Go: package main

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

// I/O
type Scanner struct {
	sc *bufio.Scanner
}

func NewScanner() *Scanner {
	sc := bufio.NewScanner(os.Stdin)
	sc.Split(bufio.ScanWords)
	sc.Buffer(make([]byte, 1024), int(1e+9))
	return &Scanner{sc}
}

func (s *Scanner) nextStr() string {
	s.sc.Scan()
	return s.sc.Text()
}

func (s *Scanner) nextInt() int {
	i, e := strconv.Atoi(s.nextStr())
	if e != nil {
		panic(e)
	}
	return i
}

func (s *Scanner) nextFloat() float64 {
	f, e := strconv.ParseFloat(s.nextStr(), 64)
	if e != nil {
		panic(e)
	}
	return f
}

func (s *Scanner) nextRuneSlice() []rune {
	return []rune(s.nextStr())
}

func (s *Scanner) nextIntSlice(n int) []int {
	res := make([]int, n)
	for i := 0; i < n; i++ {
		res[i] = s.nextInt()
	}
	return res
}

func (s *Scanner) nextFloatSlice(n int) []float64 {
	res := make([]float64, n)
	for i := 0; i < n; i++ {
		res[i] = s.nextFloat()
	}
	return res
}

// Arithmetic
func max(nums ...int) int {
	m := nums[0]
	for _, i := range nums {
		if m < i {
			m = i
		}
	}
	return m
}

func min(nums ...int) int {
	m := nums[0]
	for _, i := range nums {
		if m > i {
			m = i
		}
	}
	return m
}

func abs(x int) int {
	if x > 0 {
		return x
	}
	return -x
}

func pow(x, y int) int {
	res := 1
	for i := 0; i < y; i++ {
		res *= x
	}
	return res
}

func ceil(a, b int) int {
	if a%b == 0 {
		return a / b
	} else {
		return a/b + 1
	}
}

// Sort
type RuneSlice []rune

func (a RuneSlice) Len() int           { return len(a) }
func (a RuneSlice) Less(i, j int) bool { return a[i] < a[j] }
func (a RuneSlice) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }

const INF = 1 << 60

// Main
func main() {
	sc := NewScanner()
	wtr := bufio.NewWriter(os.Stdout)
	N := sc.nextInt()
	G := make([][]int, N)
	for i := 0; i < N; i++ {
		G[i] = make([]int, N)
		for j := 0; j < N; j++ {
			G[i][j] = sc.nextInt()
		}
	}
	d := make([]int, N)
	for i := 0; i < N; i++ {
		d[i] = INF
	}
	seen := make([]bool, N)
	d[0] = 0
	ans := 0
	for {
		cur := 0
		minCost := INF
		for i := 0; i < N; i++ {
			if !seen[i] && d[i] < minCost {
				minCost = d[i]
				cur = i
			}
		}
		if minCost == INF {
			break
		}
		seen[cur] = true
		ans += d[cur]
		for i := 0; i < N; i++ {
			cost := G[cur][i]
			if cost != -1 && cost < d[i] {
				d[i] = cost
			}
		}
	}

	fmt.Fprintln(wtr, ans)
	wtr.Flush()
}

C++: #include <iostream>
#include <string>
#include <stack>
#include <queue>
#include <vector>
#include <list>
#include <set>
#include <map>
#include <algorithm>

#include <cstdio>

using namespace std;
#define rep(i, n) for (int i = 0; i < (n); i++)
#define repc(i, s, n) for (int i = (s); i <= (n); i++)
#define rrep(i, n) for (int i = (n) - 1; i >= 0; i--)
#define rrepc(i, s, n) for (int i = (s); i >= (n); i--)
#define print_array(ary, size) cout << ary[0]; rep1(i, 1, size - 1) cout << ' ' << ary[i]; cout << endl;
typedef long long ll;
typedef unsigned long long ull;

int M[100][100];
int done[100] = { 0 };

int main()
{
	int n;
	int ans = 0;
	int mincost;
	int index = 0;

	cin >> n;
	rep(i, n) {
		rep(j, n) {
			cin >> M[i][j];
		}
	}

	done[0] = 1;

	while (true) {
		mincost = -1;

		rep(i, n) {
			rep(j, n) {
				if ((done[i] == 1 && done[j] == 0)
					&& (M[i][j] != -1)
					&& (mincost == -1 || mincost > M[i][j])) {
					mincost = M[i][j];
					index = j;
				}
			}
		}

		if (mincost == -1) break;
		else {
			done[index] = 1;
			ans += mincost;
		}
	}

	cout << ans << endl;

	return 0;
}

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

class union_find  // union find.
{
    int* U;
public:
    union_find(int n){ U = new int [n]; for(int i = 0; i < n; i++) U[i] = i; }
    int root(int a){ if(U[a] == a){ return a; } U[a] = root(U[a]); return U[a]; }
    bool same(int a, int b){ return root(a) == root(b); }
    void join(int a, int b){ int x = root(a), y = root(b); U[x] = y; }
    ~union_find(){ delete [] U; }
};

struct edge{
	int v1, v2, w;
	bool operator < (const edge &e) const{ return w > e.w; }  // const?????????????????????
};

edge init(int ver_1, int ver_2, int weight)
{
	edge e;
	e.v1 = ver_1, e.v2 = ver_2, e.w = weight;
	return e;
}

int main()
{
	int i, j, w, n;
	scanf("%d", &n);

	priority_queue<edge> ES;
	queue<edge> ST;
	union_find VS(n);

	for(i = 0; i < n; i++){
		for(j = 0; j < n; j++){
			scanf("%d", &w);
			if(i >= j || w < 0) continue;
			ES.push(init(i, j, w));
		}
	}
	int size = 0;
	edge e;
	while(!ES.empty() && size < n - 1){
		e = ES.top();
		ES.pop();
		if(!VS.same(e.v1, e.v2)){
			VS.join(e.v1, e.v2);
			ST.push(e);
			size++;
		}
	};
	int sum = 0;
	while(!ST.empty()){ e = ST.front(); sum += e.w; ST.pop(); };
	printf("%d\n", sum);

	return 0;
}

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

int main(){
  int n;
  int a[100][100],b[100]={},c[100]={};
  cin>>n;
  for(int i=0;i<n;i++)
   for(int j=0;j<n;j++)cin>>a[i][j];
  c[0]=1;

  while(1){
   int k=0.min=10000,min1=0;
   if(k==n)break;
   for(int i=0;i<n;i++)
    for(int j=0;j<n;j++)
     if(a[b[i]][j]<min && a[b[i]][j]!=-1 &&c[j]!=1){
      min=a[b[i]][j];
      b[k]=j;
    }
   c[b[k]]=1
   ans+=min;
   k++;
  }
 cout<<ans<<endl;
 return 0;
}