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>
#include <queue>

using ll = long long;
using namespace std;
int n, p, A[100][100];
int d[100], P[100], c[100];
/* initialize */

int bfs(){
    int u, minv;
    int sum;
    for (int i = 0; i < n; ++i) {
        d[i] = 1<<30;
        P[i] = -1;
        c[i] = 0;
    }
    d[0] = 0;
    
    while(true) {
        minv = 1 << 30;
        u = -1;
        for (int i = 0; i < n; ++i) {
            if (minv > d[i] && c[i] != 2) {
                u = i;
                minv = d[i];
            }
        }
        if (u == -1)break;
        c[u] = 2;
        for (int j = 0; j < n; ++j) {
            if (c[j] != 2 && A[u][j] != 1 << 30) {
                if (d[j] > A[u][j]) {
                    d[j] = A[u][j];
                    P[j] = u;
                    c[j] = 1;
                }
            }
        }
    }
        sum = 0;
        for (int i = 0; i < n; ++i) {
            if (P[i] != -1) sum += A[i][P[i]];
        }
    return sum;
}
/* main */
int main() {
    cin >> n;
    for (int i = 0; i < n; ++i) {
        for (int j = 0; j < n; ++j) {
            cin >> p;
            if (p == -1) A[i][j] = 1<<30;
            else A[i][j] = p;
        }
    }
    cout << bfs() << "\n";
}

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(true){
        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(M[u][v]<d[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 b;cin>>b;
            M[i][j]=(b==-1)? INFTY:b;
        }
    }
    
    cout << prim() << endl;
    
    return 0;
}

C++: #include <cstdio>
using namespace std;
#define MAXN 100
#define INFTY (1 << 21)
#define WHITE 0
#define GRAY 1
#define BLACK 2

int n, M[MAXN][MAXN];

int prim()
{
	int d[MAXN], p[MAXN], color[MAXN];
	int u, minv;
	for(int i = 0; i < n; i++)
	{
		d[i] = INFTY;
		p[i] = -1;
		color[i] = WHITE;
		//printf("===\n");
	}
	d[0] = 0;
	while(true)
	{
		u = -1;
		minv = INFTY;
		for(int i = 0; i < n; i++)
		{
			if(color[i] != BLACK && d[i] < minv)
			{
				u = i;
				minv = d[i];
				//printf("%d \n", u);
			}
		}
		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()
{
	scanf("%d", &n);
	for(int i = 0; i < n; i++)
	{
		for(int j = 0; j < n; j++)
		{
			int e;
			scanf("%d", &e);
			M[i][j] = (e == -1) ? INFTY : e;
		}
	}
	printf("%d\n", prim());
	return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
typedef long long ll;
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }
const ll INF = 1LL << 60;
const int MAX = 1000000010;

vector<bool> checked;
vector<int> d;
vector<int> p;

int main() {
    int N; cin >> N;
    vector<vector<int>> G(N, vector<int>(N));
    checked.assign(N, false);
    d.assign(N, MAX);
    p.resize(N);
    for (int i = 0; i < N; i++) {
        for (int j = 0; j < N; j++) {
            cin >> G[i][j];
        }
    }
    d[0] = 0;
    p[0] = -1;

    while (true) {
        int MIN = MAX;
        int u;
        for (int i = 0; i < N; i++) {
            if(!checked[i] && chmin(MIN, d[i])) u = i;
        }

        if (MIN == MAX) break;
        checked[u] = true;

        for (int v = 0; v < N; v++) {
            if (!checked[v] && G[u][v] != -1) {
                chmin(d[v], G[u][v]);
                p[v] = u;
            }
        }
    }

    int sum = 0;
    for (int i = 0; i < N; i++) {
        sum += d[i];
    }

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

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

const int MAX = 100;
const int INFINITY = 2 << 21;
int n;
int G[MAX][MAX];
int distances[MAX];
int parents[MAX];
int states[MAX];

int prim() {
  int id = 0;
  distances[id] = 0;

  while (1) {
    int min = INFINITY;

    for (int i = 0; i < n; i++) {
      if (states[i] == 0 && distances[i] < min) {
        min = distances[i];
        id = i;
      }
    }

    if (min == INFINITY) break;

    for (int i = 0; i < n; i++) {
      if (states[i] != 0 || G[id][i] == -1) continue;
      if (G[id][i] < distances[i]) {
        distances[i] = G[id][i];
        parents[i] = id;
      }
    }

    states[id] = 1;
  }

  int sum = 0;
  for (int i = 0; i < n; i++) {
    if (parents[i] != -1) sum += distances[i];
  }
  return sum;
}

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);

  cin >> n;

  for (int i = 0; i < n; i++) {
    distances[i] = INFINITY;
    parents[i] = -1;
    states[i] = 0;
    for (int j = 0; j < n; j++) {
      cin >> G[i][j];
    }
  }

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