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 <vector>
#include <queue>
#include <tuple>

int main(int argc, char const *argv[]) {
  int n;
  std::vector<std::vector<int> > a;
  std::cin >> n; // .. 100
  a = std::vector<std::vector<int> >(n, std::vector<int>(n));
  // a_ij <= 2000, a_ij = a_ji, this graph is connected;

  for (size_t i = 0; i < n; i++) {
    for (size_t j = 0; j < n; j++) {
      std::cin >> a[i][j];
    }
  }

  //ALGORITHM
  std::vector<bool> found_vtx(n, false);
  // std::tuple<int,int,int> x(1,2,3);
  std::priority_queue<
    std::tuple<int,int,int>,
    std::vector<std::tuple<int, int, int> >,
    std::greater<std::tuple<int, int, int> >
  > q;

  int new_elm = 0;
  found_vtx[new_elm] = true;
  for (size_t i = 0; i < n; i++) {
    q.push(std::make_tuple(a[new_elm][i], new_elm, i));
  }
  int mst_weight = 0;
  while(!q.empty()){
    auto x = q.top(); q.pop();
    // std::cout << "e: "
    //   << std::get<0>(x) << " "
    //   << std::get<1>(x) << " "
    //   << std::get<2>(x) << std::endl;
    if(found_vtx[std::get<2>(x)] || std::get<0>(x) < 0) continue;
    else{
      mst_weight += std::get<0>(x);
      // std::cout << std::get<0>(x) << std::endl;
      new_elm = std::get<2>(x);
      found_vtx[new_elm] = true;
      for (size_t i = 0; i < n; i++) {
        q.push(std::make_tuple(a[new_elm][i], new_elm, i));
      }
    }
  }

  std::cout << mst_weight << std::endl;
  return 0;
}

C++: #include <stdio.h>

#define N 2000
#define BLACK 0
#define WHITE 1
#define GRAY 2
#define inf 200000

int Map[N][N];
int n;
int color[N];
int d[N];
int p[N];


void init(){
	int i;
	for(i=0;i<n;i++){
		color[i]=WHITE;
		d[i] = inf;
	}
}

void prim(){
	init();
	int mincost;
	d[0] = 0;
	p[0] = -1;
	int i;
	int u;
	int v;
	while(1){
		mincost = inf;
		for(i=0;i<n;i++){
			if(color[i]!=BLACK && d[i] < mincost){
				mincost = d[i];
				u = i;
			}
		}

		if(mincost == inf){
			break;
		}
		color[u]=BLACK;
		for(v=0;v<n;v++){
			if(color[v] != BLACK && Map[u][v]!= -1){
				if(Map[u][v] < d[v]){
					d[v] = Map[u][v];
					color[v] = GRAY;
					p[v] = u;
				}
			}
		}
	}
}

int main(){
	scanf("%d",&n);
	int i,j;
	for(i=0;i<n;i++){
		for(j=0;j<n;j++){
			scanf("%d",&Map[i][j]);
		}
	}
	prim();
	int total = 0;
	for(i=0;i<n;i++){
		total += d[i];
	}
	printf("%d\n",total);
	return 0;
}

C++: #include <iostream>
#include <queue>
#include <map>
#include <functional>
#include <vector>
using namespace std;

#define V vector<vector<pair<int,int>>>

int main() {
	cin.tie(0); ios::sync_with_stdio(false);
	int n,Ans = 0; cin >> n;
	V v(2001);
	vector<int> f(n + 1,0);
	for (int i = 1; i <= n;i++) {
		for (int j = 1; j <= n; j++) {
			int a; cin >> a;
			if (a != -1) {
				v[a].push_back(make_pair(i, j));
			}
		}
	}

	for (int i = 0; i <= 2000;i++) {
		for (int j = 0; j < v[i].size(); j++) {
			int first = v[i][j].first, second = v[i][j].second;
			if (f[first] >= 2 || f[second] >= 2) continue;
			Ans += i;
			f[first]++; f[second]++;
		}
	}

	cout << Ans << endl;
}