Go: package main

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

type color int

const (
	white color = iota // 未訪問
	mst                // MSTとして確定
)

// 頂点
type node struct {
	matrix   []int // 隣接行列
	minToMst int   // MSTへの重み最小値
	parent   int   // 最小値の相手
	color    color
}

func main() {
	scanner := bufio.NewScanner(os.Stdin)
	n := eGetInt(scanner)

	nodes := make([]node, n)
	for i := 0; i < n; i++ {
		nodes[i].matrix = eAtoi((strings.Fields(eGetline(scanner))))
		nodes[i].parent = -1
		nodes[i].minToMst = math.MaxInt16
		nodes[i].color = white
	}

	// 初回はnode[0]がMSTに入るように,minToMstを設定
	nodes[0].minToMst = 0
	for { // 全部チェックするまで

		// どの頂点を新たにMSTに取り込むか
		min := math.MaxInt16
		u := -1
		for i := 0; i < n; i++ {
			if nodes[i].color == mst {
				continue
			}
			if nodes[i].minToMst < min {
				u = i
				min = nodes[i].minToMst
			}
		}

		if u == -1 {
			break
		}
		// MSTに追加する頂点決定
		nodes[u].color = mst

		// MSTに属していない頂点それぞれについて,MSTとの最小の重みを更新
		for i := 0; i < n; i++ {
			if nodes[i].color == mst {
				continue
			}
			if nodes[i].matrix[u] == -1 {
				// 接してない
				continue
			}
			if nodes[i].matrix[u] < nodes[i].minToMst {
				// node[u]との重みが最小
				nodes[i].minToMst = nodes[i].matrix[u]
				nodes[i].parent = u
			}
		}
	}

	sum := 0
	for _, v := range nodes {
		// node[0]はparentが入っていない
		if v.parent != -1 {
			sum += v.matrix[v.parent]
		}
	}
	fmt.Println(sum)
}

func eGetInt(scanner *bufio.Scanner) int {
	t := eAtoi(strings.Fields(eGetline(scanner)))
	return t[0]
}

func eGetline(p *bufio.Scanner) string {
	if !p.Scan() {
		if err := p.Err(); err != nil {
			log.Fatal(err.Error())
		}
		return ""
	}
	return p.Text()
}

func eAtoi(s []string) []int {
	ints := make([]int, len(s))
	var err error
	for i, e := range s {
		ints[i], err = strconv.Atoi(e)
		if err != nil {
			log.Fatal(err.Error())
		}
	}
	return ints
}

C++: #include<iostream>
using namespace std;
#include<stdio.h>

class union_find  // union find.
{
    int* U;
public:
    union_find(int n){ U = new int [n + 1]; for(int i = 1; i <= n; i++) U[i] = i; }
    int root(int a){ if(U[a] == a){ return a; } U[a] = root(U[a]); return U[a]; }
    bool same(int a, int b){ return root(a) == root(b); }
    void join(int a, int b){ int x = root(a), y = root(b); U[x] = y; }
    ~union_find(){ delete [] U; }
};

struct edge{ int v1, v2; int weight; };  // edge.

edge init(){
    edge e;
    e.v1 = 0; e.v2 = 0; e.weight = -1;
    return e;
}

edge init(int a, int b, int w){
    edge e;
    e.v1 = a; e.v2 = b; e.weight = w;
    return e;
}

void swap_e(edge e1, edge e2)
{
    swap(e1.v1, e2.v1); swap(e1.v2, e2.v2); swap(e1.weight, e2.weight);
}

class edge_heap  // heap.
{
    edge* E;
    int edge_n;
public:
    edge_heap(int n){ E = new edge [n + 1]; edge_n = 0; }
    void min_heap(int i);
    void insert_heap(edge e);   // sorting.
    void insert_nl(edge e);     // no sorting.
    edge delete_min();
    bool isEmpty(){ return edge_n == 0; }
    int total_weight();
    ~edge_heap(){ delete [] E; }
};

void edge_heap::min_heap(int i)
{
    int l = 2 * i, r = 2 * i + 1;
    if(l > edge_n && r > edge_n) return;
    int minid = i;
    if(l <= edge_n){ if(E[l].weight < E[minid].weight) minid = l; }
    if(r <= edge_n){ if(E[r].weight < E[minid].weight) minid = r; }
    if(minid == i) return;
    swap(E[minid], E[i]);
    min_heap(minid);
}

void edge_heap::insert_heap(edge e)
{
    edge_n++;
    int w = e.weight;
    int i = edge_n;
    while(i > 1){
        if(w < E[i / 2].weight){
        E[i] = E[i / 2];
        }else{ break; }
        i /= 2;
    };
    E[i] = e;
}
void edge_heap::insert_nl(edge e){ edge_n++; E[edge_n] = e; }

edge edge_heap::delete_min()
{
    edge e = E[1];
    E[1] = E[edge_n]; edge_n--;
    min_heap(1);
    return e;
}

int edge_heap::total_weight()
{
    int tw = 0;
    for(int i = 1; i <= edge_n; i++) tw += E[i].weight;
    return tw;
}

int main()
{
    int n, i, j, w;
    scanf("%d", &n);
    edge_heap EH(n * (n + 1) / 2); // All Edge heap.
    edge e;

    for(i = 1; i <= n; i++){
        for(j = 1; j <= n; j++){
            scanf("%d", &w);
            if(j <= i || w < 0) continue;
            e = init(i, j, w);
            EH.insert_heap(e);
        }
    }

    edge_heap TH(n);   // Tree heap.

    union_find VG(n);  // vertex group.

    while(!EH.isEmpty()){
        e = EH.delete_min();
        if(!VG.same(e.v1, e.v2)){
            VG.join(e.v1, e.v2);
            TH.insert_nl(e);
        };
    };

    printf("%d", TH.total_weight());

    return 0;
}

C++: #include <iostream>
#include <utility>
#include <algorithm>
#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++)
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 && W[size] != -1) { 
        A[size] = i;
        B[size] = j;        
        edge[size].first = W[size];
        edge[size].second = size;
        size++;
      }
    }
  }
  // cout << endl;
  rep(i,size) {
    cout << A[edge[i].second] << " " << B[edge[i].second]
         << " " << edge[i].first << endl;;
  }
  // sort by lenght of edge
  sort(edge, edge+size);
  
  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 <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define REP(i,n) for(ll i=0; i<(n); i++)
#define REP2(i,x,n) for(ll i=x; i<(n); i++)
#define REP3(i,x,n) for(ll i=x; i>(n); i--)
#define ALL(n) begin(n),end(n)

#define SENTINEL 2000000000
#define NIL -1
struct cww{cww(){ios::sync_with_stdio(false);cin.tie(0);}}star;
const long long INF = numeric_limits<long long>::max();
static const int MAX = 100;
static const int WHITE = 0;
static const int GRAY = 1;
static const int BLACK = 2;
static const int INFTY = (1<<21);

int n,M[MAX][MAX];

//primのアルゴリズム
int prim(){
    int color[MAX],d[MAX],p[MAX];
    int minv;
    for (int u = 0; u < n; u++)
    {
        color[u] = WHITE;
        d[u] = INFTY;
        p[u] = -1;
    }
    d[0]=0; 
    while(1){
        minv = INFTY;
        int u= -1;
        for (int i = 0; i < n; i++)
        {
            if(color[i]!=BLACK&&d[i]<minv){
                minv = d[i];
                u = i;
            }
        }
        if (u == -1)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]!=-1){
            sum += M[i][p[i]];
        }
    }
    return sum;
}

int main(){
    cin >> n;
    REP(i,n){REP(j,n){
        int e;
        cin >> e;
        M[i][j] = (e ==-1)?INFTY:e;    
        }
    }
    cout << prim() << endl;

    return 0;
}