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>
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(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(){
    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<cstdio>
#include<vector>
#include<algorithm>
#include<utility>
#include<numeric>
#include<iostream>
#include<array>
#include<string>
#include<sstream>
#include<stack>
#include<queue>
#include<list>
#define _USE_MATH_DEFINES

#include<math.h>
#include<map>

#define SENTINEL 1000000001

#define min(a,b) (a)>(b)?(b):(a)
#define max(a,b) (a)>(b)?(a):(b)

using namespace std;

/*int find(int x) 
{
	if (x == parent[x])return x;
	else 
	{
		return parent[x] = find(parent[x]);
	}
}

bool isSame(int x, int y)
{
	return find(x) == find(y);
}

void unite(int x, int y) 
{
	x = find(x);
	y = find(y);

	if (x == y)return;

	if (Rank[x] < Rank[y]) 
	{
		parent[x] = y;
	}
	else
	{
		parent[y] = x;
		if (Rank[x] == Rank[y])Rank[x]++;
	}
}*/

struct UnionFind
{
	vector<int> parent;
	vector<int> rank;

	UnionFind(int n) :parent(n), rank(n, 0)
	{
		for (int i = 0; i < n; i++)
		{
			parent[i] = i;
		}
	}

	int find(int x)
	{
		if (x == parent[x])
		{
			return x;
		}
		else
		{
			return parent[x] = find(parent[x]);
		}
	}

	void unite(int x, int y)
	{
		x = find(x);
		y = find(y);

		if (x == y)
		{
			return;
		}

		if (rank[x] < rank[y])
		{
			parent[x] = y;
		}
		else
		{
			parent[y] = x;
			if (rank[x] == rank[y])
			{
				rank[x]++;
			}
		}
	}

	bool same(int x, int y)
	{
		return find(x) == find(y);
	}
};

struct Edge
{
	int v1, v2, cost;
	Edge(int v1, int v2, int cost) :v1(v1), v2(v2), cost(cost){}
	bool operator<(const Edge& e)const
	{
		return cost < e.cost;
	}
};

int main()
{
	int n;
	cin >> n;

	vector<Edge> e;
	UnionFind uf(n);

	for (int i = 0; i < n; i++)
	{
		for (int j = 0; j < n; j++)
		{
			int c;
			cin >> c;
			if (c != -1)
			{
				e.push_back(Edge(i, j, c));
			}
		}
	}

	sort(e.begin(), e.end());

	int cost = 0;
	for(auto& te:e)
	{
		if (!uf.same(te.v1, te.v2))
		{
			uf.unite(te.v1, te.v2);
			cost += te.cost;
		}
	}

	cout << cost << endl;

	return 0;
}

C++: #include <iostream>
#include <algorithm>
#include <set>
#include <vector>
#include <climits>


#define MAX 100


using namespace std;


int main()
{
    vector< vector<int> > adj(MAX + 1, vector<int>(MAX, INT_MAX));
    int n;
    cin >> n;
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < n; j++) {
            int cost;
            cin >> cost;
            if (cost >= 0)
                adj[i][j] = cost;
        }
    }

    vector<int> parent(MAX);
    vector<int> in_tree(MAX, false);
    vector<int> min_dists(MAX, INT_MAX);
    int tree_count = 0;

    int acc = 0;
    min_dists[0] = 0;
    for (int tc = 0; tc < n; tc++) {
        int min_cost = INT_MAX;
        int min_v = -1;
        for (int i = 0; i < n; i++) {
            if (min_cost > min_dists[i] && !in_tree[i]) {
                min_cost = min_dists[i];
                min_v = i;
            }
        }

        acc += min_cost;
        in_tree[min_v] = true;

        for (int i = 0; i < n; i++) {
            if (min_dists[i] > adj[min_v][i] && !in_tree[i]) {
                min_dists[i] = adj[min_v][i];
                parent[i] = min_v;
            }
        }
    }

    cout << acc << endl;

    return 0;
}

C++: #include <iostream>
using namespace std;
#define INF 1000000

int main(){
  long n;
  long G[100][100],d[100];
  long minc,mi,i;
  bool flgs[i];
  long suma=0;
  
  cin >> n;
  for (i=0;i<n;i++){
    for (int j=0;j<n;j++){
      cin >> G[i][j];
      if (G[i][j]==-1) G[i][j]=INF;
    }
    d[i]=INF;
    flgs[i]=false;
  }

  d[0]=0;
  while (true){
    minc=INF;
    for (i=0;i<n;i++){
      if (!flgs[i]&&d[i]<minc){
	minc=d[i];
	mi=i;
      }
    }
    if (minc==INF) break;
    flgs[mi]=true;
    for (i=0;i<n;i++){
      if (G[mi][i]==INF)continue;
      if (!flgs[i]&&d[i]>G[mi][i]){
	d[i]=G[mi][i];
	//cout << d[i];
      }
    }
  }
  
  for (i=0;i<n;i++){
    suma+=d[i];
  }
  cout << suma<<endl;
  
  return 0;
}