Go: package main

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

var scanner = bufio.NewScanner(os.Stdin)

func nextString() string {
	scanner.Scan()
	return scanner.Text()
}

func nextInt() int {
	n, err := strconv.Atoi(nextString())
	if err != nil {
		fmt.Printf("strconv.Atoi failed: %v\n", err)
	}
	return n
}

type ElementKey int
type Node struct {
	key    ElementKey
	parent *Node
	rank   int
}

func newNode(key ElementKey) *Node {
	node := Node{}
	node.key = key
	node.parent = &node
	node.rank = 0

	return &node
}

func union(x, y *Node) {
	xSet := findSet(x)
	ySet := findSet(y)
	link(xSet, ySet)
}

func link(x, y *Node) {
	if x.rank > y.rank {
		y.parent = x
	} else {
		x.parent = y

		if x.rank == y.rank {
			y.rank++
		}
	}
}

func findSet(x *Node) *Node {
	if x != x.parent {
		x.parent = findSet(x.parent)
	}
	return x.parent
}

func newDisjointSets(nSets int) *DisjointSets {
	sets := DisjointSets{}
	sets.keyToNode = map[ElementKey]*Node{}
	for i := 0; i < nSets; i++ {
		key := ElementKey(i)
		sets.keyToNode[key] = newNode(key)
	}
	return &sets
}

type DisjointSets struct {
	keyToNode map[ElementKey]*Node
}

func (sets *DisjointSets) union(x, y ElementKey) {
	xNode := sets.keyToNode[x]
	yNode := sets.keyToNode[y]
	union(xNode, yNode)
}

func (sets DisjointSets) findSet(x ElementKey) *Node {
	xNode := sets.keyToNode[x]
	return findSet(xNode)
}

type Edge struct {
	i, j   int
	weight int
}

func main() {
	scanner.Split(bufio.ScanWords)

	n := nextInt()
	sets := newDisjointSets(n)
	edges := []Edge{}
	a := make([][]int, n)
	for i := 0; i < n; i++ {
		a[i] = make([]int, n)
		for j := 0; j < n; j++ {
			a[i][j] = nextInt()
			if a[i][j] >= 0 {
				e := Edge{
					i:      i,
					j:      j,
					weight: a[i][j],
				}
				edges = append(edges, e)
			}
		}
	}
	sort.Slice(edges, func(i, j int) bool { return edges[i].weight < edges[j].weight })
	totalWeight := 0
	for _, edge := range edges {
		i := ElementKey(edge.i)
		j := ElementKey(edge.j)
		if sets.findSet(i) != sets.findSet(j) {
			totalWeight += edge.weight
			sets.union(i, j)
		}
	}
	fmt.Println(totalWeight)
}

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;
}