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

using namespace std;
struct edge {
    edge() {};
    edge(int s, int e, int d) : start {s}, end {e}, d {d} {};
    int start;
    int end;
    int d;
};

void dfs(vector<vector<int>>& graph, bool* used, int id, int n){
    used[id] = true;
    for(int i = 0; i < n; ++i){
        if(graph[id][i] == -1 || used[i]){
            continue;
        }
        dfs(graph, used, i, n);
    }
}

bool is_connected(vector<vector<int>>& graph){
    int n = graph.size();
    bool used[n];
    for(int i = 0; i < n; ++i){
        used[i] = false;
    }
    dfs(graph, used, 0, n);
    for(int i = 0; i < n; ++i){
        if(!used[i]){
            return false;
        }
    }
    return true;
}

void delete_edges(vector<vector<int>>& graph){
    int edges_num = 0;
    vector<edge> vec {};
    int n = graph.size();
    for(int i = 0; i < n; ++i){
        for(int j = i+1; j < n; ++j){
            if(graph[i][j] != -1){
                ++edges_num;
                vec.push_back(edge(i, j, graph[i][j]));
            }
        }
    }
    sort(vec.begin(), vec.end(), [](edge x, edge y){
            return y.d < x.d;
         });

    for(auto it = vec.begin(); it != vec.end(); ++it){
        edge e = *it;
        graph[e.start][e.end] = -1;
        graph[e.end][e.start] = -1;
        if(is_connected(graph)){
            --edges_num;
            if(edges_num == n-1){
                return;
            }
        } else {
            graph[e.start][e.end] = e.d;
            graph[e.end][e.start] = e.d;
        }
    }
}

vector<vector<int>> gen_spanning_tree(vector<vector<int>>& graph){
    vector<vector<int>> tree {graph};
    delete_edges(tree);
    return tree;
}

int calc_weight(vector<vector<int>> graph){
    int sum = 0;
    int n = graph.size();
    for(int i = 0; i < n; ++i){
        for(int j = i+1; j < n; ++j){
            if(graph[i][j] == -1){
                continue;
            }
            sum += graph[i][j];
        }
    }
    return sum;
}

int main(){
    int n;
    cin >> n;
    vector<vector<int>> graph {n, vector<int>(n, 0)};
    for(int i = 0; i < n; ++i){
        for(int j = 0; j < n; ++j){
            cin >> graph[i][j];
        }
    }
    auto tree = gen_spanning_tree(graph);

    cout << calc_weight(tree) << endl;
}

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

vector<int> union_tree;

int root(int leave)
{
	int tmp = leave;
	if (leave == union_tree[leave])
	{
		return leave;
	}
	union_tree[tmp] = root(union_tree[tmp]);
	return union_tree[tmp];
}

int is_same_set(int a, int b)
{
	return root(a) == root(b) ? 1 : 0;
}

void unite(int a, int b)
{
	union_tree[root(a)] = root(b);
}

int main()
{
	int n;
	cin >> n;
	vector<tuple<int, int, int>> bridges;
	for (int i = 0; i < n; i++)
	{

		for (int j = 0; j < n; j++)
		{
			int cost;
			cin >> cost;
			if (cost == -1)
			{
				continue;
			}
			else
			{
				bridges.push_back(make_tuple(cost, i, j));
			}
		}
		union_tree.push_back(i);
	}
	sort(bridges.begin(), bridges.end());
	int index(0),cost(0);
	for (int i = 0; i < n - 1; i++)
	{
		while (is_same_set(get<1>(bridges[index]), get<2>(bridges[index])))
		{
			index++;
		}
		cost += get<0>(bridges[index]);
		unite(get<1>(bridges[index]), get<2>(bridges[index]));
	}
	cout << cost << 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];
	fill(visit, visit + n, false);
	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;
		q.pop();
		if (visit[s]) continue;
		visit[s] = true;
		res += f;
		for (int i = 0; i < n; i++) {
			if (!visit[i]) {
				if (m[s][i] != -1) {
					if (m[s][i] < d[i]) {
						d[i] = m[s][i];
						q.push(P(d[i], i));
					}
				}
				}
			}
		}
	cout << res << endl;
	delete[] d, visit;
	return 0;
}

C++: #include<cstdio>
#include<vector>
#define N_MAX 105
#define INF 32700
using namespace std;
short adj[N_MAX][N_MAX];
struct Node{
    char parent;//父节点
    short weight;//权值
    bool state;//节点状态
}nodes[N_MAX];
char n;
int solve() {
    int res = 0;
    nodes[0].weight = 0;
    nodes[0].parent = -1;
    nodes[0].state = true;
    char nowParent = 0;
    while(true) {
        for(char j = 0; j < n; j++) {
            if(nodes[j].weight > adj[nowParent][j]&&!nodes[j].state) {
                nodes[j].weight = adj[nowParent][j];
            }
        }
        char minId = n;
        for(char i = 0; i < n; i++) {
            if(!nodes[i].state) {
                if(nodes[i].weight < nodes[minId].weight) {
                    minId = i;
                }
            }
        }
        if(minId == n) break;
        else nodes[minId].state = true;
        nodes[minId].parent=nowParent;
        nowParent=minId;
    }
    for(char i = 0; i < n; i++) {
        res += nodes[i].weight;
    }
    return res;
}

int main(){
    scanf("%d", &n);
    for(char i=0;i<n;i++){
        for(char j=0;j<n;j++){
            scanf("%d", &adj[i][j]);
            if(adj[i][j]==-1) adj[i][j]=INF;
        }
        nodes[i].weight=INF;//初始权值
    }
    nodes[n].weight=INF;
    printf("%d\n", solve());
    return 0;
}