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;
int main(){
    int n;
    cin>>n;
    int G[n][n];
    int find[n];
    for(int i=0;i<n;i++){
        for(int j=0;j<n;j++){
            cin>>G[i][j];
        }
    }
    for(int i=0;i<n;i++){
        find[i]=-1;
    }
    find[0]=0;
    int end=0;
    int mind;
    int sum=0;
    while(true){
        int min=2001;
        for(int i=0;i<n;i++){
            if (find[i]!=-1){
                for(int j=0;j<n;j++){
                    if (G[i][j]!=-1&&G[i][j]<min){
                        if (find[j]==-1){
                            min=G[i][j];
                            mind=j;
                        }
                    }
                }
            }
        }
        find[mind]=mind;
        sum+=min;
        end+=1;
        if(end==n-1) break;
    }cout<<sum<<endl;
    return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
#define ll long long
#define rep(i, n) for (ll i = 0; i < (ll)n; ++i)
ll dx[4] = { 0, 1, 0, -1 };
ll dy[4] = { 1, 0, -1, 0 };
static const ll INF = 1 << 21;
static const ll MOD = 1e9 + 7;
static const int MAX = 100;
static const int WHITE = 0; // V-T
static const int BLACK = 1; // T

int n, M[MAX][MAX];

int prim()
{
    int u, minv;
    int d[MAX]; // d[u]: Tに属する頂点とV-Tに属するuから頂点を繋ぐ辺の中で重みが最小の辺の重みを記録
    int p[MAX]; // p[u]: MSTにおける頂点uの親を記録
    int color[MAX]; // 訪問状態を記録

    // initialize
    rep(i, n)
    {
        d[i] = INF;
        p[i] = -1;
        color[i] = WHITE;
    }
    d[0] = 0;

    while (1) {
        minv = INF;
        u = -1;
        rep(i, n)
        {
            // uは最小の経路がある かつ uへは未完了
            if (d[i] < minv && color[i] != BLACK) {
                u = i;
                minv = d[i];
            }
        }

        // 更新できる辺がないときに終了
        if (u == -1) {
            break;
        }

        color[u] = BLACK;
        rep(v, n)
        {
            // vは未完了 かつ u->vに道はある
            if (color[v] != BLACK && M[u][v] != INF) {
                // vへの経路を最小に更新
                if (M[u][v] < d[v]) {
                    d[v] = M[u][v];
                    p[v] = u;
                }
            }
        }
    }

    int sum = 0;
    rep(i, n)
    {
        if (p[i] != -1) {
            sum += M[i][p[i]];
        }
    }
    return sum;
}

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(0);

    cin >> n;
    rep(i, n)
    {
        rep(j, n)
        {
            int e;
            cin >> e;
            M[i][j] = (e == -1) ? INF : e;
        }
    }

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

C++: #include <bits/stdc++.h>
using namespace std;
 
#define INF (1<<29)
 
int G[100][100], n;
 
int prim(){
  int p[100], d[100], c[100];
  int u;
 
  for(int i = 0; i < n; i++){
    d[i] = INF;
    p[i] = -1;
    c[i] = 0;
  }
 
  d[0] = 0;
 
  while(1){
    int minv = INF;
    u = -1;
    for(int i = 0; i < n; i++){
      if(minv > d[i] && c[i] != 2){
        u = i;
        // cout << u << endl;
        minv = d[i];
      }
    }
 
    if(u == -1) break;
    c[u] = 2;
    for(int v = 0; v < n; v++){
      if(c[v] != 2 && G[u][v] != INF){
        if(d[v] > G[u][v]){
          d[v] = G[u][v];
          p[v] = u;
          c[v] = 1;
        }
      }
    }
  }
 
  int sum = 0;
  for(int i = 0; i < n; i++){
    if(p[i] != -1) sum+=G[i][p[i]];
    // cout << p[i] << endl;
  }
  return sum;
}
 
 
  int main(){
 
    cin >> n;
    for(int i = 0; i < n; i++){
      for(int j = 0; j < n; j++){
        int v;
        cin >> v;
        G[i][j] = v;
        if(v == -1) G[i][j] = INF;
      }
      // cout << endl;
    }
 
    cout << prim() << endl;
 
  }

C++: #include <cstdio>
#include <vector>
#include <algorithm>
using namespace std;
bool vis[107];
struct edge {
	int u, v, w;
	edge(int u, int v, int w) :u(u), v(v), w(w) {}
	bool operator < (const edge &rhs) const {
		return w < rhs.w;
	}
};
vector<edge> e;
int main() {
    int n, ans = 0;
    scanf("%d", &n);
    for (int i = 0; i < n; i++) {
    	for (int j = 0; j < n; j++) {
    		int tmp;
    		scanf("%d", &tmp);
    		if (tmp >= 0 && i >= j) e.push_back(edge(i, j, tmp));
		}
	}
	sort(e.begin(), e.end());
	vis[e[0].u] = true;
	bool flag = true;
	int cnt = e.size();
	while (flag) {
		flag = false;
		for (int i = 0; i < cnt; i++) {
			if (vis[e[i].u] != vis[e[i].v]) {
				vis[e[i].u] = true;
				vis[e[i].v] = true;
				ans += e[i].w;
				flag = true;
				break;
			}
		}
	}
	printf("%d\n", ans);
	return 0;
}