Go: package main

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

var scanner = bufio.NewScanner(os.Stdin)

func nextString() string {
	scanner.Scan()
	return scanner.Text()
}

func nextInt() int {
	n, err := strconv.Atoi(nextString())
	if err != nil {
		fmt.Printf("strconv.Atoi failed: %v\n", err)
	}
	return n
}

type ElementKey int
type Node struct {
	key    ElementKey
	parent *Node
	rank   int
}

func newNode(key ElementKey) *Node {
	node := Node{}
	node.key = key
	node.parent = &node
	node.rank = 0

	return &node
}

func union(x, y *Node) {
	xSet := findSet(x)
	ySet := findSet(y)
	link(xSet, ySet)
}

func link(x, y *Node) {
	if x.rank > y.rank {
		y.parent = x
	} else {
		x.parent = y

		if x.rank == y.rank {
			y.rank++
		}
	}
}

func findSet(x *Node) *Node {
	if x != x.parent {
		x.parent = findSet(x.parent)
	}
	return x.parent
}

func newDisjointSets(nSets int) *DisjointSets {
	sets := DisjointSets{}
	sets.keyToNode = map[ElementKey]*Node{}
	for i := 0; i < nSets; i++ {
		key := ElementKey(i)
		sets.keyToNode[key] = newNode(key)
	}
	return &sets
}

type DisjointSets struct {
	keyToNode map[ElementKey]*Node
}

func (sets *DisjointSets) union(x, y ElementKey) {
	xNode := sets.keyToNode[x]
	yNode := sets.keyToNode[y]
	union(xNode, yNode)
}

func (sets DisjointSets) findSet(x ElementKey) *Node {
	xNode := sets.keyToNode[x]
	return findSet(xNode)
}

type Edge struct {
	i, j   int
	weight int
}

func main() {
	scanner.Split(bufio.ScanWords)

	n := nextInt()
	sets := newDisjointSets(n)
	edges := []Edge{}
	a := make([][]int, n)
	for i := 0; i < n; i++ {
		a[i] = make([]int, n)
		for j := 0; j < n; j++ {
			a[i][j] = nextInt()
			if a[i][j] >= 0 {
				e := Edge{
					i:      i,
					j:      j,
					weight: a[i][j],
				}
				edges = append(edges, e)
			}
		}
	}
	sort.Slice(edges, func(i, j int) bool { return edges[i].weight < edges[j].weight })
	totalWeight := 0
	for _, edge := range edges {
		i := ElementKey(edge.i)
		j := ElementKey(edge.j)
		if sets.findSet(i) != sets.findSet(j) {
			totalWeight += edge.weight
			sets.union(i, j)
		}
	}
	fmt.Println(totalWeight)
}

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;
}

C++: #include<iostream>
#include<list>
#include<vector>
const int INF = 2001;
using namespace std;
list<int> L;
bool ok(int i) {
    for (list<int>::iterator it = L.begin(); it != L.end(); it++) if (i == *it) return 0;
    return 1;
}
int main() {
    L.push_back(0);
    int n; cin >> n;
    int M[100][100];
    int tmp = INF;
    int id = 0;
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < n; j++) {
            cin >> M[i][j];
            if (i == 0 && M[i][j] != -1) {
                if (M[i][j] < tmp) {
                    tmp = M[i][j];
                    id = j;
                }
            }
        }
    }
    L.push_back(id);
    int ans = tmp;
    tmp = INF;
    int OK[100];
    while (1) {
        tmp = INF;
        for (int i = 0; i < n; i++) OK[i] = ok(i); //reached?
        for (list<int>::iterator it = L.begin(); it != L.end(); it++ ) { //from reached
            for (int i = 0; i < n; i++) {
                if (M[*it][i] < tmp && M[*it][i] != -1 && OK[i]) {
                    tmp = M[*it][i];
                    id = i;
                }
            }
        }
        if (tmp == INF) break;
        ans += tmp;
        L.push_back(id);
    }
    cout << ans << endl;
    return 0;
    
}