Go: package main

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

const BIG_COST = 3000

const (
	ST_NOT_YET = iota
	ST_JOINED
)

func main() {
	sc := bufio.NewScanner(os.Stdin)
	sc.Split(bufio.ScanWords)

	nNodes := scanInt(sc)
	costs := make([][]int, nNodes)

	for r := 0; r < nNodes; r++ {
		costs[r] = make([]int, nNodes)
		for c := 0; c < nNodes; c++ {
			costs[r][c] = scanInt(sc)
		}
	}

	states := make([]int, nNodes)
	minCosts := make([]int, nNodes)

	for i := 1; i < nNodes; i++ {
		minCosts[i] = BIG_COST
	}

	for {
		var u int
		minCost := BIG_COST

		for i := 0; i < nNodes; i++ {
			if states[i] != ST_JOINED && minCosts[i] < minCost {
				minCost = minCosts[i]
				u = i
			}
		}

		// 全てのノードが MST に JOIN できたので終了。
		if minCost == BIG_COST {
			break
		}

		states[u] = ST_JOINED

		for v := 0; v < nNodes; v++ {
			cost := costs[u][v]

			// MST内の u と MST外の v との辺の重みをそれぞれ minCosts[v] として
			// 保存しておく。このうち辺の重みが最小となる v が次のループで MST に JOIN する。
			if states[v] != ST_JOINED && cost != -1 && cost < minCosts[v] {
				minCosts[v] = cost
			}
		}
	}

	sum := 0
	for _, cost := range minCosts {
		sum += cost
	}
	fmt.Println(sum)
}

func scanInt(sc *bufio.Scanner) int {
	sc.Scan()
	v, _ := strconv.Atoi(sc.Text())
	return v
}

C++: #include <bits/stdc++.h>
using namespace std;
int prim(void);

#define MAX 100
#define WHITE 0
#define GRAY 1
#define BLACK 2

int A[MAX][MAX];
int n;

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

int prim(){
  int sum = 0;
  int i,j,minV;
  int d[MAX],p[MAX],color[MAX];
  int u,v; 
  for(i = 0; i < n; i++){
    d[i] = INFINITY;
    p[i] = -1;
    color[i] = WHITE;
  }
  d[0] = 0;
  while(1){
    minV = INFINITY;
    u = -1;
    for(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(v = 0; v < n; v++){
      if(color[v] != BLACK && A[u][v] != INFINITY){
        if(d[v] > A[u][v]){
          d[v] = A[u][v];
          p[v] = u;
          color[v] = GRAY;
        }
      }
    }
  }
  //sum = 0;
  for(i = 0; i < n; i++){
    if(p[i] != -1){
      sum += A[i][p[i]];
    }
  }
  return sum;
}

C++: #include <climits>
#include <iostream>
using namespace std;

#define MAX_N 2000
int n;
int A[MAX_N][MAX_N];

class Node
{
    public:
        int cost = INT_MAX;;
        bool used = false;
};

int prim()
{
    int i, u, v, minv, sum = 0;
    Node *nodes = new Node[n];
    nodes[0].cost = 0;
    while (true) {
        u = -1; minv = INT_MAX;
        for (i = 0; i < n; i++) {
            if (nodes[i].cost < minv && !nodes[i].used) {
                u    = i;
                minv = nodes[i].cost;
            }
        }
        if (-1 == u) break;
        nodes[u].used = true;
        sum += nodes[u].cost;
        for (v = 0; v < n; v++) {
            if (!nodes[v].used && A[u][v] != -1) {
                nodes[v].cost = A[u][v] < nodes[v].cost ? A[u][v] : nodes[v].cost;
            }
        }
    }
    return sum;
}

int main()
{
    cin >> n;
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < n; j++) {
            cin >> A[i][j];
        }
    }
    cout << prim() << endl;
    return 0;
}

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;
}