Go: package main

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

func getScanner(fp *os.File) *bufio.Scanner {
	scanner := bufio.NewScanner(fp)
	scanner.Split(bufio.ScanWords)
	scanner.Buffer(make([]byte, 500001), 500000)
	return scanner
}

func getNextString(scanner *bufio.Scanner) string {
	scanner.Scan()
	return scanner.Text()
}

func getNextInt(scanner *bufio.Scanner) int {
	i, _ := strconv.Atoi(getNextString(scanner))
	return i
}

func main() {
	fp := os.Stdin
	if len(os.Args) > 1 {
		fp, _ = os.Open(os.Args[1])
	}

	scanner := getScanner(fp)
	writer := bufio.NewWriter(os.Stdout)

	n := getNextInt(scanner)

	g := make([][]int, n)
	ans := make([][]int, n)

	inf := 1 << 30
	for i := 0; i < n; i++ {
		g[i] = make([]int, n)
		ans[i] = make([]int, n)
		for ii := 0; ii < n; ii++ {
			g[i][ii] = getNextInt(scanner)
			if g[i][ii] == -1 {
				g[i][ii] = inf
			}
			ans[i][ii] = -1
		}
	}

	im := make(IntMap, 1)

	im.increment(1)

	for len(im) < n {
		min := inf
		u := -1
		v := -1
		for i := range im {
			for ii := 0; ii < n; ii++ {
				if im.get(ii) != 0 {
					continue
				}
				if min > g[i][ii] {
					min = g[i][ii]
					u = i
					v = ii
				}
			}
		}
		im.increment(v)
		ans[u][v] = min
		ans[v][u] = min
	}

	sum := 0
	for i := 0; i < n-1; i++ {
		for ii := i + 1; ii < n; ii++ {
			if ans[i][ii] != -1 {
				sum += ans[i][ii]
			}
		}
	}

	fmt.Fprintln(writer, sum)
	writer.Flush()
}

// IntMap ...
type IntMap map[int]int

func (m IntMap) get(key int) int {
	if _, ok := m[key]; ok {
		return m[key]
	}
	return 0
}
func (m IntMap) increment(key int) {
	if _, ok := m[key]; ok == false {
		m[key] = 0
	}
	m[key]++
}

C++: #include<iostream>
#include<cstdio>
#define INF 1000000
#define NIL -2
#define GRAY -3
#define WHITE -4
#define BRACK -5

using namespace std;

int n;
int M[101][101];
int d[101];//重み
int p[101];//親
int color[101];
int sum = 0;

void Prim(int u) {
	color[u] = BRACK;

	for (int i = 1; i <= n; i++) {
		if (color[i] != BRACK && M[u][i] != -1) {
			color[i] = GRAY;
			if (d[i] > M[u][i]) {
				d[i] = M[u][i];
				p[i] = u;
			}
		}
	}

	sum += d[u];

	//全体のうち重みが一番小さいものについて再帰
	//color[v] = WHITE かつ　d[v]が一番小さい点v について再帰
	//かつGRAYの点とつながっている
	int min = 0;
	d[min] = 100000;


	for (int v = 1; v <= n; v++) {
		if (color[v] == GRAY) {
			if (d[v] < d[min]) {
				min = v;
			}
		}
	}

	if (min != 0) {
		Prim(min);
	}
}

int main() {
	cin >> n;

	for (int i = 1; i <= n; i++) {
		for (int j = 1; j <= n; j++) {
			M[i][j] = 0;
		}
	}
	for (int i = 1; i <= n; i++) {
		for (int j = 1; j <= n; j++) {
			cin >> M[i][j];
		}
	}
	for (int i = 1; i <= n; i++) {
		color[i] = WHITE;
	}

	for (int i = 1; i <= n; i++) {
		d[i] = INF;
		p[i] = NIL;
	}
	d[1] = 0;

	//関数
	Prim(1);

	cout << sum << endl;


	return 0;
}

C++: #include <stdio.h>

#define NMAX 100
#define WMAX 2000
#define INF WMAX + 1
#define NIL -1

int n;
int M[NMAX][NMAX];
int d[NMAX]; // distance to mst
int p[NMAX]; // parent
int c[NMAX]; // c[i]=1 if i in mst

int solve() { // find total cost of mst
    for (int i = 0; i < n; i++) {
        c[i] = 0;
        p[i] = NIL;
        d[i] = INF;
    }
    d[0] = 0; // root
    p[0] = NIL;
    int total = 0;
    while (1) {
        // find i s.t d[i] is min
        int mincost = INF;
        int u;
        for (int i = 0; i < n; i++) {
            // if i not in mst and d[i] < mincost
            if (!c[i] && d[i] < mincost) {
                mincost = d[i];
                u = i;
            }
        }

        if (mincost == INF)
            break;

        // printf("include %d to mst!\n", u);
        c[u] = 1; // include u to mst
        total += mincost;

        // update d[i] for M[u][i]!=-1
        for (int i = 0; i < n; i++) {
            if (!c[i] && M[u][i] < d[i]) {
                d[i] = M[u][i];
                p[i] = u;
            }
        }
    }
    return total;
}

int main() {
    scanf("%d", &n);
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < n; j++) {
            int w;
            scanf("%d", &w);
            if (w != -1) {
                M[i][j] = w;
            } else {
                M[i][j] = INF;
            }
        }
    }
    printf("%d\n", solve());
    return 0;
}

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( 1 ){
   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>
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, min_value;
	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) {
		min_value = INFTY;
		u = -1;
		for (int i = 0; i < n; i++) {
			if (min_value > d[i] && color[i] != BLACK) {
				u = i;
				min_value = 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++: /*
* @Author: Samson
* @Date:   2018-06-12 18:24:44
* @Last Modified by:   Samson
* @Last Modified time: 2018-06-13 11:56:23
*/
//   @URL : http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=ALDS1_12_A
#include<bits/stdc++.h>
#include<algorithm>
#include <cstdlib>
#define white 0
#define gray 1
#define black 2
#define INF 0x3f3f3f3f
using namespace std;
typedef long long LL;
const int MAXN = 1e3+10;

int p[MAXN],color[MAXN],a[MAXN][MAXN],t,n,d[MAXN];

int prim()
{
	int minv,u,v;
	for(int i = 1; i <= n; ++i)
	{
		d[i] = INF;
		p[i] = -1;
		color[i] = white;
	}
	
	d[0] = 0;

	while(1)
	{
		minv = INF;
		u = -1;
		for(int i = 1; i <= n; ++i)
		{
			if(minv > d[i] && color[i] != black)
				minv = d[i],u = i;
		}
		if(u == -1)	break;
		color[u] = black;
		for(int v = 1; v <= n; ++v)
		{
			if(color[v] != black && a[u][v] != INF)
			{
				if(d[v] > a[u][v])
				{
					d[v] = a[u][v];
					p[v] = u;
					color[v] = gray;
				}
			}
		}
	}
	int ans = 0;
	for(int i = 1; i <= n; ++i)
		if(p[i] != -1)
			ans += a[i][p[i]];
	return ans;
}

int main(void)
{
	ios::sync_with_stdio(false); 
	//cin.tie(0);
	cin>>n;
	for(int i = 1; i <= n; ++i)
	{
		for(int j = 1; j <= n; ++j)
		{
			cin>>t;
			a[i][j] = t==-1?INF:t;
		}
	}
	cout<<prim()<<'\n';
	return 0;
}