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 <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;
}

C++: #include <bits/stdc++.h>
using namespace std;
#define WHITE 0
#define GRAY 1
#define BLACK 2

int M[101][101];
int n;
vector<int> q;

int check(int point){
    if(find(q.begin(), q.end(), point) != q.end()){
        //printf("%d in q\n",point);
        return 1;
    }
    else{
        //printf("%d not in q\n",point);
        return 0;
    }
}

int prim(int point){
    q.push_back(point);
    int dis = INT_MAX;
    int next_point;
    int sum = 0;
    while(q.size()!=n){
        dis = INT_MAX;
        for(int i=0;i<q.size();++i){
            for(int j=0;j<n;++j){
                if(M[q[i]][j] != INT_MAX && M[q[i]][j] < dis && check(j)==0){
                    dis = M[q[i]][j];
                    next_point = j;
                }
            }
        }
        q.push_back(next_point);
        //printf("push %d\n",next_point);
        sum += dis;
    }
    return sum;
}

int main(){
    int u, z, y;
    int value;
    scanf("%d",&n);
    for(int i=0;i<n;++i){
        for(int j=0;j<n;++j){
            cin>>value;
            M[i][j] = (value==-1)?INT_MAX:value;
            M[j][i] = (value==-1)?INT_MAX:value;
        }
    }
    int sum = prim(0);

    printf("%d\n",sum);
    return 0;
}