio.NewScanner(os.Stdin)

func nextString() string {
	scanner.Scan()
	return scanner.Text()
}

func nextInt() int {
	n, err := strconv.Atoi(nextString())
	if err != nil {
		fmt.Printf("strconv.Atoi failed: %v\n", err)
	}
	return n
}

type ElementKey int
type Node struct {
	key    ElementKey
	parent *Node
	rank   int
}

func newNode(key ElementKey) *Node {
	node := Node{}
	node.key = key
	node.parent = &node
	node.rank = 0

	return &node
}

func union(x, y *Node) {
	xSet := findSet(x)
	ySet := findSet(y)
	link(xSet, ySet)
}

func link(x, y *Node) {
	if x.rank > y.rank {
		y.parent = x
	} else {
		x.parent = y

		if x.rank == y.rank {
			y.rank++
		}
	}
}

func findSet(x *Node) *Node {
	if x != x.parent {
		x.parent = findSet(x.parent)
	}
	return x.parent
}

func newDisjointSets(nSets int) *DisjointSets {
	sets := DisjointSets{}
	sets.keyToNode = map[ElementKey]*Node{}
	for i := 0; i < nSets; i++ {
		key := ElementKey(i)
		sets.keyToNode[key] = newNode(key)
	}
	return &sets
}

type DisjointSets struct {
	keyToNode map[ElementKey]*Node
}

func (sets *DisjointSets) union(x, y ElementKey) {
	xNode := sets.keyToNode[x]
	yNode := sets.keyToNode[y]
	union(xNode, yNode)
}

func (sets DisjointSets) findSet(x ElementKey) *Node {
	xNode := sets.keyToNode[x]
	return findSet(xNode)
}

type Edge struct {
	i, j   int
	weight int
}

func main() {
	scanner.Split(bufio.ScanWords)

	n := nextInt()
	sets := newDisjointSets(n)
	edges := []Edge{}
	a := make([][]int, n)
	for i := 0; i < n; i++ {
		a[i] = make([]int, n)
		for j := 0; j < n; j++ {
			a[i][j] = nextInt()
			if a[i][j] >= 0 {
				e := Edge{
					i:      i,
					j:      j,
					weight: a[i][j],
				}
				edges = append(edges, e)
			}
		}
	}
	sort.Slice(edges, func(i, j int) bool { return edges[i].weight < edges[j].weight })
	totalWeight := 0
	for _, edge := range edges {
		i := ElementKey(edge.i)
		j := ElementKey(edge.j)
		if sets.findSet(i) != sets.findSet(j) {
			totalWeight += edge.weight
			sets.union(i, j)
		}
	}
	fmt.Println(totalWeight)
}

C++: #include <iostream>
using namespace std;
static const int MAX = 100;
static const int INFINITY = (1<<21);
static const int WHITE = 0;
static const int GRAY = 1;
static const int BLACK = 2;

int M[MAX][MAX];
int n;

int prim(){
    int u, minv;
    int d[MAX], p[MAX], color[MAX];

    for(int i = 0; i < n; i ++){
        d[i] = INFINITY;
        p[i] = -1;
        color[i] = WHITE;
    }

    d[0] = 0;

    while(1){

        // for(int i = 0; i < n; i++){ //for check
        //     printf("%d ", d[i]);
        // }

        // printf("\n");

        minv = INFINITY;
        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; //全てのノードが確定している場合、探索終了
        }

        color[u] = BLACK;

        for(int v = 0; v < n; v++){
            if (color[v] != BLACK && M[u][v] != INFINITY){
                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(void){
    cin >> n;

    for (int i = 0; i < n; i++){
        for (int j = 0; j < n; j++){
            int e;
            cin >> e;
            if (e != -1){ 
                M[i][j] = e;
            }else{
                M[i][j] = INFINITY; //無限大を挿入
            }
        }
    }

    cout << prim() << endl;

    return 0;

}

/*
5
-1 2 3 1 -1
2 -1 -1 4 -1
3 -1 -1 1 1
1 4 1 -1 3
-1 -1 1 3 -1
 */

C++: #include<iostream>
#include<queue>
#include<map>
using namespace std;
typedef pair< int, int > Pi;
int info[100][100];
bool used[100];

int prim(const int& V) {
    priority_queue< Pi > que;
    que.push(Pi());
    int ret = 0;
    while (!que.empty()) {
        Pi p = que.top(); que.pop();
        if (!~- p.first || used[p.second]++) continue;
        ret -= p.first;
        for (int i = 0; i < V; i++) que.push(Pi(-info[p.second][i], i));
    }
    return ret;
}

int main() {
    int N;
    cin >> N;
    for (int i = 0; i < N; i++) {
        for (int j = 0; j < N; j++) {
            cin >> info[i][j];
        }
    }
    cout << prim(N) << endl;
}

C++: //kruskal tree
#include <iostream>
#include <algorithm>
using namespace std;
#define M 9999
int parent[M],a[M],b[M];
pair<int,int>node[M];
int root(int a){return parent[a]==a?a:parent[a]=root(parent[a]);}
int unite(int a,int b){
	int x=root(a),y=root(b);
	if(x==y)return 0;
	parent[x]=y;
	return 1;
}
int main(){
	int i,j,s,n,m,x;
	cin>>n;
	for(m=i=0;i<n;i++)for(j=0;j<n;j++)if(cin>>x,~x)a[m]=i,b[m]=j,node[m].first=x,node[m].second=m++;
	sort(node,node+m);
	for(i=0;i<n;i++)parent[i]=i;
	for(s=i=0;i<m;i++)if(unite(a[node[i].second],b[node[i].second]))s+=node[i].first;
	cout<<s<<endl;
}

C++: #define _CRT_SECURE_NO_WARNINGS
#define _USE_MATH_DEFINES
//#include "MyMath.h"
//#include "MyDisjointset.h"
#include <iostream>
#include <vector>
#include <algorithm>
#include <queue>
#include <functional>
#include <stdio.h>
using namespace std;
typedef pair<int, int> P;
const int INF = 2000000000;
int main() {
	int n; cin >> n;
	int m[101][101];
	for (int i = 0; i < n; i++)for (int j = 0; j < n; j++)
		cin >> m[i][j];
	int res = 0;
	int *d = new int[n];
	fill(d, d + n, INF); 
	d[0] = 0;
	bool *visit = new bool [n];
	priority_queue <P, vector<P>, greater<P> > q;
	q.push(P(d[0], 0));
	while (!q.empty()) {
		int f = q.top().first, s = q.top().second;
		visit[s] = true;
		res += f;
		q.pop();
		for (int i = 0; i < n; i++)
			if (!visit[i] && m[s][i] != -1 && m[s][i] > d[i]) {
				d[i] = m[s][i];
				q.push(P(d[i], i));
			}
	}
	cout << res << endl;
	delete[] d, visit;
	return 0;
}