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 <vector>
#include <algorithm>

using namespace std;
struct edge {
    edge() {};
    edge(int s, int e, int d) : start {s}, end {e}, d {d} {};
    bool operator<(edge y){
        return d < y.d || start < y.start || end < y.end;
    };
    bool operator>(edge y){
        return d > y.d || start > y.start || end > y.end;
    };
    bool operator==(edge y){
        return d == y.d || start == y.start || end || y.end;
    };
    int start;
    int end;
    int d;
};

void dfs(vector<vector<int>> graph, bool* used, int id, int n){
    used[id] = true;
    for(int i = 0; i < n; ++i){
        if(graph[id][i] != -1 || used[i]){
            continue;
        }
        dfs(graph, used, i, n);
    }
}

bool is_connected(vector<vector<int>>& graph){
    int n = graph.size();
    bool used[n];
    for(int i = 0; i < n; ++i){
        used[i] = false;
    }
    dfs(graph, used, 0, n);
    for(int i = 0; i < n; ++i){
        if(!used[i]){
            return false;
        }
    }
    return true;
}

void delete_edges(vector<vector<int>>& graph){
    int edges_num = 0;
    vector<edge> vec {};
    int n = graph.size();
    for(int i = 0; i < n; ++i){
        for(int j = i+1; j < n; ++j){
            if(graph[i][j] != -1){
                ++edges_num;
                vec.push_back(edge(i, j, graph[i][j]));
            }
        }
    }
    sort(vec.begin(), vec.end(), [](edge x, edge y){
            return y.d < x.d;
         });

    for(auto it = vec.begin(); it != vec.end(); ++it){
        edge e = *it;
        graph[e.start][e.end] = -1;
        graph[e.end][e.start] = -1;
        if(is_connected(graph)){
            --edges_num;
            if(edges_num == n-1){
                return;
            }
        } else {
            graph[e.start][e.end] = e.d;
            graph[e.end][e.start] = e.d;
        }
    }
}

vector<vector<int>> gen_spanning_tree(vector<vector<int>>& graph){
    vector<vector<int>> tree {graph};
    delete_edges(tree);
    return tree;

}

int calc_weight(vector<vector<int>> graph){
    int sum = 0;
    int n = graph.size();
    for(int i = 0; i < n; ++i){
        for(int j = i+1; j < n; ++j){
            if(graph[i][j] == -1){
                continue;
            }
            sum += graph[i][j];
        }
    }
    return sum;
}

int main(){
    int n;
    cin >> n;
    vector<vector<int>> graph {n, vector<int>(n, 0)};
    for(int i = 0; i < n; ++i){
        for(int j = 0; j < n; ++j){
            cin >> graph[i][j];
        }
    }
    auto tree = gen_spanning_tree(graph);
    cout << calc_weight(tree) << endl;
}

C++: #include<bits/stdc++.h>
using namespace std;
#define MAX 100
#define INF 1<<25
int n,cost[MAX][MAX];
int mincost[MAX];
bool used[MAX];
int prim(){
  for(int i=0;i<n;i++)mincost[i]=INF,used[i]=false;
    mincost[0]=0;
  int res=0;
  while(1){
    int v=-1;
    for(int u=0;u<n;u++)
      if(!used[u]&&(v==-1||mincost[u]<mincost[v]))v=u;
    if(v==-1)break;
    used[v]=1;
    res+=mincost[v];
    for(int u=0;u<n;u++)mincost[u]=min(mincost[u],cost[v][u]);
  }
return res;
}
int main(){
  cin>>n;
  for(int i=0;i<n;i++){
    for(int j=0;j<n;j++){
      int e;
      cin>>e;
      cost[i][j]=(e==-1)?INF:e;
    }
  }
  cout<<prim()<<endl;
}

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

using namespace std;

// int search_xyz(int e);
int tree[100];

int root(int a);
bool is_same_set(int p, int q);
void joint(int x, int y);

int main()
{
    int n, tempa, k = 0;
    cin >> n;
    int e = n * (n - 1) / 2;
    pair<int, pair<int, int>

         >
        a[e];
    for (int i = 0; i < n; i++)
    {
        tree[i] = i;
        for (int j = 0; j < n; j++)
        {
            cin >> tempa;
            if (i < j)
            {
                a[k] = make_pair(tempa, make_pair(i, j));
                k++;
            }
        }
    }
    sort(a, a + e);
    int result = 0;
    for (int i = 0; i < e; i++)
    {
        if (a[i].first == -1)
            continue;

        if (is_same_set(a[i].second.first, a[i].second.second))
        {
            continue;
        }
        else
        {
            joint(a[i].second.first, a[i].second.second);
            result += a[i].first;
        }
    }
    cout << result << endl;
    return 0;
}

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

bool is_same_set(int p, int q)
{
    return root(p) == root(q);
}

void joint(int x, int y)
{
    tree[root(x)] = root(y);
}