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 <utility>
#include <string>

using namespace std;


int P[10010]; // 0 ?????? 10000 ?????§????????????????????±?????????
void init(int N) { // ????????? ??????????????¨???????????????????????????
  for (int i=0; i<=N; ++i) P[i] = i;
  // ??¨: ????????? [0,N-1] ????????? [1,N] ?????????????????????
  // ???????????§???????????§???????????????????????§??? [0,N] ???????????¨??????
}
int root(int a) { // a ??? root(??£??¨???) ????±???????
  if (P[a] == a) return a; // a ??? root
  return (P[a] = root(P[a])); // a ????????? root ????±???????a ????????¨??????
}
bool is_same_set(int a, int b) { // a ??¨ b ???????????°?????????????±???????????
  return root(a) == root(b);
}
void unite(int a, int b) { // a ??¨ b ???????????°?????????????????¨??????
  P[root(a)] = root(b);
}


int N=0;
int A[100+10][100+10] = {0};

int cost[10100] = {0};
int src[10100] = {0};
int dst[10100] = {0};

pair<int,int> c_n[10100];



int main() {
  cin >> N;
  init(N+10);
  
  int k=0;
  for(int j=0;j<N;++j){
    for(int i=0;i<N;++i){
      cin >> A[i][j];
      //cout << A[i][j] << "|";
      if (A[i][j] != -1){
	cost[k] = A[i][j];
	src[k] = i;
	dst[k] = j;
	c_n[k] = make_pair(cost[k],k);
	++k;
      }
    }
    //cout << endl;
  }

  
  /*
  for (int i=0;i<20;++i){
    cout << c_n[i].first << " " << c_n[i].second << endl;
  }

  cout << endl << "SORT" << endl;

  
  for (int i=0;i<20;++i){
    cout << c_n[i].first << " " << c_n[i].second << endl;
  }*/

  sort(c_n, c_n+k);
  /*
  c_n[0].second
  ???????????????
  is_same_set
  ??????
  unite(src[],dst[])
?????????
  c_n[1].second????????????k???????????§????????°??????
  */

  int num = 0;
  int sum = 0;
  for(int i=0;i<k;++i){
    num = c_n[i].second;
    if(is_same_set(src[num], dst[num]) == 0){
      unite(src[num], dst[num]);
      sum += cost[num];
      //cout << num << " " << sum << endl;
    }
  }
  cout << sum << endl;
}

C++: #include <iostream>
using namespace std;
#define INF 1000000

int main(){
  int n;
  int G[100][100],d[100];
  int minc,mi,i;
  bool flg[i];
  int sum=0;
  
  cin >> n;
  for (i=0;i<n;i++){
    for (int j=0;j<n;j++){
      cin >> G[i][j];
      if (G[i][j]==-1) G[i][j]=INF;
    }
    d[i]=INF;
    flg[i]=false;
  }

  d[0]=0;
  while (true){
    minc=INF;
    for (i=0;i<n;i++){
      if (!flg[i]&&d[i]<minc){
	minc=d[i];
	mi=i;
      }
    }
    if (minc==INF) break;
    flg[mi]=true;
    for (i=0;i<n;i++){
      if (G[mi][i]==INF)continue;
      if (!flg[i]&&d[i]>G[mi][i]){
	d[i]=G[mi][i];
	//cout << d[i];
      }
    }
  }
  
  for (i=0;i<n;i++){
    sum+=d[i];
  }
  cout << sum<<endl;
  
  return 0;
}

C++: #include<iostream>
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;
	}

	d[0] = 0;

	while(1){
		minv = INFTY;
		u = -1;
		for(int 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(int v=0;v<n;v++){
			if(color[v] != BLACK && M[u][v] !=INFTY){
				if(d[v] > M[u][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;

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

	cout << prim() << endl;

	return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;

class union_find {
  vector<int> uf;

public:
  union_find(int sz)
  {
    uf = vector<int>(sz);
    for (int i = 0; i < sz; i++)
      uf[i] = i;
  }
  
  int root(int i)
  {
    if (uf[i] == i) return i;
    return uf[i] = root(uf[i]);
  }

  void unite(int i, int j)
  {
    int ri = root(i);
    int rj = root(j);
    if (ri != rj)
      uf[rj] = ri;
  }

  bool same(int i, int j)
  {
    return root(i) == root(j);
  }
};

typedef pair<int, int> edge;
typedef pair<edge, int> V;
priority_queue<V, vector<V>, function<bool(V, V)>> que(
  [](V a, V b) { return a.second >= b.second; }
);

#define make_V(i, j, w) make_pair(make_pair((i), (j)), (w))

int solve(int n)
{
  union_find uf(n);

  int ans = 0;
  while (!que.empty()){
    int i, j, w;
    V v = que.top(); que.pop();
    i = v.first.first, j = v.first.second;
    w = v.second;

    if (uf.same(i, j)) continue;
    uf.unite(i, j);
    ans += w;

    // printf("(%d, %d) %d\n", v.first.first, v.first.second, v.second);
  }

  return ans;
}

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

  for (int i = 0; i < n; i++) {
    for (int j = 0; j < n; j++) {
      int weight; cin >> weight;
      if (j > i && weight != -1)
        que.push(make_V(i, j, weight));
    }
  }

  cout << solve(n) << endl;

  return EXIT_SUCCESS;
}