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 && m[s][i] > d[i]) {
				m[s][i] = d[i];
				q.push(P(d[i], i));
			}
	}
	cout << res << endl;
	delete[] d, visit;
	return 0;
}

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

using namespace std;
using Graph=vector<vector<int>>;



struct Edge {
	int node1, node2, weight;

	Edge() {}

	Edge(int n1, int n2, int w) :node1(n1), node2(n2), weight(w) {}

	void numset(int n1, int n2, int w) {
		node1 = n1; node2 = n2; weight = w;
	}

};

void eswap(Edge& e1, Edge& e2) {
	Edge temp = e1;   e1 = e2;  e2 = temp;
}

void bubbleSort(vector<Edge>& edge) {
	bool flag;
	for (int i = 0; i < edge.size(); i++) {
		flag = true;
		for (int j = 0; j < edge.size()-i-1; j++) {
			if (edge.at(j).weight > edge.at(j + 1).weight) {
				eswap(edge.at(j), edge.at(j + 1));
				flag = false;
			}
		}
		if (flag) break;
	}
}

int kruskal(Graph &spntree,vector<Edge>& edgeset) {
	int size = spntree.size();
	queue<int> unconnected;
	vector<bool> connected(size,false);
	
	if (edgeset.size() == 0) return 0;
	bubbleSort(edgeset);

	connected.at(edgeset.at(0).node1)=true;
	connected.at(edgeset.at(0).node2) = true;
	spntree.at(edgeset.at(0).node1).push_back(edgeset.at(0).node2);
	spntree.at(edgeset.at(0).node2).push_back(edgeset.at(0).node1);
	int index=0,count = 2,cost=edgeset.at(0).weight;
	Edge ed;
	edgeset.erase(edgeset.begin());

	while (count<size) {
		ed = edgeset.at(index);
		if (connected.at(ed.node1) == true && connected.at(ed.node2) == false) {
			connected.at(ed.node2) = true;
			spntree.at(ed.node1).push_back(ed.node2);
			spntree.at(ed.node2).push_back(ed.node1);
			edgeset.erase(edgeset.begin() + index);
			count++; cost += ed.weight;
			index = 0; continue;
		}
		else if (connected.at(ed.node2) == true && connected.at(ed.node1) == false) {
			connected.at(ed.node1) = true;
			spntree.at(ed.node1).push_back(ed.node2);
			spntree.at(ed.node2).push_back(ed.node1);
			edgeset.erase(edgeset.begin() + index);
			count++; cost += ed.weight;
			index = 0; continue;
		}
		index++;
	}
	return cost;


}

int main(void) {

	int size, temp,cost,edgenum=0;
	Edge etemp;
	vector<Edge> edgeset;
	cin >> size;
	Graph spntree(size);
	for (int i = 0; i < size; i++) {
		for (int j = 0; j < size; j++) {
			cin >> temp;
			if (i < j) {
				if (temp != -1) {
					edgenum++;
					etemp.numset(i, j, temp);
					edgeset.push_back(etemp);
				}
			}
		}
	}

	cost=kruskal(spntree, edgeset);
	cout << cost << endl;
}

C++: #include <iostream>
#include <cstdio>
#include <cstring>
#include <stack>
#include <vector>
using namespace std;
const int maxx = 1010;
const int INF = 1 << 30;
int n, m[maxx][maxx];
int u, num, v;
int flag[maxx];

int prim() {
	// d??°????????????V-T??? p??°????????°???MST???
	int d[maxx], p[maxx];
	for (int i = 0; i < n; i++) {
		d[i] = INF;
		p[i] = -1;
		flag[i] = 0;
	}

	d[0] = 0;
	while (1) {
		int minv = INF;
		u = -1;
		// ???????????????????°???????
		for (int i = 0; i < n; i++) {
			if (minv > d[i] && flag[i] == 0) {
				u = i;
				minv = d[i];
			}
		}

		if (u == -1)
			break;

		// ?????°???????????\MST
		flag[u] = 1;

		// ??´??°?????????????????????????????????????????´???????°???????????????????????????????
		for (int v = 0; v < n; v++) {
			if (flag[v] == 0 && m[u][v] != INF && d[v] > m[u][v]) {
				d[v] = m[u][v];
				p[v] = u;
			}
		}
	}


	int ans = 0;
	for (int i = 0; i < n; i++)
		if (p[i] != -1)
			ans += m[i][p[i]];

	return ans;
}
int main()
{
	memset(flag, 0, sizeof(flag));
	scanf("%d", &n);
	
	for (int i = 0; i < n; i++) {
		for (int j = 0; j < n; j++) {
			scanf("%d", &m[i][j]);
			if (m[i][j] == -1)
				m[i][j] = INF;
		}
	}
	printf("%d\n", prim());
	return 0;
}