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 <vector>
#include <iostream>

class DisjointSet
{
  private:
    std::vector<int> P;
    int root(int a);

  public:
    DisjointSet(int N);
    ~DisjointSet();
    void unite(int a, int b);
    bool is_same_set(int a, int b);

    void show_all(int a);
};

DisjointSet::DisjointSet(int N)
{
    for (int i = 0; i <= N; ++i)
    {
        P.push_back(i);
    }
}

// a の木の root の値を返す
int DisjointSet::root(int a)
{
    if (P[a] == a)
        return a;
    return (P[a] = root(P[a]));
}

// a の属する木を b の木に合体する
void DisjointSet::unite(int a, int b)
{
    P[root(a)] = root(b);
    return;
}

// a と b が同じ集合に属しているかどうか
bool DisjointSet::is_same_set(int a, int b)
{
    return root(a) == root(b);
}

// デバッグ用：a から root までの値をすべて表示する
void DisjointSet::show_all(int a)
{
    if (P[a] != a)
        show_all(P[a]);
    std::cout << "P[" << a << "] = " << P[a] << std::endl;
    return;
}

//=========================

#include <iostream>
#include <vector>
#include <utility>
#include <algorithm>

using namespace std;
typedef pair<int, int> P;

int main()
{
    int n;
    cin >> n;
    vector<int> cost;
    vector<pair<int, P>> edges;

    for (int row = 0; row < n; row++)
    {
        for (int col = 0; col < n; col++)
        {
            int c;
            cin >> c;
            if (row > col && c > -1)
            {
                edges.push_back(
                    make_pair(
                        c, make_pair(row, col)));
            }
        }
    }

    sort(edges.begin(), edges.end());
    DisjointSet *tree = new DisjointSet(n);

    int ans = 0;
    for (int i = 0, size = edges.size(); i < size; i++)
    {
        int cost = edges[i].first;
        int src = edges[i].second.first;
        int dst = edges[i].second.second;

        if (!tree->is_same_set(src, dst))
        {
            tree->unite(src, dst);
            ans += cost;
        }
    }
    cout << ans << endl;
    return 0;
}

C++: #include <iostream>
#include <string>
#include <vector>
#include <stdio.h>
#include <math.h>
#include <map>
#include <algorithm>

using namespace std;

#define FAR INT_MAX
#define N 100

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


int n, color[N], d[N], p[N];
int m[N][N];

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

	d[0] = 0;
	int dist = FAR;

	while (1)
	{
		int u = -1;
		dist = FAR;
		for (int i = 0; i < n; ++i) {
			if (color[i] != BLACK && d[i] < dist){
				dist = d[i];
				u = i;
			}
		}

		if (-1 == u) break;
		color[u] = BLACK;

		for (int v = 0; v < n; ++v){
			if (color[v] != BLACK && (m[u][v] < d[v])){
				d[v] = m[u][v];
				p[v] = u;
				color[v] = GRAY;
			}
		}
	}
	dist = 0;
	for (int i = 0; i < n; ++i){
		if (p[i] != -1) dist += m[i][p[i]];
	}
	return dist;
}

int main(){
	cin >> n;
	for (int i = 0; i < n; ++i) {
		for (int j = 0; j < n; ++j) {
			int val; cin >> val;
			m[i][j] = (val == -1) ? FAR : val;
		}
	}
 
	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]];
		}
	}
		
	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++: #include<bits/stdc++.h>
using namespace std;
#define MAX 100
#define INF 1<<25
int n,cost[MAX][MAX];
int mincost[MAX];
bool used[MAX];
int prim(){
  for(int i=0;i<n;i++)mincost[i]=INF,used[i]=false;
    mincost[0]=0;
  int res=0;
  while(1){
    int v=-1;
    for(int u=0;u<n;u++)
      if(!used[u]&&(v==-1||mincost[u]<mincost[v]))v=u;
    if(v==-1)break;
    used[v]=1;
    res+=mincost[v];
    for(int u=0;u<n;u++)mincost[u]=min(mincost[u],cost[v][u]);
  }
}
int main(){
  cin>>n;
  for(int i=0;i<n;i++){
    for(int j=0;j<n;j++){
      int e;
      cin>>e;
      cost[i][j]=(e==-1)?INF:e;
    }
  }
  cout<<prim()<<endl;
}

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 W[MAX];
int n;

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

int main() {

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

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

	MST();

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

	return 0;
}