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 <bits/stdc++.h>
using namespace std;

int main() {

  int n;
  cin >> n;

  int INF = 10000;
  int graph[n][n];
  for(int i = 0; i < n; i++) {
    for(int j = 0; j < n; j++) {
      int v;
      cin >> v;
      if (v == -1) {
        graph[i][j] = INF;
      } else {
        graph[i][j] = v;
      }
    }
  }

  int answer = 0;

  vector<bool> visited(n);
  vector<int> cost(n);

  visited[0] = 1;

  for (int i = 0; i < n; i++) {
    cost[i] = graph[0][i];
  }

  while (1) {
    int v = -1;
    for (int i = 0; i < n; i++) {
      if (visited[i] || cost[i] < 0) {
        continue;
      }
      if (v < 0 || cost[i] < cost[v]) {
        v = i;
      }
    }
    if (v < 0) {
      break;
    }
    visited[v] = 1;
    answer += cost[v];
    for (int i = 0; i < n; i++) {
      if (graph[v][i] < 0) {
        continue;
      }
      if (cost[i] < 0 || graph[v][i] < cost[i]) {
        cost[i] = graph[v][i];
      }
    }
  }

  cout << answer << endl;

  return 0;
}

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

const int INF = 2001;
const int MAX_N = 100;

int N;
int A[MAX_N][MAX_N];

void solve() {
  int cost , d[MAX_N], p[MAX_N];
  bool c[MAX_N];

  for (int i = 0; i < N; i++) {
    c[i] = false;
    d[i] = INF;
    p[i] = -1;
  }

  d[0] = 0;
  
  while (1) {
    int cost = INF;
    int from = -1;

    for (int i = 0; i < N; i++) {
      if (d[i] < cost && !c[i]) {
        cost = d[i];
        from = i;
      }
    }

    if (from == -1) break;

    c[from] = true;

    for (int dest = 0; dest < N; dest++) {
      if (!c[dest] && A[from][dest] != -1) {
        if (A[from][dest] < d[dest]) {
          d[dest] = A[from][dest];
          p[dest] = from;
        }
      }
    }
  }

  int sum = 0;
  for (int i = 0; i < N; i++)
    if (p[i] != -1) sum += A[i][p[i]];

  cout << sum << endl;
}

int main() {
  cin >> N;
  for (int i = 0; i < N; i++)
    for (int j = 0; j < N; j++)
      cin >> A[i][j];
  solve();
}

C++: #include<bits/stdc++.h>

using namespace std;

class UnionFind{
    int N;
    vector<int> par;
    vector<int> rank;
    public:
    UnionFind(int N):N(N),par(N),rank(N,0){iota(par.begin(),par.end(),0);}
    int find(int x);
    void unite(int x,int y);
    bool same(int x,int y){return find(x)==find(y);}
};

int UnionFind::find(int x){
    if(par[x]==x) return x;
    return par[x]=find(par[x]);
}
void UnionFind::unite(int x,int y){
    x=find(x),y=find(y);
    if(x==y) return;
    if(rank[x]>=rank[y]){
        if(rank[x]==rank[y]) rank[x]++;
        par[y]=x;
    }else{
        par[x]=y;
    }
}
using Weight=long long;
struct Edge{
    int from,to;
    Weight weight;
};
bool operator<(const Edge& lhs,const Edge& rhs){
    return lhs.weight<rhs.weight;
}

using Edges=vector<Edge>;
using Graph=vector<Edges>;


Edges MinimumSpanningTree(int n,Edges edges){
    Edges res;
    UnionFind u(n);
    for(int i=0;i<edges.size();i++){
        if(!u.same(edges[i].from,edges[i].to)){
            res.push_back(edges[i]);
            u.unite(edges[i].from,edges[i].to);
        }
    }
    return res;
}

Edges MinimumSpanningTree(const Graph& g){
    Edges edges;
    for(int i=0;i<g.size();i++){
        copy(g[i].begin(),g[i].end(),back_inserter(edges));
    }
    auto res=MinimumSpanningTree(g.size(),edges);
    return res;
}

int main(){
    int n;
    cin>>n;
    Graph g(n);
    for(int i=0;i<n;i++){
        for(int j=0;j<n;j++){
            Weight c;
            cin>>c;
            if(c==-1) continue;
            g[i].push_back({i,j,c});
        }
    }
    auto mst=MinimumSpanningTree(g);
    cout<<accumulate(mst.begin(),mst.end(),Weight(0),[](const Weight& lhs,const Edge& rhs){return lhs+rhs.weight;})<<endl;
    return 0;
}

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;
}