Go: package main

import (
	"bufio"
	"fmt"
	"log"
	"math"
	"os"
	"strconv"
	"strings"
)

type color int

const (
	white color = iota // 未訪問
	mst                // MSTとして確定
)

// 頂点
type node struct {
	matrix   []int // 隣接行列
	minToMst int   // MSTへの重み最小値
	parent   int   // 最小値の相手
	color    color
}

func main() {
	scanner := bufio.NewScanner(os.Stdin)
	n := eGetInt(scanner)

	nodes := make([]node, n)
	for i := 0; i < n; i++ {
		nodes[i].matrix = eAtoi((strings.Fields(eGetline(scanner))))
		nodes[i].parent = -1
		nodes[i].minToMst = math.MaxInt16
		nodes[i].color = white
	}

	// 初回はnode[0]がMSTに入るように,minToMstを設定
	nodes[0].minToMst = 0
	for { // 全部チェックするまで

		// どの頂点を新たにMSTに取り込むか
		min := math.MaxInt16
		u := -1
		for i := 0; i < n; i++ {
			if nodes[i].color == mst {
				continue
			}
			if nodes[i].minToMst < min {
				u = i
				min = nodes[i].minToMst
			}
		}

		if u == -1 {
			break
		}
		// MSTに追加する頂点決定
		nodes[u].color = mst

		// MSTに属していない頂点それぞれについて,MSTとの最小の重みを更新
		for i := 0; i < n; i++ {
			if nodes[i].color == mst {
				continue
			}
			if nodes[i].matrix[u] == -1 {
				// 接してない
				continue
			}
			if nodes[i].matrix[u] < nodes[i].minToMst {
				// node[u]との重みが最小
				nodes[i].minToMst = nodes[i].matrix[u]
				nodes[i].parent = u
			}
		}
	}

	sum := 0
	for _, v := range nodes {
		// node[0]はparentが入っていない
		if v.parent != -1 {
			sum += v.matrix[v.parent]
		}
	}
	fmt.Println(sum)
}

func eGetInt(scanner *bufio.Scanner) int {
	t := eAtoi(strings.Fields(eGetline(scanner)))
	return t[0]
}

func eGetline(p *bufio.Scanner) string {
	if !p.Scan() {
		if err := p.Err(); err != nil {
			log.Fatal(err.Error())
		}
		return ""
	}
	return p.Text()
}

func eAtoi(s []string) []int {
	ints := make([]int, len(s))
	var err error
	for i, e := range s {
		ints[i], err = strconv.Atoi(e)
		if err != nil {
			log.Fatal(err.Error())
		}
	}
	return ints
}

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;
}