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<algorithm>
#define INF 99999
using namespace std;

int G[110][110] = {};

int prim(int n)
{
    int mini[110];
    int used[110];

    //init
    for(int i = 0;i < n;i++){
        mini[i] = INF;
        used[i] = 0;
    }
    mini[0] = 0;
    int ans = 0;

    while(true)
    {
        int v = -1;
        for(int i = 0;i < n;i++){
            if(!used[i] && (v == -1 || mini[i] < mini[v])) v = i;
        }
        if(v == -1) break;
        //ans = -1;

        used[v] = 1;
        ans += mini[v];
        //cout << ans << endl;

        for(int i = 0;i < n;i++){
            mini[i] = min(mini[i], G[v][i]);
        }
    }
    return ans;
}

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

    for(int i = 0;i < n;i++){
        for(int j = 0;j < n;j++){
            int cost;
            cin >> cost;
            if(cost == -1) G[i][j] = INF;
            else G[i][j] = cost;
        }
    }

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

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


int P[10010]; // 0 から 10000 までの頂点を取り扱い可能
void init(int N) { // 初期化 はじめは全ての頂点はバラバラ
  for (int i=0; i<N; i++) P[i] = i;
}
int root (int a) { //aの大表現を求める
  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);
}


int main(){
  int n;
  cin >> n;
  tuple<int,int,int> cost[10010]; //Array for cost
  int count = 0;
  for (int i = 0; i < n; i ++){
    for (int j = 0; j < n; j++){
      int t;
      cin >> t;
      if (t >= 0){
      get<1>(cost[count]) = i;
      get<2>(cost[count]) = j;
      get<0>(cost[count]) = t;
      count++;
    }
    }
  }
  sort(cost,cost + n*n);//sortする

  init(n + 10);

  int ans = 0;
  int anscount = 0;//このcountがnになった時点で終了する
  int k = 0;
  while (anscount != n-1){
    if (is_same_set(get<1>(cost[k]), get<2>(cost[k]))) {
      k ++;
    }
    else{
      unite(get<1>(cost[k]), get<2>(cost[k]));
      ans += get<0>(cost[k]);
      k++;
      anscount++;
    }
  }


  cout << ans << endl;
  return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;
  static const int MAX=100;
  static const int INF=(1<<21);
  static const int WHI=0;
  static const int GRA=1;
  static const int BLA=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]=INF;
      p[i]=-1;
      color[i]=WHI;
    }
    
    d[0]=0;
    while(1){
      minv=INF;
      u=-1;
      for(int i=0;i<n;i++){
	if(minv>d[i]&&color[i]!=BLA){
	  u=i;
	  minv=d[i];
	}
      }

      if(u==-1){
	break;
      }
      color[u]=BLA;

      for(int v=0;v<n;v++){
	if(color[v]!=BLA&&M[u][v]!=INF){
	  if(M[u][v]<d[v]){
	    d[v]=M[u][v];
	    p[v]=u;
	    color[v]=GRA;
	  }

	}
      }
    }

    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;
	if(e==-1){
	  M[i][j]=INF;
	}else{
	  M[i][j]=e;
	}
      }
    }
    
    cout<<prim()<<endl;
    return 0;
  }

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

int P[10010];
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);
}

signed main() {
	int g;
	cin >> g;
	vector<pair<int, pair<int, int>>> a;
	int weight;

	init(g);

	for (int i = 0; i < g; ++i) {
		for (int j = 0; j < g; ++j) {
			cin >> weight;
			if (i > j and weight != -1) {
				a.push_back(make_pair(weight, make_pair(i, j)));
			}
		}
	}

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

	int ans = 0;

	for (auto e : a) {
		//cout << e.first << ' ' << e.second.first << ' ' << e.second.second << endl;
		if (!is_same_set(e.second.first, e.second.second)) {
			unite(e.second.first, e.second.second);
			ans += e.first;
		}
		else {
			continue;
		}
	}
	cout << ans << endl;
}