Go: package main

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

func getScanner(fp *os.File) *bufio.Scanner {
	scanner := bufio.NewScanner(fp)
	scanner.Split(bufio.ScanWords)
	scanner.Buffer(make([]byte, 500001), 500000)
	return scanner
}

func getNextString(scanner *bufio.Scanner) string {
	scanner.Scan()
	return scanner.Text()
}

func getNextInt(scanner *bufio.Scanner) int {
	i, _ := strconv.Atoi(getNextString(scanner))
	return i
}

func main() {
	fp := os.Stdin
	if len(os.Args) > 1 {
		fp, _ = os.Open(os.Args[1])
	}

	scanner := getScanner(fp)
	writer := bufio.NewWriter(os.Stdout)

	n := getNextInt(scanner)

	g := make([][]int, n)
	ans := make([][]int, n)

	inf := 1 << 30
	for i := 0; i < n; i++ {
		g[i] = make([]int, n)
		ans[i] = make([]int, n)
		for ii := 0; ii < n; ii++ {
			g[i][ii] = getNextInt(scanner)
			if g[i][ii] == -1 {
				g[i][ii] = inf
			}
			ans[i][ii] = -1
		}
	}

	im := make(IntMap, 1)

	im.increment(1)

	for len(im) < n {
		min := inf
		u := -1
		v := -1
		for i := range im {
			for ii := 0; ii < n; ii++ {
				if im.get(ii) != 0 {
					continue
				}
				if min > g[i][ii] {
					min = g[i][ii]
					u = i
					v = ii
				}
			}
		}
		im.increment(v)
		ans[u][v] = min
		ans[v][u] = min
	}

	sum := 0
	for i := 0; i < n-1; i++ {
		for ii := i + 1; ii < n; ii++ {
			if ans[i][ii] != -1 {
				sum += ans[i][ii]
			}
		}
	}

	fmt.Fprintln(writer, sum)
	writer.Flush()
}

// IntMap ...
type IntMap map[int]int

func (m IntMap) get(key int) int {
	if _, ok := m[key]; ok {
		return m[key]
	}
	return 0
}
func (m IntMap) increment(key int) {
	if _, ok := m[key]; ok == false {
		m[key] = 0
	}
	m[key]++
}

C++: #include <iostream>
#include <queue>

using namespace std;




struct link { //link connecting x and y vertices
  int x;
  int y;
  int weight;
  bool operator<(const link& other) const
  {
    return weight > other.weight; // for min priority queue, trick the operator
  }
};

priority_queue<link> links;
int roots[100];

int root(int x) {
  if (x == roots[x]) {
    return x;
  } else {
    //roots[x] = root(roots[x]); //attach all to root
    return root(roots[x]);
  }
}

bool same(int x, int y) {
  return (root(x) == root(y));
}

void unite(int x, int y) {
  roots[root(x)] = root(y);
}


  
int main() {
  int min_cost = 0;
  int n;
  cin >> n;


  for (int i = 0; i < n; i++) {
    roots[i] = i; // initialize roots
    for (int j = 0; j < n; j++) {
      
      int weight;
      cin >> weight;
      if (j <= i ) continue;
      if (weight != -1) {
	link new_link = {i, j, weight};
	links.push(new_link);
	//cout << "link added has i j weight: " << i << " " << j << " " << weight << endl;
      }
    }
  }

  while (!links.empty()) {
    link cur_link = links.top();
    //cout << "cur link weight: " << cur_link.weight << " x: " << cur_link.x << " y: " << cur_link.y << endl;
    if (!same(cur_link.x, cur_link.y)) {
      min_cost += cur_link.weight;
      unite(cur_link.x, cur_link.y);
    }
    links.pop();
  }

  cout << min_cost << endl;
      
    
}

C++: #include <iostream>

using namespace std;

static const int INFTY = 10000000;

int a[124][124] = {{}};
int n;


int prim(){

  int d[124] = {};
  int p[124] = {};
  int flag[124] = {};
  int min = INFTY;
  int u;
  int sum = 0;

  for(int i = 0;i < n;i++){
    d[i] = INFTY;
    p[i] = -1;
  }

  d[0] = 0;
  while(1){
    u = -1;
    min = INFTY;
    for(int i = 0;i < n;i++){
      if(flag[i] == 0 && d[i] <= min){
        u = i;
        min = d[i];
      }
    }
    //cout << "u = " << u << endl;
    //cout << "min = " << min << endl;

    if(u == -1) break;
    sum += min;
    flag[u] = 1;

    for(int i = 0;i < n;i++){
      if(flag[i] == 0 && a[u][i] != INFTY){
        if(d[i] > a[u][i]){
          d[i] = a[u][i];
          p[i] = u;
        }
      }
    }
  }
  //for(int i = 0;i < n;i++) if(a[p[i]][i] != INFTY)sum += a[p[i]][i];
  return sum;
}

int main(){

  cin >> n;

  for(int i = 0;i < n;i++){
    for(int j = 0;j < n;j++){
      cin >> a[i][j];
      if(a[i][j] == -1) a[i][j] = INFTY;
    }
  }

  cout << prim() << endl;
}

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 <iostream>
using namespace std;
static const int MAX = 100;
static const int INFTY = (1<<21);
static const int WHITH = 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] = WHITH;
  }

  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 <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;
}