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 <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define FOR(I,A,B) for(int I = (A); I < (B); ++I)
#define CLR(mat) memset(mat, -1, sizeof(mat))
#define N 101
struct edge{
	int to,cost;
};
int par[N];
int a,n,ans=0;
priority_queue< pair<int,pair<int,int> > > Q;

void init(int n){
	FOR(i,1,n+1)par[i]=i;
}

int root(int x){
	if(par[x]==x){
		return x;
	}else{
		return par[x] = root(par[x]);
	}
}

void unite(int x,int y){
	x=root(x);
	y=root(y);
	if(x==y)return;
	par[x]=y;
}


int main(){
	ios::sync_with_stdio(false);
	cin.tie(0);
	cin>>n;
	init(n);
	FOR(i,1,n+1){
		FOR(j,1,n+1){
			cin>>a;
			if(a==(-1))a=2001;
			Q.push(make_pair(-a,make_pair(i,j)));
		}
	}
	while(Q.size()){
		pair<int,int> e;
		int cost,x,y;
		pair<int,pair<int,int> > q;
		q=Q.top();
		e=q.second;
		cost=-q.first;
		x=e.first;
		y=e.second;
		Q.pop();
		if(root(x)==root(y))continue;
		unite(x,y);
		ans+=cost;
	}
	cout<<ans<<endl;
	return 0;
}

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

struct edge{ int to,cost; };
const int N = 1e5+10;
vector< edge > G[N];

int prim(){
  typedef pair<int, int> P;
  const int INF = 1 << 30;

  priority_queue<P,vector<P>,greater<P>> que;
  vector<bool> visited(N,false);

  que.push({0,0}); //{距離,頂点}
  int total = 0;

  while( !que.empty() ){
    P p = que.top(); que.pop();
    int v = p.second;
    if(visited[v]) continue;
    visited[v] = true;
    total += p.first;
    for(auto e : G[v]){
      if(!visited[e.to]){
        que.push( {e.cost,e.to} );
      }
    }
  }
  return total;
}

signed main(){
  cin.tie(0);
  ios::sync_with_stdio(false);

  int V; cin >> V;
  for(int i = 0; i < V; i++){
    for(int j = 0; j < V; j++){
      int cost; cin >> cost;
      if(cost == -1) continue;
      G[i].push_back({j,cost});
    }
  }
  cout << prim() << endl;

  return 0;
}

C++: #include <stdio.h>
#include <iostream>
using namespace std;

#define NUM 100
#define BLACK 0
#define GREY 1
#define WHITE 2
#define NIL -1
#define INF 1000000000
int A[NUM][NUM];

int V[NUM];
int color[NUM];

void init(){
    for(int i = 0; i < NUM; i++){
        color[i] = WHITE;
        V[i] = INF;
    }

}

int main(void){
    init();

    int n;
    scanf("%d", &n);

    for(int i = 0; i < n; i++){
        for(int j = 0; j < n; j++){
            scanf("%d", &A[i][j]);
        }
    }

    int next_node = 0;
    int sum = 0;
    while(1){
        color[next_node] = BLACK;
        for(int i = 0; i < n; i++){
            if(A[next_node][i] == NIL){
                continue;
            }
            V[i] = min(V[i], A[next_node][i]);
        }

        next_node = NIL;
        int min_dist = INF;
        for(int i = 0; i < n; i++){
            if(color[i] == BLACK){
                continue;
            }
            if(V[i] < min_dist){
                next_node = i;
                min_dist = V[i];
            }
        }
        if(min_dist == INF){
            break;
        }else{
            sum += min_dist;
        }
    }

    printf("%d\n", sum);
}

C++: #include<iostream>
using namespace std;
static const int MAX = 100;
static const int INFTY = (1 << 21);
static const int WHITE = 0;
static const int GRAY = 1;
static const int BLACK = 2;

int n, M[MAX][MAX];

int prim() {
	int u, minv;
	int d[MAX], p[MAX], color[MAX];

	for (int i = 0; i < n; i++) {
		d[i] = INFTY;
		p[i] = -1;
		color[i] = WHITE;
	}

	d[0] = 0;

	while (true) {
		minv = INFTY;
		u = -1;
		for (int i = 0; i < n; i++) {
			if (minv > d[i] && color[i] != BLACK) {
				u = i;
				minv = d[i];
			}
		}
		if (u == -1)break;
		color[u] = BLACK;
		for (int v = 0; v < n; v++) {
			if (color[v] != BLACK&&M[u][v] != INFTY) {
				if (d[v] > M[u][v]) {
					d[v] = M[u][v];
					p[v] = u;
					color[v] = GRAY;
				}
			}
		}
	}
	int sum = 0;
	for (int i = 0; i < n; i++) {
		if (p[i] != -1)sum += M[i][p[i]];
	}

	return sum;	
}

int main() {
	cin >> n;
	for (int i = 0; i < n; i++) {
		for (int j = 0; j < n; j++) {
			int e; cin >> e;
			M[i][j] = (e == -1) ? INFTY : e;
		}
	}
	cout << prim() << endl;
	return 0;
}

C++: #include <iostream>
#include <set>

#define rep(i, n) for (int i = 0; i < (int)(n); ++i)

const int INF = 1 << 21;
const int SIZE = 101;
int a[SIZE][SIZE];

int prim(int n) {
  int d[n], p[n], state[n];

  rep(i, n) {
    d[i] = INF;
    p[i] = -1;
    state[i] = 0;
  }

  d[0] = 0;

  while (1) {
    int minv = INF, u = -1;

    rep(i, n) {
      if (d[i] < minv && state[i] != 1) {
        minv = d[i];
        u = i;
      }
    }

    if (u == -1) break;
    state[u] = 1;

    rep(i, n) {
      if (a[u][i] != INF && state[i] != 1) {
        if (d[i] > a[u][i]) {
          d[i] = a[u][i];
          p[i] = u;
        }
      }
    }
  }

  int sum = 0;
  rep(i, n) {
    if (p[i] != -1) { sum += a[i][p[i]]; }
  }

  return sum;
}

int main() {
  int n;
  std::cin >> n;

  rep(i, n) rep(j, n) {
    int m;
    std::cin >> m;
    if (m == -1) m = INF;
    a[i][j] = m;
  }

  std::cout << prim(n) << '\n';

  return 0;
}