Go: package main

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

const (
	WHITE int = iota
	GRAY
	BLACK
)

type graph struct {
	G [][]int
	C []int
	D []int
	P []int
}

var INFTY = 1 << 32

func constructGraph() *graph {
	sc := bufio.NewScanner(os.Stdin)
	sc.Scan()
	n, _ := strconv.Atoi(sc.Text())

	g := make([][]int, n)
	c := make([]int, n)
	d := make([]int, n)
	p := make([]int, n)

	for i := 0; i < n; i++ {
		sc.Scan()
		fields := strings.Fields(sc.Text())
		g[i] = toIntSlice(fields)
	}

	return &graph{g, c, d, p}
}

func (g *graph) initMst() {
	for i := 0; i < len(g.G); i++ {
		g.C[i] = WHITE
		g.D[i] = INFTY
	}
	g.D[0] = 0
	g.P[0] = -1
}

func (g *graph) primMst() int {
	g.initMst()

	var u int
	for {
		mincost := INFTY
		for i := 0; i < len(g.G); i++ {
			if g.C[i] != BLACK && g.D[i] < mincost {
				mincost = g.D[i]
				u = i
			}
		}

		if mincost == INFTY {
			break
		}

		g.C[u] = BLACK

		for i := 0; i < len(g.G); i++ {
			if g.C[i] != BLACK && g.existEdge(u, i) {
				if g.G[u][i] < g.D[i] {
					g.D[i] = g.G[u][i]
					g.P[i] = u
				}
			}
		}
	}

	totalCost := 0
	for _, cost := range g.D {
		totalCost += cost
	}
	return totalCost
}

func (g *graph) existEdge(u, v int) bool {
	return g.G[u][v] != INFTY && g.G[v][u] != INFTY
}

func toIntSlice(s []string) []int {
	a := make([]int, len(s))
	for i, v := range s {
		num, _ := strconv.Atoi(v)
		if num == -1 {
			num = INFTY
		}
		a[i] = num
	}
	return a
}

func main() {
	g := constructGraph()
	fmt.Println(g.primMst())
}

C++: #include <iostream>
#include <utility>
#include <string>
#include <algorithm>
#include <set>
#include <deque>
#include <vector>
#define MAX 150
#define rep(i,p) for(int i=0;i<p;i++)
#define rep1(i,p) for(int i=1;i<=p;i++)
#define pb push_back
using namespace std;
int n,temp,a,b,size,ra,rb;
int A[MAX];
int B[MAX];
int W[MAX];
int V[MAX];
pair<int,int> edge[MAX];
int root (int child) {
  int a = child;
  while (V[a] != a) {
    a = V[a];
  }
  return a;
}
int main(){
  size = 0;
  cin >> n;
  rep(i,n) 
    rep(j,n) {
      cin >> W[size];
      if (i <= j) {        
        A[size] = i;
        B[size] = j;
        edge[size].first = W[size];
        edge[size].second = size;
        size++;
      }
    }
  // sort by lenght of edge
  sort(edge, edge+size+1);
  // printing
  // rep(i,size+1) {
  //   cout << A[edge[i].second] << " " << B[edge[i].second]
  //        << " " << edge[i].first << endl;;
  // }
  int sum = 0;
  rep(i,n) V[i] = i;
  rep(i,size) {
    if (edge[i].first == -1) continue;
    temp = edge[i].second;
    // a , b : vertex
    a = A[temp];
    b = B[temp];
    ra = root(a);
    rb = root(b);
    if (ra != rb) {
      // ?????????????????????
      sum += edge[i].first;
      V[ra] = rb;
      // if (ra != a && rb == b){     
      //   V[b] = ra;
      // } else if (ra == a && rb != b) {
      //   V[a] = rb;
      // } else if (ra == a && rb == b) {        
      //   V[a] = b;
      // } else if (ra != a && rb != b) {                
      //   V[ra] = rb;
      // }
    }
  }
  cout << sum << endl;
  return 0;
}

C++: #include<iostream>
#define MAX 100
#define INFTY 1<<21
#define WHITE 0
#define GRAY 1
#define BLACK 2
#define NIL -1

int M[MAX][MAX];
int d[MAX], p[MAX], color[MAX];
int n;

// プリムのアルゴリズムを用いて、最小全域木の辺の重みの総和を返す
int prim()
{
	for (int i = 0; i < n; i++)
	{
		d[i] = INFTY;
		p[i] = NIL;
		color[i] = WHITE;
	}

	d[0] = 0;
	color[0] = GRAY;

	while (true)
	{
		int u = NIL;
		int minv = INFTY;
		for (int i = 0; i < n; i++)
		{
			if (color[i] != BLACK && d[i] < minv)
			{
				u = i;
				minv = d[i];
			}
		}

		if (u == NIL) break;

		color[u] = BLACK;

		for (int v = 0; v < n; v++)
		{
			if (color[v] != BLACK && M[u][v] != INFTY)
			{
				if (M[u][v] < d[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] != NIL) sum += d[i];
	}

	return sum;
}

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

	int x;
	for (int i = 0; i < n; i++)
	{
		for (int j = 0; j < n; j++)
		{
			std::cin >> x;
			M[i][j] = x == -1 ? INFTY : x;
		}
	}

	std::cout << prim() << std::endl;

}

/*
木とは、閉路を持たないグラフ。
全域木とは、グラフのすべての頂点を含み木である限りできるだけ多くの辺を持つグラフ。
最小全域木とは、全域木の中で辺の重みの総和が最小のグラフ。
*/

C++: //Minimu Spanning Tree
#include<iostream>
using namespace std;

static const int  MAX = 100;
static const int  INF = (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];
    
    //d is INF, p is -1, color IS WHITE;
    for(int i = 0; i < n; i++){
        d[i] = INF;
        p[i] = -1;
        color[i] = WHITE;
    }
    //d[0] = 0;
    d[0] = 0;
    
    while(true){
        minv = INF;
        u = -1;
        
        //????°???¨????????????V - S????????¢???????????????S?????£??\??????????????????
        for(int i = 0; i < n; i++){
            if(minv > d[i] && color[i] != BLACK){
                minv = d[i];
                u = i;
            }
        }
        
        //???????????????????????¨????????????????????????S=V??¨?????£???
        if(u == -1) break;
        
        color[u] = BLACK;
        for(int v = 0; v < n; v++){
            if(color[v] != BLACK && M[u][v]){
                if(M[u][v] < d[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(void){
    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) ? INF : e;
        }
    }
    
    cout << prim() << endl;
    
    return 0;
}

C++: #include <iostream>
#include <cmath>
#include <vector>
#include <string>
#include <algorithm>

constexpr int INF = 3000;

int main() {
    std::ios::sync_with_stdio(false);
    std::cin.tie(0);
    int n, aij, mincost, minu, sum=0;
    std::cin >> n;
    std::vector<std::vector<int>> a(n);
    std::vector<int> d(n), p(n), color(n); // white:0, gray:1, black:2
    for (int i=0; i<n; i++) {
        for (int j=0; j<n; j++) {
            std::cin >> aij;
            a[i].push_back(aij);
        }
        d[i] = INF;
    }
    d[0]=0;
    p[0]=-1;
    while (true) {
        mincost = INF;
        for (int i=0; i<n; i++) {
            if (color[i] < 2 and d[i] < mincost) {
                mincost = d[i];
                minu = i;
            }
        }
        if (mincost == INF) {
            break;
        }
        color[minu] = 2;
        for (int v=0; v<n; v++) {
            if (color[v] < 2 and a[minu][v] >= 0 and d[v] > a[minu][v]) {
                d[v] = a[minu][v];
                p[v] = minu;
                color[v] = 1;
            }
        }
    }

    for (int i=0; i<n; i++) {
        sum += d[i];
    }
    std::cout << sum << "\n";

    return 0;
}

C++: #include <stdio.h>
#define rep(i, n) for(int i = 0; i < n; ++i)

int main(void) {
  int n, i, j, ans = 0;
  scanf("%d", &n);
  int a[n][n];
  char f[n];
  rep(i, n) rep(j, n) scanf("%d", &a[i][j]);
  rep(i, n) f[i] = 0;
  f[0] = 1;
  rep(i, n - 1) {
    int min = 10000, minx, miny;
    rep(j, n) {
      rep(k, n) {
        if(f[j] && !f[k] && a[j][k] > 0 && a[j][k] < min) min = a[j][k], minx = j, miny = k;
      }
    }
    ans += min;
    f[miny] = 1;
  }
  printf("%d\n", ans);
  return 0;
}