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 <queue>
using namespace std;
int main() {
    int n;
    cin >> n;
    int a[100][100];
    for (int i = 0; i < n; ++i)
        for (int j = 0; j < n; ++j)
            cin >> a[i][j];
    struct Edge {
        int from, to, weight;
        Edge(int from, int to, int weight) : from(from), to(to), weight(weight) {}
        bool operator < (const Edge& e) const {
            return weight > e.weight;
        }
    };
    int sm = 0;
    priority_queue<Edge> q;
    q.push(Edge(-1, 0, 0));
    bool visited[100]{};
    while (q.size()) {
        auto e = q.top();
        q.pop();
        if (visited[e.to]) continue;
        visited[e.to] = true;
        sm += e.weight;
        for (int i = 0; i < n; ++i) {
            if (a[e.to][i] >= 0 && !visited[i])
                q.push(Edge(e.to, i, a[e.to][i]));
        }
    }
    cout << sm << endl;
    return 0;
}

C++: #include<stdio.h>
#include<limits.h>
#define NIL -1
static const int MAX = 500;
static const int INFTY = (1<<21);
int n, i, j, e, v, sum, u , minV;
main(){
 
  int M[MAX][MAX], key[MAX], L[MAX],A[MAX];
  
  scanf("%d", &n);
  
  for ( i = 0; i< n; i++ ){
    for ( j = 0; j < n; j++ ){
      scanf("%d", &e);
      M[i][j] = (e==-1)?INFTY:e;
    }
  }

  for(i = 0; i < n; i++){
    key[i] = INT_MAX;
    L[i] = NIL;
    A[i] = NIL;
  }
  
  key[0]=0;
  
  for(;;){
    
    u = NIL;
    
    minV = INT_MAX;
    
    for(i = 0;  i < n; i++){
      
      if(key[i] < minV && A[i] == NIL){
        
        u = i;
        
        minV = key[i];
        
      }
      
    }  
    if(u == NIL)break;
    
    A[u] = u; 
    
    for(v = 0; v < n; v++){
      if(M[u][v] < key[v] && A[v] == NIL){
        key[v] = M[u][v];
        L[v] = u;
      }
    }
  }
  
  sum = 0;
  
  for ( i = 0; i < n; i++ ){
    if ( L[i] != -1 ) sum +=M[L[i]][i];
  }
  
  printf("%d\n", sum);
  
  
  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;
}

C++: //
// Created by 李青坪 on 2018/11/6.
//

#include <cstdio>
#include <algorithm>
#include <set>
#define MAXN 100
#define INFINITY 1<<20
#define NIL -1
using namespace std;

int n;
int sum;
int graph[MAXN][MAXN];
int d[MAXN];
int parent[MAXN];
set<int> T;
set<int> V_T;

void init(){
    fill(d, d+n, INFINITY);
    fill(parent, parent+n, NIL);
    T.insert(0);
    for (int i = 1; i < n; ++i) {
        V_T.insert(i);
    }
    for (int j = 0; j < n; ++j) {
        d[j] = graph[0][j];
    }
}

void updateMinWeight(){

}

pair<int, pair<int, int> > findMinEdge(){
    int min = INFINITY;
    pair<int, int> r;
    set<int >::iterator itr;
    for (itr = T.begin(); itr != T.end(); itr++){
        for (set<int>::iterator itr1 = V_T.begin(); itr1 != V_T.end(); itr1++){
            if (graph[*itr][*itr1] < min){
                min = graph[*itr][*itr1];
                r = make_pair(*itr, *itr1);
            }
        }
    }
    return make_pair(min, r);
}

void findMinSpanTree(){
    while (!V_T.empty()){
        pair<int, pair<int, int> > v = findMinEdge();
        sum += v.first;
        parent[v.second.second] = v.second.first;
        T.insert(v.second.second);
        V_T.erase(V_T.find(v.second.second));
    }
}

int main(){
    scanf("%d", &n);
    for (int i = 0; i < n; ++i) {
        int w;
        for (int j = 0; j < n; ++j) {
            scanf("%d", &w);
            if (w == -1) graph[i][j] = INFINITY;
            else graph[i][j] = w;
        }
    }
    init();
    findMinSpanTree();
    printf("%d\n", sum);
}
/*
5
 -1 2 3 1 -1
 2 -1 -1 4 -1
 3 -1 -1 1 1
 1 4 1 -1 3
 -1 -1 1 3 -1
 */