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 <stack>
#include <string>
#include <vector>
using namespace std;
static const int Max = 100;
static const int Infinit = 1000000; // ??????
static const int Nil = -1; // 

enum Status {
	Not_Searched = 0,
	Searched = 1,
	Mts = 2 
};

int n;
int M[Max][Max] = {};
Status st[Max] = {}; 
int min_cost[Max] = {}; // i ??????????????????????????§???????°??????????
int parent[Max] = {}; // ??????index

int prim_search() {
	for (int i = 0; i < n; i++) {
		min_cost[i] = Infinit;
		parent[i] = -1;
		st[i] = Not_Searched;
	}
	min_cost[0] = 0;

	while (1) {
		int mincost = Infinit;
		int mincost_idx = -1;

		for (int i = 0; i < n; i++) {
			if (st[i] != Mts && min_cost[i] < mincost) {
				mincost = min_cost[i];
				mincost_idx = i;
			}
		}
		if (mincost_idx == -1) {
			break;
		}
		st[mincost_idx] = Mts;
		for (int i = 0; i < n; i++) {
			if (st[i] != Mts && M[mincost_idx][i] != Infinit) {
				if (min_cost[i] > M[mincost_idx][i]) {
					min_cost[i] = M[mincost_idx][i];
					parent[i] = mincost_idx;
					st[i] = Searched;
				}
			}
		}
	}
	int sum = 0;
	for (int i = 0; i < n; i++) {
		if (parent[i] != Nil) sum += M[i][parent[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) ? Infinit : e;
		}
	}

	cout << prim_search() << endl;
	return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int (i)=0;(i)<(n);(i)++)
#define WHITE 0
#define GRAY 1
#define BLACK 2
#define INF 2100

int main() {
  int n; cin >> n;
  vector<vector<int>> G(n, vector<int>(n));
  rep(i, n)rep(j, n) cin >> G.at(i).at(j);

  vector<int> color(n); vector<int> mindis(n);
  rep(i, n) mindis.at(i) = INF;
  vector<int> par(n);
  int ans = 0;
  mindis.at(0) = 0;
  color.at(0) = GRAY;
  par.at(0) = -1;

  while (true) {
    int m = INF;
    int now = 0;
    rep(i, n) {
      if (color.at(i) == GRAY && mindis.at(i) < m) {
        m = mindis.at(i);
        now = i;
      }
    }

    if (m == INF) break;

    color.at(now) = BLACK;
    if (par.at(now) != -1) {
      ans += G.at(par.at(now)).at(now);
    }

    rep(i, n) {
      if (G.at(now).at(i) < mindis.at(i) && G.at(now).at(i) != -1 && color.at(i) != BLACK) {
        mindis.at(i) = G.at(now).at(i);
        color.at(i) = GRAY;
        par.at(i) = now;
      }
    }
  }
  cout << ans << endl;
}

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

const int INF = 1000000000;

int main() {
    int n;
    cin >> n;
    vector< vector<int> > cost(n, vector<int>(n, INF));
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < n; j++) {
            int a;
            cin >> a;
            cost[i][j] = (a == -1) ? INF : a;
        }
    }

    vector<int> mincost(n, INF);
    vector<bool> used(n, false);
    mincost[0] = 0;

    int res = 0;
    while (true) {
        int v = -1;
        for (int u = 0; u < n; u++) {
            if (!used[u] && (v == -1 || mincost[u] < mincost[v])) {
                v = u;
            }
        }
        if (v == -1) break;
        used[v] = true;
        res += mincost[v];

        for (int u = 0; u < n; u++) {
            mincost[u] = min(mincost[u], cost[u][v]);
        }
    }

    cout << res << endl;
    return 0;
}

C++: #include <iostream>
#include <utility>
#include <algorithm>
#define MAX 3000
#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++;
      }
    }
  }
  // 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) {
    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;
    }
  }
  cout << sum << endl;
  return 0;
}

C++: #include<bits/stdc++.h>
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,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){
    minv=INF; 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]!=INF){
        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;
      if(e==-1)M[i][j]=INF;
      else M[i][j]=e;
    }
  }
  cout << prim() << endl;
}