Go: package main

import (
	"sort"
	"bufio"
	"os"
	"fmt"
	"strconv"
)

// Define disjoint-set forests

type vertexNo int

type vertex struct {
	number vertexNo
	rank int
	parent *vertex
}

func makeSet(number vertexNo) vertex {
	return vertex {
		number: number,
		rank: 0,
		parent: nil,
	}
}

func union(x, y *vertex) *vertex {
	return link(findSet(x), findSet(y))
}

func link(x, y *vertex) *vertex {
	if x.rank > y.rank {
		y.parent = x
		return x
	} else {
		x.parent = y
		if x.rank == y.rank {
			y.rank += 1
		}
		return y
	}
}

func findSet(v *vertex) *vertex {
	if v.parent == nil {
		return v
	}
	v.parent = findSet(v.parent)
	return v.parent
}

// Function to calculate minimum spanning tree

type matrix [][]int

type edge struct {
	one vertexNo
	two vertexNo
	weight int
}

const(
	MAX_VERTICES = 100
)

func newEdgesFromMatrix(matrix matrix) []edge {
	edges := make([]edge, 0, MAX_VERTICES * MAX_VERTICES)

	for i := 0; i < len(matrix); i++ {
		row := matrix[i]
		for j := i; j < len(row); j++ {
			if row[j] >= 0 {
				edge := edge{one: vertexNo(i+1), two: vertexNo(j+1), weight: row[j]}
				edges = append(edges, edge)
			}
		}
	}

	return edges
}

func solve(matrix matrix) int {
	nVertices := len(matrix)

	// initialize vertices
	vertices := make([]vertex, nVertices)
	for i := 0; i < nVertices; i++ {
		vertices[i] = makeSet(vertexNo(i+1))
	}

	// prepare edges sorted in the ascending order of weight
	edges := newEdgesFromMatrix(matrix)
	sort.Slice(edges, func(i, j int) bool {
		return edges[i].weight < edges[j].weight
	})

	unionCount := 0
	weightSum := 0
	for unionCount < nVertices-1 {
		for i := 0; i < len(edges); i++ {
			edge := edges[i]
			v1 := &vertices[edge.one-1]
			v2 := &vertices[edge.two-1]
			rep1 := findSet(v1)
			rep2 := findSet(v2)
			if rep1.number == rep2.number {
				continue
			}
			union(rep1, rep2)
			edges = append(edges[0:i], edges[i+1:]...)
			weightSum += edge.weight
			unionCount++
			break
		}
	}

	return weightSum
}

// main

func main() {
	sc := bufio.NewScanner(os.Stdin)
	sc.Split(bufio.ScanWords)

	n, err := nextInt(sc)
	if err != nil {
		panic(err)
	}

	matrix := parseMatrix(n, sc)

	edges := newEdgesFromMatrix(matrix)
	sort.Slice(edges, func(i, j int) bool {
		return edges[i].weight < edges[j].weight
	})

	fmt.Println(solve(matrix))
}

func parseMatrix(n int, sc *bufio.Scanner) [][]int {
	matrix := make([][]int, n)
	for i := 0; i < n; i++ {
		matrix[i] = make([]int, n)
		for j := 0; j < n; j++ {
			x, err := nextInt(sc)
			if err != nil {
				panic(err)
			}
			matrix[i][j] = x
		}
	}
	return matrix
}

func nextText(sc *bufio.Scanner) string {
	sc.Scan()
	return sc.Text()
}

func nextInt(sc *bufio.Scanner) (int, error) {
	return strconv.Atoi(nextText(sc))
}

C++: #include <iostream>
#include <iomanip>
#include <algorithm>
#include <cfloat>
#include <climits>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <functional>
#include <sstream>
#include <string>
#include <utility>
#include <map>
#include <memory>
#include <set>
#include <vector>
#include <deque>
#include <list>
#include <stack>
#include <queue>
// #include <array>

using namespace std;

class MSTree
{
public:
  int solve();
};

int MSTree::solve()
{
  int n;
  cin >> n;
  
  vector<vector<int>> adj(n, vector<int>(n));
  list<int> t, vt;

  for ( auto i = 0; i < n; ++i )
    for ( auto j = 0; j < n; ++j )
      cin >> adj[i][j];

  t.push_back( 0 );

  for ( auto i = 1; i < n; ++i )
    vt.push_back( i );

  int w = 0;

  while ( vt.size() )
    {
      int mw = 4000;
      list<int>::iterator itm;
      
      for ( auto ita = t.begin(); ita != t.end(); ++ita )
        for ( auto itb = vt.begin(); itb != vt.end(); ++itb )
          {
            if ( adj[*ita][*itb] >= 0 && mw > adj[*ita][*itb] )
              {
                itm = itb;
                mw = adj[*ita][*itb];
              }
          }
      
      w += mw;
      t.push_back( *itm );
      vt.erase( itm );
    }

  cout << w << endl;
    
  return 0;
}

int main()
{
  MSTree m;

  m.solve();

  return 0;
}

C++: #include<iostream>
#include<algorithm>
#include<queue>
#include<utility>
#include<vector>

using namespace std;



int main() {
	int n,tmp;
	cin >> n;
	pair<int, pair<int, int>>* sorted_weight = new pair<int, pair<int, int>>[n * n];
	int* equiv = new int[n];


	int cnt_equiv = 0;
	int cnt_edge = 0;
	int cost = 0;
	
	for (int i = 0;i < n;i++) {
		for (int j = 0;j < n;j++) {
			cin >> tmp;
			sorted_weight[i * n + j] = make_pair(tmp, make_pair(i, j));
		}
		equiv[i] = 0;
	}
	sort(sorted_weight, sorted_weight + n * n);
	for (int i = upper_bound(sorted_weight, sorted_weight + n * n, make_pair(-1, make_pair(n, n))) - sorted_weight;cnt_edge<n-1;i++) {
		if (equiv[sorted_weight[i].second.second] == 0 || equiv[sorted_weight[i].second.first] == 0) {
			cnt_edge++;
			cost += sorted_weight[i].first;
			if (equiv[sorted_weight[i].second.second] == 0 && equiv[sorted_weight[i].second.first] == 0) {
				cnt_equiv++;
				equiv[sorted_weight[i].second.second] = equiv[sorted_weight[i].second.first] = cnt_equiv;
			}
			else if (equiv[sorted_weight[i].second.second] == 0) {
				equiv[sorted_weight[i].second.second] = equiv[sorted_weight[i].second.first];
			}
			else {
				equiv[sorted_weight[i].second.first] = equiv[sorted_weight[i].second.second];
			}
		}
		else if (equiv[sorted_weight[i].second.second] != equiv[sorted_weight[i].second.first]) {
			cnt_edge++;
			cost += sorted_weight[i].first;
			tmp = equiv[sorted_weight[i].second.second];
			for (int j = 0;j < n;j++) {
				if (equiv[j] == tmp)equiv[j] = equiv[sorted_weight[i].second.first];
			}
		}
	}
	
	cout << cost << endl;
}

C++: #include <stdio.h>
#define INF 100000000
#define BLACK 0
#define WHITE 1
#define N 101
 
int n,a[N][N];

int prim(void)
{
  int u,v,i,mincost,d[N],pi[N],total = 0,color[N];
 
  for(u = 1;u <= n;u++){
    d[u] = INF;
    pi[u] = -1;
    color[u] = WHITE;
  }
 
  d[1] = 0;
     
  while(1){
    mincost = INF;
    for(i = 1;i <= n;i++){
      if(color[i] != BLACK && d[i] < mincost){
	mincost = d[i];
	u = i;
      }
    }
     
    if(mincost == INF) break;
    
    color[u] = BLACK;
     
    for(v = 1;v <= n;v++){
      if(color[v] != BLACK && a[u][v] < d[v]){
	pi[v] = u;
	d[v] = a[u][v];
      }
    }
    color[v] = BLACK;
  }
 
  for(i = 1;i <= n; i++){
    if(pi[i] != -1) total += a[i][pi[i]];
  }
 
  return total;
}

 
int main()
{
  int i,j;
 
  scanf("%d",&n);
 
  for(i = 1;i <= n;i++){
    for(j = 1;j <= n;j++){
      scanf("%d",&a[i][j]);
      if(a[i][j] == -1) a[i][j] = INF;
    }
  }
 
   printf("%d\n",prim());
    
   return 0;
 
}

C++: #include <iostream>

const int MAX   = 100;
const int INFTY = (1<<21);
const int WHITE = 0;
const int GRAY  = 1;
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() {
    std::cin >> n;

    for (int i = 0; i < n; i++) {
        for (int j = 0; j < n; j++) {
            int e;
            std::cin >> e;
            M[i][j] = (e == -1) ? INFTY : e;
        }
    }

    std::cout << prim() << std::endl;

    return 0;
}