Go: package main

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

const (
	MAX = 100
	INF = 1 << 21
	WHITE = 0
	GRAY = 1
	BLACK = 2
)

var (
	n int
	M [MAX][MAX]int
)

func prim() int {
	var u, minv int
	var d [MAX]int
	var p [MAX]int
	var color [MAX]int

	for i := 0; i < n; i++ {
		d[i] = INF
		p[i] = -1
		color[i] = WHITE
	}
	d[0] = 0
	for {
		minv = INF
		u = -1
		for 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 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
				}
			}
		}
	}
	sum := 0
	for i := 0; i < n; i++ {
		if p[i] != -1 {
			sum += M[i][p[i]]
		}
	}
	return sum
}

func main() {
	scanner := bufio.NewScanner(os.Stdin)
	scanner.Split(bufio.ScanWords)
	scanner.Scan()
	n, _ = strconv.Atoi(scanner.Text())
	for i := 0; i < n; i++ {
		for j := 0; j < n; j++ {
			scanner.Scan()
			e, _ := strconv.Atoi(scanner.Text())
			if e == -1 {
				M[i][j] = INF
			} else {
				M[i][j] = e
			}
		}
	}
	fmt.Println(prim())
}

C++: #include <iostream>
#include <cstring>
using namespace std;
#define int long long

main(){
  int n;
  cin>>n;
  int a[n][n];
  for(int i=0;i<n;i++)
    for(int j=0;j<n;j++)
      cin>>a[i][j];
  int ans=0;
  bool used[n];
  memset(used,0,sizeof(used));
  used[0]=1;
  int cost[n];
  for(int i=0;i<n;i++) cost[i]=a[0][i];
  while(1){
    int v=-1;
    for(int i=0;i<n;i++){
      if(used[i]||cost[i]<0) continue;
      if(v<0||cost[i]<cost[v]) v=i;
    }
    if(v<0) break;
    used[v]=1;
    ans+=cost[v];
    for(int i=0;i<n;i++){
      if(a[v][i]<0) continue;
      if(cost[i]<0||a[v][i]<cost[i]) cost[i]=a[v][i];
    }
  }
  cout<<ans<<endl;
}

C++: #include<iostream>
#include<list>
#include<vector>
const int INF = 2001;
using namespace std;
list<int> L;
bool ok(int i) {
    for (list<int>::iterator it = L.begin(); it != L.end(); it++) if (i == *it) return 0;
    return 1;
}
int main() {
    L.push_back(0);
    int n; cin >> n;
    int M[100][100];
    int tmp = INF;
    int id = 0;
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < n; j++) {
            cin >> M[i][j];
            if (i == 0 && M[i][j] != -1) {
                if (M[i][j] < tmp) {
                    tmp = M[i][j];
                    id = j;
                }
            }
        }
    }
    L.push_back(id);
    int ans = tmp;
    tmp = INF;
    int OK[100];
    while (1) {
        tmp = INF;
        for (int i = 0; i < n; i++) OK[i] = ok(i); //reached?
        for (list<int>::iterator it = L.begin(); it != L.end(); it++ ) { //from reached
            for (int i = 0; i < n; i++) {
                if (M[*it][i] < tmp && M[*it][i] != -1 && OK[i]) {
                    tmp = M[*it][i];
                    id = i;
                }
            }
        }
        if (tmp == INF) break;
        ans += tmp;
        L.push_back(id);
    }
    cout << ans << endl;
    return 0;
    
}