Go: package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
)

const (
	MAX_N      = 200
	MAX_WEIGHT = 2000
)

type Edge struct {
	start  int
	end    int
	weight int
}

type ShortestInfo struct {
	edges    []Edge
	shortest int
}

func main() {
	sc := bufio.NewScanner(os.Stdin)
	sc.Split(bufio.ScanWords)

	n, err := nextInt(sc)
	if err != nil {
		panic(err)
	}

	shortestList := make([]ShortestInfo, n)
	for i := 1; i < n; i++ {
		shortestList[i] = ShortestInfo{edges: []Edge{}, shortest: MAX_N*MAX_WEIGHT + 1}
	}

	adjacentMatrix := make([][]int, n)
	for i := 0; i < n; i++ {
		adjacentMatrix[i] = make([]int, n)
		for j := 0; j < n; j++ {
			v, err := nextInt(sc)
			if err != nil {
				panic(err)
			}
			adjacentMatrix[i][j] = v
		}
	}

	mst([]int{1}, shortestList, adjacentMatrix)
	fmt.Println(calculateMinWeight(shortestList))
}

func calculateMinWeight(shortestList []ShortestInfo) int {
	visitedList := make([]bool, len(shortestList))
	visitedList[0] = true
	sumWeight := 0
	for !visitedAll(visitedList) {
		shortestInfo := shortestList[0]
		shortestList = shortestList[1:]
		edges := shortestInfo.edges
		for _, edge := range edges {
			if !visitedList[edge.end-1] {
				visitedList[edge.end-1] = true
				sumWeight += edge.weight
			}
		}
	}
	return sumWeight
}

func visitedAll(visitedList []bool) bool {
	for _, v := range visitedList {
		if !v {
			return false
		}
	}
	return true
}

func mst(nodeStack []int, shortestList []ShortestInfo, adjacentMatrix [][]int) {
	for len(nodeStack) > 0 {
		currentNode := nodeStack[0]
		nodeStack = nodeStack[1:]
		currentShortest := getShortest(currentNode, shortestList)
		adjacents := findAdjacents(currentNode, adjacentMatrix)
		for i := 0; i < len(adjacents); i++ {
			adj := adjacents[i]
			s := getShortest(adj.end, shortestList)
			if currentShortest.shortest+adj.weight < s.shortest || (currentShortest.shortest+adj.weight == s.shortest && len(currentShortest.edges)+1 > len(s.edges)) {
				setShortest(adj.end, adj, currentShortest, shortestList)
				nodeStack = append(nodeStack, adj.end)
			}
		}
	}
}

func getShortest(node int, shortestList []ShortestInfo) ShortestInfo {
	return shortestList[node-1]
}

func setShortest(node int, edge Edge, currentShortest ShortestInfo, shortestList []ShortestInfo) {
	newEdges := make([]Edge, len(currentShortest.edges)+1)
	for i := 0; i < len(newEdges)-1; i++ {
		newEdges[i] = currentShortest.edges[i]
	}
	newEdges[len(newEdges)-1] = edge
	shortestInfo := ShortestInfo{edges: newEdges, shortest: currentShortest.shortest + edge.weight}
	shortestList[node-1] = shortestInfo
}

func findAdjacents(currentNode int, adjacentMatrix [][]int) []Edge {
	adjacents := adjacentMatrix[currentNode-1]
	edges := []Edge{}
	for i, v := range adjacents {
		if v > 0 {
			edges = append(edges, Edge{start: currentNode, end: i + 1, weight: v})
		}
	}
	return edges
}

func nextInt(sc *bufio.Scanner) (int, error) {
	sc.Scan()
	return strconv.Atoi(sc.Text())
}

C++: #include <iostream>
#include <vector>
#include <stack>
#include <queue>

using namespace std;

static const long long FAR = (1LL << 21);
static const int N = 100;

int n, m[N][N];
bool visited[N];
int d[N], p[N];

int prim(){
	for (int i = 0; i < n; ++i) {
		d[i] = FAR;
		p[i] = -1;
		visited[i] = false;
	}

	d[0] = 0;
	while (true)
	{
		int mi = FAR;
		int u = -1;
		for (int i = 0; i < n; ++i)	{
			if (!visited[i] && d[i] < mi){
				u = i;
				mi = d[i];
			}
		}

		if (-1 == u) break;
		visited[u] = true;

		for (int v = 0; v < n; ++v) {
			if (!visited[v] && m[u][v] < d[v]){
				p[v] = u;
				d[v] = m[u][v];
			}
		}
	}

	int res = 0;
	for (int i = 0; i < n; ++i) {
		if (p[i] != -1) res += m[i][p[i]];
	}
	return res;
}

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) ? FAR : e);
		}
	}

	cout << prim() << endl;

	return 0;
}

C++: #include <iostream>
#include <tuple>
#include <vector>
#include <algorithm>

int P[10000];

void init(int N)
{
    for (int i = 0; i < N; i++)
    {
        P[i] = i;
    }
}

int root(int a)
{
    if (P[a] == a)
    {
        return a;
    }
    else
    {
        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, weight = 0;
    std::cin >> n;
    init(n);
    std::vector<std::tuple<int, int, int>> edge;
    for (int i = 0; i < n; i++)
    {
        for (int j = 0; j < n; j++)
        {
            int a;
            std::cin >> a;
            if (j >= i + 1 && a != -1)
            {
                edge.push_back(std::make_tuple(a, i + 1, j + 1));
            }
        }
    }
    sort(edge.begin(), edge.end());
    for (int i = 0; i < edge.size(); i++)
    {
        //        std::cout << std::get<0>(edge.at(i)) << " " << std::get<1>(edge.at(i)) << " " << std::get<2>(edge.at(i)) << std::endl;
        if (is_same_set(std::get<1>(edge.at(i)), std::get<2>(edge.at(i))) == 0)
        {
            weight += std::get<0>(edge.at(i));
            unite(std::get<1>(edge.at(i)), std::get<2>(edge.at(i)));
        }
    }
    std::cout << weight << std::endl;
    return 0;
}

C++: #include<cstdio>
static const int kMaxVertices = 100;
static const int kInfinity = 3000;
static int Adjacency_matrix[kMaxVertices][kMaxVertices];

void Prim(int n){
	bool has_not_been_discovered[kMaxVertices];
	int lowest_cost_edge[kMaxVertices];
	int parent[kMaxVertices];
	for (int i = 0; i < n; ++i)	{
		has_not_been_discovered[i] = true;
		lowest_cost_edge[i] = kInfinity;
	}
	lowest_cost_edge[0] = 0;
	parent[0] = -1;

	while(true) {
		int mincost = kInfinity;
		int vertex_to_add;
		for (int i = 0; i < n; ++i)	{
			if(has_not_been_discovered[i] && lowest_cost_edge[i] < mincost){
				mincost = lowest_cost_edge[i];
				vertex_to_add = i;
			}
		}

		if(mincost == kInfinity)
			break;

		has_not_been_discovered[vertex_to_add] = false;
		int vertex_added = vertex_to_add;

		for (int i = 0; i < n; ++i)	{
			if(has_not_been_discovered[i] && Adjacency_matrix[vertex_added][i] < lowest_cost_edge[i]) {
				lowest_cost_edge[i] = Adjacency_matrix[vertex_added][i];
				parent[i] = vertex_added;
			}
		}
	}
	int total_cost = 0;
	for (int i = 0; i < n; ++i)
		total_cost += lowest_cost_edge[i];
	printf("%d\n", total_cost);
}

int main() {
	int number_of_vertices;
	scanf("%d", &number_of_vertices);
	for (int vertex_from = 0; vertex_from < number_of_vertices; ++vertex_from) {
		for (int vertex_to = 0; vertex_to < number_of_vertices; ++vertex_to)	{
			int tmp;
			scanf("%d", &tmp);
			Adjacency_matrix[vertex_from][vertex_to] = (tmp < 0) ? kInfinity : tmp;
		}
	}
	Prim(number_of_vertices);
	return 0;
}

C++: #include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <iostream>
#include <algorithm>
#define N 110
#define INF 0x3f3f3f3f
using namespace std;
typedef struct {
    int w;
    int s;
    int e;
}Edge;
Edge edge[N*N];
int m[N][N];
int father[N];
int n;
int k = 0;  //edge的下标
int result = 0;
int findfather(int x){
    return x == father[x] ? x : findfather(father[x]);
}
int cmp(Edge a, Edge b){
    return a.w < b.w;
}
void Kruscal(){
    for(int u = 0; u < k; u++){
        int s = edge[u].s;
        int e = edge[u].e;
        if(findfather(s) != findfather(e)){
            father[findfather(e)] = s;
            result += edge[u].w;
        }
    }
    printf("%d\n", result);
}
int main(){

    scanf("%d", &n);
    for(int i = 1; i <= n; i++) father[i] = i;
    for(int i = 1; i <= n; i++){
        for(int j = 1; j <= n; j++){
            int b;
            scanf("%d", &b);
            m[i][j] = b;
        }
    }
    for(int i = 1; i <= n; i++){
        for(int j = i; j <= n ;j++){
            if(m[i][j] != -1) {
                edge[k].w = m[i][j];
                edge[k].s = i;
                edge[k].e = j;
                k++;
            }
        }
    }
    sort(edge, edge+k, cmp);
    Kruscal();
    return 0;
}