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 <bits/stdc++.h>
using namespace std;

#define INF 1000000
#define N_MAX 100
#define WH 0
#define GR 1
#define BK 2

int main() {
    int n;
    cin >> n;
    int A[N_MAX][N_MAX] = {};
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < n; j++) {
            int a;
            cin >> a;
            if (a == -1) {
                A[i][j] = INF;
            }
            else {
                A[i][j] = a;
            }
        }
    }

    int color[N_MAX] = {};
    int d[N_MAX];
    for (int i = 0; i < n; i++) {
        d[i] = INF;
    }
    int p[N_MAX] = {};

    while (true) {
        int u;
        int min_cost = INF;
        d[0] = 0;
        p[0] = -1;

        for (int i = 0; i < n; i++) {
            if (color[i] != BK && d[i] < min_cost) {
                min_cost = d[i];
                u = i;
            }
        }
        if (min_cost == INF) {
            break;
        }
        color[u] = BK;

        for (int v = 0; v < n; v++) {
            if (color[v] != BK && A[u][v] != INF) {
                if (A[u][v] < d[v]) {
                    d[v] = A[u][v];
                    p[v] = u;
                    color[v] = GR;
                }
            }
        }
    }

    int ans = 0;
    for (int i = 0; i < n; i++) {
        ans += d[i];
    }
    cout << ans << 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 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;
}

C++: #include <bits/stdc++.h>
 
using namespace std;

int n;
const int INF = 100100100;

int a[105][105];

int u,minv;
int d[105],p[105],color[105];

int prim(){

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

  while(1){
    minv = INF;
    u = -1;
    for(int i = 0; i < n; i++){
      if(minv > d[i] && color[i] != 2){
        minv = d[i];
        u = i;
      }
    }
    if(u==-1) break;

    color[u] = 2;

    for(int i = 0; i < n; i++){
      if(color[i]!= 2 && a[u][i] != INF){
        if(d[i] > a[u][i]){
          d[i] = a[u][i];
          p[i] = u;
          color[i] = 1;
        }
      }
    }
  }
  int sum = 0;
  for(int i = 0; i < n; i++){
    if(p[i] != -1) sum+=a[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;
      a[i][j] = (e == -1) ? INF:e;
    }
  }
  cout << prim() << endl;
  return 0;
}