Go: package main

import (
	"bufio"
	"fmt"
	"log"
	"math"
	"os"
	"strconv"
	"strings"
)

type color int

const (
	white color = iota // 未訪問
	mst                // MSTとして確定
)

// 頂点
type node struct {
	matrix   []int // 隣接行列
	minToMst int   // MSTへの重み最小値
	parent   int   // 最小値の相手
	color    color
}

func main() {
	scanner := bufio.NewScanner(os.Stdin)
	n := eGetInt(scanner)

	nodes := make([]node, n)
	for i := 0; i < n; i++ {
		nodes[i].matrix = eAtoi((strings.Fields(eGetline(scanner))))
		nodes[i].parent = -1
		nodes[i].minToMst = math.MaxInt16
		nodes[i].color = white
	}

	// 初回はnode[0]がMSTに入るように,minToMstを設定
	nodes[0].minToMst = 0
	for { // 全部チェックするまで

		// どの頂点を新たにMSTに取り込むか
		min := math.MaxInt16
		u := -1
		for i := 0; i < n; i++ {
			if nodes[i].color == mst {
				continue
			}
			if nodes[i].minToMst < min {
				u = i
				min = nodes[i].minToMst
			}
		}

		if u == -1 {
			break
		}
		// MSTに追加する頂点決定
		nodes[u].color = mst

		// MSTに属していない頂点それぞれについて,MSTとの最小の重みを更新
		for i := 0; i < n; i++ {
			if nodes[i].color == mst {
				continue
			}
			if nodes[i].matrix[u] == -1 {
				// 接してない
				continue
			}
			if nodes[i].matrix[u] < nodes[i].minToMst {
				// node[u]との重みが最小
				nodes[i].minToMst = nodes[i].matrix[u]
				nodes[i].parent = u
			}
		}
	}

	sum := 0
	for _, v := range nodes {
		// node[0]はparentが入っていない
		if v.parent != -1 {
			sum += v.matrix[v.parent]
		}
	}
	fmt.Println(sum)
}

func eGetInt(scanner *bufio.Scanner) int {
	t := eAtoi(strings.Fields(eGetline(scanner)))
	return t[0]
}

func eGetline(p *bufio.Scanner) string {
	if !p.Scan() {
		if err := p.Err(); err != nil {
			log.Fatal(err.Error())
		}
		return ""
	}
	return p.Text()
}

func eAtoi(s []string) []int {
	ints := make([]int, len(s))
	var err error
	for i, e := range s {
		ints[i], err = strconv.Atoi(e)
		if err != nil {
			log.Fatal(err.Error())
		}
	}
	return ints
}

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]);
  }
}
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 <cstdio>
#include <iostream>
#include <queue>
#include <utility>

using namespace std;

const int NIL = 2001;
const int MAX = 100;
int G[MAX][MAX];
int W[MAX];
int n;

void MST() {
	priority_queue<pair<int, int>> PQ;
	PQ.push(make_pair(0, 0));
	W[0] = 0;
	while (!PQ.empty()) {
		int u = PQ.top().second;
		PQ.pop();
		for (int i = 0; i < n; ++i) {
			if (G[u][i] != -1 && G[u][i] < W[i]) {
				PQ.push(make_pair(G[u][i] * -1, i));
				W[i] = G[u][i];
			}
		}
	}
}

int main() {

	scanf("%d", &n);
	for (int i = 0; i < n; ++i) {
		for (int j = 0; j < n; ++j)
			G[i][j] = NIL;
		W[i] = NIL;
	}

	for (int i = 0; i < n; ++i) {
		for (int j = 0; j < n; ++j) {
			int a;
			scanf("%d", &a);
			G[i][j] = a;
		}
	}

	MST();

	int sum = 0;
	for (int i = 0; i < n; ++i) {
		sum += W[i];
	}
	cout << sum << endl;

	return 0;
}

C++: #define _CRT_SECURE_NO_WARNINGS
#define _USE_MATH_DEFINES
//#include "MyMath.h"
//#include "MyDisjointset.h"
#include <iostream>
#include <vector>
#include <algorithm>
#include <queue>
#include <functional>
#include <stdio.h>
using namespace std;
typedef pair<int, int> P;
const int INF = 2000000000;
int main() {
	int n; cin >> n;
	int m[101][101];
	for (int i = 0; i < n; i++)for (int j = 0; j < n; j++)
		cin >> m[i][j];
	int res = 0;
	int *d = new int[n];
	fill(d, d + n, INF); 
	d[0] = 0;
	bool *visit = new bool [n];
	priority_queue <P, vector<P>, greater<P> > q;
	q.push(P(d[0], 0));
	while (!q.empty()) {
		int f = q.top().first, s = q.top().second;
		visit[s] = true;
		res += f;
		q.pop();
		for (int i = 0; i < n; i++)
			if (!visit[i] && m[s][i] != -1 && d[s] > d[i] + m[s][i]) {
				d[s] = d[i] + m[s][i];
				q.push(P(d[i], i));
			}
	}
	cout << res << endl;
	delete[] d, visit;
	return 0;
}

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

class TEdge;
struct TNode
{
	int TreeIndex;
	std::vector<const TEdge*> Connected;
	
	TNode();
	void SetIndex(int Index, const TEdge* Edge);
};

class TEdge
{
	std::pair<TNode*, TNode*> FEnds;
	int FWeight;
public:
	TEdge(TNode* EndA, TNode* EndB, int Weight);
	
	bool operator<(const TEdge& Other)const;
	int  Weight()const {return FWeight;}
	bool ToMekeCycle()const;
	
	void JoinTree(int TreeIndex)const;
};

TNode::TNode()
:TreeIndex(0)
{
}

void TNode::SetIndex(int Index, const TEdge* Edge)
{
	if(TreeIndex >= Index) return;
	TreeIndex = Index;
	for(std::vector<const TEdge*>::const_iterator It = Connected.begin(); It != Connected.end(); ++It){
		(*It)->JoinTree(Index);
	}
	Connected.push_back(Edge);
}

TEdge::TEdge(TNode* EndA, TNode* EndB, int Weight)
:FEnds(EndA, EndB), FWeight(Weight)
{
}

bool TEdge::operator<(const TEdge& Other)const
{
	return Weight() < Other.Weight();
}

bool TEdge::ToMekeCycle()const
{
	return FEnds.first->TreeIndex > 0 && FEnds.first->TreeIndex == FEnds.second->TreeIndex;
}

void TEdge::JoinTree(int TreeIndex)const
{
	FEnds.first->SetIndex(TreeIndex, this);
	FEnds.second->SetIndex(TreeIndex, this);
}

class TGraph
{
	std::vector<TNode> FNodes;
	std::vector<TEdge> FEdges;
public:
	TGraph(int NodeCount);
	
	void AddEdge(int NodeIndexA, int NodeIndexB, int Weight);
	int WeightMinimumSpanningTree();
};

TGraph::TGraph(int NodeCount)
:FNodes(NodeCount)
{
}

void TGraph::AddEdge(int NodeIndexA, int NodeIndexB, int Weight)
{
	if(NodeIndexA <= NodeIndexB || Weight < 0) return;
	FEdges.push_back(TEdge(&FNodes[NodeIndexA], &FNodes[NodeIndexB], Weight));
}

int TGraph::WeightMinimumSpanningTree()
{
	std::sort(FEdges.begin(), FEdges.end());
	unsigned int TreeCount = 0;
	int Weight = 0;
	for(std::vector<TEdge>::iterator It = FEdges.begin(); It != FEdges.end() && TreeCount < FNodes.size() - 1; ++It){
		if(It->ToMekeCycle()) continue;
		It->JoinTree(TreeCount + 1);
		Weight += It->Weight();
		++TreeCount;
	}
	return Weight;
}

int main()
{
	int NodeCount;
	std::cin >> NodeCount;
	TGraph Graph(NodeCount);
	
	for(int i = 0; i < NodeCount; ++i)
	{
		for(int j = 0; j < NodeCount; ++j){
			int Weight;
			std::cin >> Weight;
			Graph.AddEdge(i, j, Weight);
		}
	}
	
	std::cout << Graph.WeightMinimumSpanningTree() << std::endl;
	return 0;
}

C++: #include <iostream>
#include <cstring>
using namespace std;
#define int long long

main(){
  int n;
  cin>>n;
  int a[n][n];
  for(int i=0;i<n;i++)
    for(int j=0;j<n;j++)
      cin>>a[i][j];
  int ans=0;
  bool used[n];
  memset(used,0,sizeof(used));
  used[0]=1;
  int cost[n];
  for(int i=0;i<n;i++) cost[i]=a[0][i];
  while(1){
    int v=-1;
    for(int i=0;i<n;i++){
      if(used[i]||cost[i]<0) continue;
      if(v<0||cost[i]<cost[v]) v=i;
    }
    if(v<0) break;
    used[v]=1;
    ans+=cost[v];
    for(int i=0;i<n;i++){
      if(a[v][i]<0) continue;
      if(cost[i]<0||a[v][i]<cost[i]) cost[i]=a[v][i];
    }
  }
  cout<<ans<<endl;
}