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<bits/stdc++.h>

using namespace std;
#define fordebug int hoge;cin>>hoge;
#define DEKAI 1000000007
#define INF (1<<28)
#define lp(i,n) for(int i=0;i<n;i++)
#define lps(i,j,n) for(int i=j;i<n;i++)
#define floot10 cout<<fixed<<setprecision(10);
#define int long long
int n,m[1000][1000],col[1000],d[1000],cent[1000];

void prim(){
	lp(i,n){
		d[i]=INF;
	}
	d[0]=0;
	cent[0]=-1;
	while(1){
		int mini=INF,hoge;
		lp(i,n){
			if(col[i]!=2&&d[i]<mini){
				mini=d[i];
				hoge=i;
			}
		}
		if(mini==INF) return;
		col[hoge]=2;
		lp(i,n){
			if(col[i]!=2){
				if(m[hoge][i]==INF) continue;
				if(m[hoge][i]<d[i]){
					d[i]=m[hoge][i];
					cent[i]=hoge;
					col[i]=1;
				}
			}
		}
	}
}

signed main(){
	cin>>n;
	lp(i,n){
		lp(j,n){
			cin>>m[i][j];
			if(m[i][j]==-1) m[i][j]=INF;
		}
	}
	prim();
	int ans=0;
	lp(i,n){
		if(cent[i]==-1) continue;
		ans+=m[i][cent[i]];
	}
	cout<<ans<<endl;
    return 0;
}

C++: #include <iostream>
#include <string>
#include <vector>
#include <cstring>
#include <climits>
#include <algorithm>
#include <map>

using namespace std;

struct edge {
    int to, cost;
};
vector<edge> G[100];

int main() {
    int n;
    cin >> n;
    int cost;
    for (int i=0; i<n; ++i) {
        for (int j=0; j<n; ++j) {
            cin >> cost;
            if (0 <= cost) {
                G[i].push_back((edge){j, cost});
                G[j].push_back((edge){i, cost});
            }
        }
    }
    vector<int> V(1, 0);
    bool used[100] = {};
    used[0] = true;
    int ret = 0;
    while (1) {
        edge e = (edge){-1, INT_MAX};
        for (int i=0; i<(int)V.size(); ++i) {
            for (int j=0; j<(int)G[V[i]].size(); ++j) {
                if ( ! used[G[V[i]][j].to] && G[V[i]][j].cost < e.cost) {
                    e = G[V[i]][j];
                }
            }
        }
        if (e.to < 0) break;
        V.push_back(e.to);
        used[e.to] = true;
        ret += e.cost;
    }

    cout << ret << endl;
    return 0;
}

C++: #include <iostream>
#include <climits>
using namespace std;

int n;
int g[101][101];

int d[101];
char c[101];
int p[101];

void prim() {
    for (int i = 0; i < n; ++i) {
        d[i] = INT_MAX;
        c[i] = 'w';
    }
    c[0] = 'b';
    d[0] = 0;
    while (true) {
        int u, v;
        int mine = INT_MAX;
        for (int j = 0; j < n; ++j) {
            if (c[j] == 'b') {
                u = j;
                for (int k = 0; k < n; ++k) {
                    if(g[u][k] > -1 && g[u][k] <= d[k] && c[k] != 'b' && g[u][k] <= mine) {
                        d[k] = g[u][k];
                        mine = d[k];
                        v = k;
                    }
                }
            }
        }
        p[v] = u;
        c[v] = 'b';
        if(mine == INT_MAX) break;
    }
}

int main() {
    cin >> n;
    for (int i = 0; i < n; ++i) {
        for (int j = 0; j < n; ++j) {
            int w;
            cin >> w;
            g[i][j] = w;
        }
    }
    prim();
    int sum;
    for (int k = 0; k < n; ++k) {
        sum += d[k];
    }
    cout << sum << endl;


    return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int, int> P;
constexpr int di[] = { 0, 1, 0, -1 }, dj[] = { 1, 0, -1, 0 };
#define rep(i, n) for (int i = 0; i < n; i++)
#define repr(i, n) for (int i = n; i >= 0; i--)
#define SORT(v) sort((v).begin(), (v).end())
#define SORTR(v) sort((v).rbegin(), (v).rend())
#define all(v) (v).begin(), (v).end()
#define MAX 100
#define vmax 10000
#define sentinel 2000000000
#define INF (1<<30)
constexpr long long int inf = 1000000000000;
constexpr long long int rinf = -(INT64_MAX);
static const int NIL=-1;

//cout<<fixed<<setprecision(20);cin.tie(0);ios::sync_with_stdio(false);

struct edge
{
    int to,cost;
};


int V;                //Vは頂点数
bool used[MAX+1];         //頂点iがxに含まれているか
vector<edge> g[MAX+1];


int prim(int s){
    for(int i=1;i<=V;i++){
        used[i]=false;
    }
    int res=0;
    priority_queue<P,vector<P>,greater<P>> que;
    que.push(P(0,s));
    while(!que.empty()){
        P p=que.top();que.pop();
        int v=p.second;
        if(used[v]) continue;
        res+=p.first;
        used[v]=true;
        for(auto e:g[v]){
            que.push(P(e.cost,e.to));
        }
    }
    return res;
}

int main() {
    int x;
    cin>>V;
    for(int i=1;i<=V;i++){
        for(int j=1;j<=V;j++){
            cin>>x;
            if(x!=-1){
                g[i].push_back({j,x});
            }
        }
    }
    cout<<prim(1)<<endl;
    return 0;
}