Go: package main

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

// Define disjoint-set forests

type vertexNo int

type vertex struct {
	number vertexNo
	rank int
	parent *vertex
}

func makeSet(number vertexNo) vertex {
	return vertex {
		number: number,
		rank: 0,
		parent: nil,
	}
}

func union(x, y *vertex) *vertex {
	return link(findSet(x), findSet(y))
}

func link(x, y *vertex) *vertex {
	if x.rank > y.rank {
		y.parent = x
		return x
	} else {
		x.parent = y
		if x.rank == y.rank {
			y.rank += 1
		}
		return y
	}
}

func findSet(v *vertex) *vertex {
	if v.parent == nil {
		return v
	}
	v.parent = findSet(v.parent)
	return v.parent
}

// Function to calculate minimum spanning tree

type matrix [][]int

type edge struct {
	one vertexNo
	two vertexNo
	weight int
}

const(
	MAX_VERTICES = 100
)

func newEdgesFromMatrix(matrix matrix) []edge {
	edges := make([]edge, 0, MAX_VERTICES * MAX_VERTICES)

	for i := 0; i < len(matrix); i++ {
		row := matrix[i]
		for j := i; j < len(row); j++ {
			if row[j] >= 0 {
				edge := edge{one: vertexNo(i+1), two: vertexNo(j+1), weight: row[j]}
				edges = append(edges, edge)
			}
		}
	}

	return edges
}

func solve(matrix matrix) int {
	nVertices := len(matrix)

	// initialize vertices
	vertices := make([]vertex, nVertices)
	for i := 0; i < nVertices; i++ {
		vertices[i] = makeSet(vertexNo(i+1))
	}

	// prepare edges sorted in the ascending order of weight
	edges := newEdgesFromMatrix(matrix)
	sort.Slice(edges, func(i, j int) bool {
		return edges[i].weight < edges[j].weight
	})

	unionCount := 0
	weightSum := 0
	for unionCount < nVertices-1 {
		for i := 0; i < len(edges); i++ {
			edge := edges[i]
			v1 := &vertices[edge.one-1]
			v2 := &vertices[edge.two-1]
			rep1 := findSet(v1)
			rep2 := findSet(v2)
			if rep1.number == rep2.number {
				continue
			}
			union(rep1, rep2)
			edges = append(edges[0:i], edges[i+1:]...)
			weightSum += edge.weight
			unionCount++
			break
		}
	}

	return weightSum
}

// main

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

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

	matrix := parseMatrix(n, sc)

	edges := newEdgesFromMatrix(matrix)
	sort.Slice(edges, func(i, j int) bool {
		return edges[i].weight < edges[j].weight
	})

	fmt.Println(solve(matrix))
}

func parseMatrix(n int, sc *bufio.Scanner) [][]int {
	matrix := make([][]int, n)
	for i := 0; i < n; i++ {
		matrix[i] = make([]int, n)
		for j := 0; j < n; j++ {
			x, err := nextInt(sc)
			if err != nil {
				panic(err)
			}
			matrix[i][j] = x
		}
	}
	return matrix
}

func nextText(sc *bufio.Scanner) string {
	sc.Scan()
	return sc.Text()
}

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

C++: #include <iostream>
#include <list>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <numeric>
#include <algorithm>
#include <cmath>
#include <string>
using namespace std;
typedef long long lli;
typedef vector<lli> vll;
typedef vector<bool> vbl;
typedef vector<vector<lli> > mat;
typedef vector<vector<bool> > matb;
typedef vector<string> vst;
typedef pair<lli,lli> pll;
typedef pair<double,double> pdd;

vll par;
vll rnk;
void init(lli n){
    par = vll(n);
    rnk = vll(n);
    for(lli i = 0;i < n;i++) par[i] = i;
}
lli find(lli x){
    if(par[x] == x) return x;
    else return par[x] = find(par[x]);
}
void unite(lli x,lli y){
    x = find(x);y = find(y);
    if(x == y) return;
    if(rnk[x] < rnk[y]) par[x] = y;
    else {
        par[y] = x;
        if(rnk[x] == rnk[y]) rnk[x]++;
    }
}
bool same(lli x,lli y){
    return find(x) == find(y);
}

lli n;
vll c,r,l;
vll p;
mat g;
lli ans = 0;

int main(){
    cin >> n;
    g = mat(n,vll(n));
    for(lli i = 0;i < n;i++){
        for(lli j = 0;j < n;j++){
            cin >> g[i][j];
        }
    }
    for(lli i = 0;i < n;i++){
        for(lli j = i+1;j < n;j++){
            if(g[i][j] >= 0) c.push_back(g[i][j]),r.push_back(i),l.push_back(j);
        }
    }
    p = vll(c.size());
    for(lli i = 0;i < p.size();i++) p[i] = i;
    sort(p.begin(),p.end(),[](lli a,lli b){
        return c[a] < c[b];
    });
    init(n);
    for(lli i = 0;i < p.size();i++){
        if(!same(r[p[i]],l[p[i]])) unite(r[p[i]],l[p[i]]),ans += c[p[i]];
    }
    cout << ans << endl;
    return 0;
}

C++: #include<bits/stdc++.h>
//#include"UnionFind.hpp"
using namespace std;

using Weight=long long;
struct Edge{
    int from,to;
    Weight weight;
};
bool operator<(const Edge& lhs,const Edge& rhs){
    return lhs.weight<rhs.weight;
}

using Edges=vector<Edge>;
using Graph=vector<Edges>;


Edges MinimumSpanningTree(int n,Edges edges){
    Edges res;
    UnionFind u(n);
    for(int i=0;i<edges.size();i++){
        if(!u.same(edges[i].from,edges[i].to)){
            res.push_back(edges[i]);
            u.unite(edges[i].from,edges[i].to);
        }
    }
    return res;
}

Edges MinimumSpanningTree(const Graph& g){
    Edges edges;
    for(int i=0;i<g.size();i++){
        copy(g[i].begin(),g[i].end(),back_inserter(edges));
    }
    auto res=MinimumSpanningTree(g.size(),edges);
    return res;
}

int main(){
    int n;
    cin>>n;
    Graph g(n);
    for(int i=0;i<n;i++){
        for(int j=0;j<n;j++){
            Weight c;
            cin>>c;
            if(i==j) continue;
            g[i].push_back({i,j,c});
        }
    }
    auto mst=MinimumSpanningTree(g);
    cout<<accumulate(mst.begin(),mst.end(),Weight(0),[](const Weight& lhs,const Edge& rhs){return lhs+rhs.weight;})<<endl;
    return 0;
}

C++: #include<iostream>
#include<vector>
#include<algorithm>
#define INFTY 10000
#define PII pair<int,int>
#define MPII(a,b) make_pair(a,b)
using namespace std;
class NODE
{
public:
	vector<PII> v;
	int index = 0;
	bool Isdone = false;
};
class Tree
{
public:
	vector<int> V;
	int cost = 0;
};

int main()
{
	NODE* nodes;
	nodes = new NODE[101];
	Tree T;
	int n;
	cin >> n;
	for (int i = 1; i <= n; i++)
	{
		for (int j = 1; j <= n; j++)
		{
			int t;
			cin >> t;
			if (t != -1)
			{
				PII p = MPII(t, j);
				nodes[i].v.push_back(p);
			}
		}
		sort(nodes[i].v.begin(), nodes[i].v.end());
	}
	T.V.push_back(1);
	nodes[1].Isdone = true;
	while (1)
	{
		if (T.V.size() == n)break;
		int mincost=INFTY;
		int addv=-1;
		int index;
		for (auto itr : T.V)
		{
			if (nodes[itr].v.size() <= nodes[itr].index)continue;
			while (1)
			{
				if (nodes[nodes[itr].v[nodes[itr].index].second].Isdone == false)break;
				nodes[itr].index++;
				if (nodes[itr].v.size() <= nodes[itr].index)break;
			}
			if (nodes[itr].v.size() <= nodes[itr].index)continue;
			if (mincost >= nodes[itr].v[nodes[itr].index].first)
			{
				mincost = nodes[itr].v[nodes[itr].index].first;
				addv = nodes[itr].v[nodes[itr].index].second;
				index = itr;
			}
		}
		
		nodes[index].index++;
		T.V.push_back(addv);
		nodes[addv].Isdone = true;
		T.cost += mincost;
	}
	cout << T.cost << endl;
	delete[] nodes;
	return 0;
}