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 <iomanip>
#include <vector>
#include <set>
#include <string>
#include <queue>
#include <algorithm>
#include <map>
#include <cmath>
#include <list>
#include <stack>
#include <cstdio>
#include <cstdlib>
#include <cstring>

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

using namespace std;
typedef vector<int> vi;
typedef pair<int, int> pii;

const int INF = 1 << 30;
const int MOD = 1000000007;
const int MAX = 100000;
const int N = 100;

vector<pii> G[N];
bool T[N];

int main() {
	int n; cin >> n;
	rep(i, n) { rep(j, n) {
		int a; cin >> a;
		if (a == -1) continue;
		G[i].push_back(pii(a, j));
	} }
	rep(i, n) T[i] = false;

	int ans = 0;
	priority_queue<pii, vector<pii>, greater<pii>> Q;
	Q.push(pii(0, 0));
	while (!Q.empty()) {
		pii p = Q.top(); Q.pop();
		if (T[p.second]) continue;
		ans += p.first;
		T[p.second] = true;
		for (auto i : G[p.second]) Q.push(i);
	}
	cout << ans << endl;
}

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

const int NOTV = 0;
const int VIST = 0;
const int NMAX = 100;
const int VMAX = 2000;
const int NILL = -1;
const int INFTY = 1 << 21;
int n;
int a[NMAX][NMAX];  //??£??\??????
int v[NMAX];  //????°???¨?????¨?????????
int d[NMAX];  //????°???¨?????¨??????????????????
int vis[NMAX];
//priority_queue <greater<int>> pq;

int prim() {
	for (int i = 0; i < n; i++) {
		vis[i] = NOTV;
		d[i] = INFTY;
		v[i] = -1;
	}
	d[0] = 0;
	int u = 0;
	while (1) {
		int mincost = INFTY;
		u = -1;
		for (int i = 0; i < n; i++) {
			if (vis[i] == NOTV and mincost > d[i]) {
				mincost = d[i];
				u = i;
			}
		}
		if (u==-1) break;
		vis[u] = VIST;
		for (int ver = 0; ver < n; ver++) {
			if (vis[ver] == NOTV and a[u][ver] != INFTY) {
				if (a[u][ver] < d[ver]) {
					d[ver] = a[u][ver];
					v[ver] = u;
					vis[ver] = NOTV;
				}
			}
		}
		cout << u << endl;
	}
	int ans = 0;
	for (int i = 0; i < n; i++) {
		if (v[i] != -1) ans += a[i][v[i]];
	}
	return ans;
}

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

C++: #include<bits/stdc++.h>
#define rep(i,n) for(int i=0;i< (n);i++)
using namespace std;
typedef long long ll;
const int INF = 1e+7;

struct prim{
    vector<int> color;
    vector<vector<int>> M;
    vector<int> d;
    vector<int> p;
    int n_;
    prim(int n):color(n,0),M(n,vector<int>(n,-1)),d(n,INF),p(n,-1){
        n_ = n;
    }
    void init(){
        rep(i,n_){
            rep(j,n_){
                int tmp;cin>>tmp;
                M[i][j] = tmp;
            }
        }
    }

    void solve(){
        init();
        d[0] = 0;
        p[0] = -1;

        while(true){
            int mincost = INF;
            int u;
            rep(i,n_){
                if(color[i] != 2 && d[i] < mincost ){
                    mincost = d[i];
                    u = i;
                }
            }
            if( mincost == INF){
                break;
            }
            color[u] = 2;

            rep(v,n_){
                if(color[v] != 2 && M[u][v] != -1){
                    if(M[u][v] < d[v]){
                        d[v] = M[u][v];
                        p[v] = u;
                        color[v] = 1;
                    }
                }
            }
        }
        int ans = 0;
        rep(i,n_){
            ans += d[i];
        }
        cout<<ans<<endl;
    }
};

int main(){
    int n;cin >> n;
    prim p(n);
    p.solve();
    return 0;
}