Go: package main

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

// I/O
type Scanner struct {
	sc *bufio.Scanner
}

func NewScanner() *Scanner {
	sc := bufio.NewScanner(os.Stdin)
	sc.Split(bufio.ScanWords)
	sc.Buffer(make([]byte, 1024), int(1e+9))
	return &Scanner{sc}
}

func (s *Scanner) nextStr() string {
	s.sc.Scan()
	return s.sc.Text()
}

func (s *Scanner) nextInt() int {
	i, e := strconv.Atoi(s.nextStr())
	if e != nil {
		panic(e)
	}
	return i
}

func (s *Scanner) nextFloat() float64 {
	f, e := strconv.ParseFloat(s.nextStr(), 64)
	if e != nil {
		panic(e)
	}
	return f
}

func (s *Scanner) nextRuneSlice() []rune {
	return []rune(s.nextStr())
}

func (s *Scanner) nextIntSlice(n int) []int {
	res := make([]int, n)
	for i := 0; i < n; i++ {
		res[i] = s.nextInt()
	}
	return res
}

func (s *Scanner) nextFloatSlice(n int) []float64 {
	res := make([]float64, n)
	for i := 0; i < n; i++ {
		res[i] = s.nextFloat()
	}
	return res
}

// Arithmetic
func max(nums ...int) int {
	m := nums[0]
	for _, i := range nums {
		if m < i {
			m = i
		}
	}
	return m
}

func min(nums ...int) int {
	m := nums[0]
	for _, i := range nums {
		if m > i {
			m = i
		}
	}
	return m
}

func abs(x int) int {
	if x > 0 {
		return x
	}
	return -x
}

func pow(x, y int) int {
	res := 1
	for i := 0; i < y; i++ {
		res *= x
	}
	return res
}

func ceil(a, b int) int {
	if a%b == 0 {
		return a / b
	} else {
		return a/b + 1
	}
}

// Sort
type RuneSlice []rune

func (a RuneSlice) Len() int           { return len(a) }
func (a RuneSlice) Less(i, j int) bool { return a[i] < a[j] }
func (a RuneSlice) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }

const INF = 1 << 60

// Main
func main() {
	sc := NewScanner()
	wtr := bufio.NewWriter(os.Stdout)
	N := sc.nextInt()
	G := make([][]int, N)
	for i := 0; i < N; i++ {
		G[i] = make([]int, N)
		for j := 0; j < N; j++ {
			G[i][j] = sc.nextInt()
		}
	}
	d := make([]int, N)
	for i := 0; i < N; i++ {
		d[i] = INF
	}
	seen := make([]bool, N)
	d[0] = 0
	ans := 0
	for {
		cur := 0
		minCost := INF
		for i := 0; i < N; i++ {
			if !seen[i] && d[i] < minCost {
				minCost = d[i]
				cur = i
			}
		}
		if minCost == INF {
			break
		}
		seen[cur] = true
		ans += d[cur]
		for i := 0; i < N; i++ {
			cost := G[cur][i]
			if cost != -1 && cost < d[i] {
				d[i] = cost
			}
		}
	}

	fmt.Fprintln(wtr, ans)
	wtr.Flush()
}

C++: #include <iostream>
#include <vector>
#include <list>
#include <algorithm>
#include <queue>
#include <stack>
#include <math.h>

#define ll long long
#define Int int
#define loop(x, start, end) for(Int x = start; x < end; x++)
#define loopdown(x, start, end) for(int x = start; x > end; x--)

using namespace std;

#define MAX_N 100
#define MAX_INT (1 << 21)


Int prim(Int W[MAX_N][MAX_N], Int n) {
  Int parent[n], key[n];
  bool mst_set[n];
  
  loop(i,0,n) key[i] = MAX_INT, mst_set[i] = false;
  
  key[0] = 0, parent[0] = -1;
  
  // why n-1 ?
  loop(count,0,n-1) {
      Int u = -1;
      Int min_ = MAX_INT;
      loop(v,0,n) {
          if (mst_set[v] == false && key[v] < min_) {
              min_ = key[v], u = v;
          }
      }
      
      mst_set[u] = true;
      
      loop(v,0,n) {
          if (W[u][v] != MAX_INT && mst_set[v] == false && W[u][v] < key[v]) {
              parent[v] = u, key[v] = W[u][v];
          }
      }
  }

  Int sum = 0;
  loop(v,1,n) {
    sum += W[v][parent[v]];
  }

  return sum;
}


int main(void) {
  Int n, w;
  Int W[MAX_N][MAX_N];
  cin >> n;
  loop(x,0,n) {
    loop(y,0,n) {
      cin >> w;
      W[x][y] = W[y][x] = (w == -1) ? MAX_INT : w;
    }
  }

  cout << prim(W, n) << endl;
}

C++: // 最小全域木 ( プリム法 )
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<stack>
#include<algorithm>
#include<iostream>
#include<cmath>
#include<vector>

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;
    }

    // 初期値の設定
    d[0] = 0;

    while(1)
    {
        minv = INFTY;
        // 次に移動するべき頂点
        u = -1;

        for(int i = 0; i < n; i++)
        {
            if( minv > d[i] && color[i] != BLACK )
            {
                // 未訪問の頂点の中で, 最小のコストを持つ辺を探索する
                u = i;
                minv = d[i];
            }
        }

        if( u == -1 )
        {
            // 最初全域木が完成した
            break;
        }

        color[u] = BLACK;

        for(int v = 0; v < n; v++)
        {
            if( color[v] != BLACK && M[u][v] != INFTY )
            {
                // u → v の辺の中で, 最小コストの辺のコストを記録する
                if( d[v] > M[u][v] )
                {
                    // コストの記録
                    d[v] = M[u][v];
                    // 頂点 v の親を記録する
                    p[v] = u;
                    // 訪問状態の記録
                    color[v] = GRAY;
                }
            }
        }
    }

    int sum = 0;

    for(int i = 0; i < n; i++)
    {
        if( p[i] != -1 )
        {
            // 最小コストの辺の和を求める
            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;

            // 隣接行列の設定
            cin >> e;

            M[i][j] = ( e == -1 ) ? INFTY : e;
        }
    }

    // 結果の表示
    cout << prim() << endl;

    return 0;

}

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

int n, M[MAX][MAX];

int prim(){
	int d[MAX], p[MAX], color[MAX];
	for (int i = 0; i < n; i++){
		d[i] = INF;
		p[i] = -1;
		color[i] = WHITE;
	}
	d[0] = 0;
	while (1){
		int u = -1;
		int minv = INF;
		for (int v = 0; v < n; v++){
			if (minv > d[v] && color[v] != BLACK){
				minv = d[v];
				u = v;
			}
		}
		if (u == -1)break;
		color[u] = BLACK;
		for (int v = 0; v < n; v++){
			if (color[v] != BLACK && M[u][v] != INF){
				if (d[v] > M[u][v]){
					d[v] = M[u][v];
					p[v] = u;
					color[v] = GLAY;
				}
			}
		}
	}
	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;
	int e;
	for (int i = 0; i < n; i++){
		for (int j = 0; j < n; j++){
			cin >> e;
			M[i][j] = (e == -1) ? INF : e;
		}
	}
	cout << prim() << endl;
}

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

class Side {
public:
	int r, l;
	int w;
	Side(int right, int left, int weight) {
		r = right;
		l = left;
		w = weight;
	};
	bool operator>=(Side s) {
		return w >= s.w;
	}
	bool operator<(Side s) {
		return w < s.w;
	}
};

class Node {
public:
	int p; /// parent
	int d; /// depth
	Node() {
		p = -1;
		d = 1;
	}
};

int Find(std::vector<Node> n, int i) {
	int a = i;
	while (1) {
		if (n[a].p == -1) return a;
		a = n[a].p;
	}
}

bool Union(std::vector<Node> &n, int i, int j) {
	int a = Find(n, i);
	int b = Find(n, j);
	if (a == b) return false;
	if (n[a].d <= n[b].d) {
		n[b].p = a;
		n[a].d = n[b].d + 1;
	}
	else {
		n[a].p = b;
		n[b].d = n[a].d + 1;
	}
	return true;
}

int main() {
	int n;
	std::cin >> n;
	std::vector<Side> side;
	std::vector<Node> node(n);

	for (int i = 0; i < n; i++) {
		for (int j = 0; j < n; j++) {
			int a;
			std::cin >> a;
			if (a != -1) {
				side.push_back(Side(i, j, a));
			}
		}
	}

	std::sort(side.begin(), side.end());
	int m = side.size();
	int cnt = 0;
	int weight = 0;


	for (int i = 0; i < m; i++) {
		if (Union(node, side[i].r, side[i].l)) {
			cnt++;
			weight += side[i].w;
		}
		if (cnt == n - 1) break;
	}

	std::cout << weight << std::endl;
}