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 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 (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;
}

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 a, min;
   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 ) {
      min = INFTY;
      a = -1;
  
      for ( int i = 0; i < n; i++ ) {
         if ( min > d[i] && color[i] != BLACK ) {
            a = i;
            min = d[i];
         }
      }
  
      if ( a == -1 ) break;
      color[a] = BLACK;
      for ( int v = 0; v < n; v++ ) {
         if ( color[v] != BLACK && M[a][v] != INFTY ) {
            if ( d[v] > M[a][v] ) {
               d[v] = M[a][v];
               p[v] = u;
               color[v] = GRAY;
            }
         }
      }
   }
  
   int sum = 0;
   for ( int i = 0; i < n; i++ ) {
      if ( p[i] != -1 ) sum = 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 ll = long long;
using namespace std;
using pii = pair<int, int>;

int INF = 1000000000;
int n, x;
std::vector<std::vector<pii>> g;
priority_queue<pii, std::vector<pii>, std::greater<pii>> pq;
std::vector<int> dist;
std::vector<bool> used;

int main() {
    std::cin >> n;
    dist.assign(n, INF);
    used.assign(n, false);
    g.assign(n, std::vector<pii>(0));
    for (int i = 0; i < n; i++) {
        for (int k = 0; k < n; k++) {
            std::cin >> x;
            if (x == -1) {
                continue;
            }
            g[i].push_back(make_pair(x, k));
        }
    }
    pq.push(std::make_pair(0, 0));
    used[0] = true;
    dist[0] = 0;
    while (!pq.empty()) {
        auto e = pq.top();
        pq.pop();
        used[e.second] = true;
        for (int i = 0; i < g[e.second].size(); i++) {
            if (used[g[e.second][i].second] == false && dist[g[e.second][i].second] > g[e.second][i].first) {
                dist[g[e.second][i].second] = g[e.second][i].first;
                pq.push(g[e.second][i]);
            }
        }
    }
    ll ans = 0;
    for (int i = 0; i < dist.size(); i++) {
        ans += dist[i];
    }
    std::cout << ans << std::endl;
}

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


struct edge
{
   int cost;
   int src;
   int dst;
};

edge e[5000];
int A[100][100];
int M=0;
int P[100];

void init(int N)
{
  for (int i=0; i<=N; ++i) P[i] = i;
}

int root(int a)
{
  if (P[a] == a) return a;
  return (P[a] = root(P[a]));
}

bool is_same_set(int a, int b)
{
  return root(a) == root(b);
}

void unite(int a, int b)
{
  P[root(a)] = root(b);
}

void checkEdges(int n)
{
    for(int r=0;r<n;r++)
        for(int c=r+1;c<n;r++)
    {
        if(A[r][c]>-1)
        {
            e[M].cost=A[r][c];
            e[M].src=r;
            e[M].dst=c;
            M+=1;
        }
    }
}

bool cmp(edge e1,edge e2)
{
    return e1.cost<=e2.cost;
}
int main()
{
    int n,count=0;
    cin>>n;
    init(n);
    for(int i=0;i<n;i++)
    {
        for(int j=0;j<n;j++){
                cin>>A[i][j];
        }
    }
    checkEdges(n);
    sort(e,e+M,cmp);
    for (int i=0; i<M; ++i)
    {
        if(is_same_set(e[i].src,e[i].dst))
        {
            continue;
        }
        unite(e[i].dst,e[i].src);
        count+=e[i].cost;
    }
    cout<<count<<endl;
    //system("pause");
    return 0;
}