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>
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;
		minv = d[i];
		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 (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;

int main(){
  int n;
  int a[100][100],b[100]={},c[100]={};
  c[0]=1;
  cin>>n;
  for(int i=0;i<n;i++)
   for(int j=0;j<n;j++)cin>>a[i][j];

  int k=1,min=10000,min1=0,ans=0;
  while(1){
   for(int i=0;i<k;i++)
    for(int j=0;j<n;j++)
     if(a[b[i]][j]<min && a[b[i]][j]!=-1 &&c[j]!=1){
      min=a[b[i]][j];
      min1=j;
    }
   if(k==n)break;
   b[k]=min1;
   c[min1]=1
   ans+=min;
   k++;
  }

 cout<<ans<<endl;
 return 0;
}

C++: #include <iostream>
#include <vector>
#include <climits>

using namespace std;

const int kSize = 100001;

vector<int> V;

struct Node {
    int to;
    int weight;

    Node(int _to, int _weight) {
        to = _to;
        weight = _weight;
    }
};

int n;
vector<Node> adj[kSize];
bool isMarked[kSize];
int tree[kSize];
int ans = 0;

pair<int,int> GetMinWeight(int v) {
    int ans = INT_MAX;
    int vertex = -1;
    for (int i = 0; i < adj[v].size(); ++i) {
        int u = adj[v][i].to;
        if (!isMarked[u]) {
            //ans = min(ans, adj[v][i].weight);
            //vertex = u;
            if (adj[v][i].weight <= ans) {
                ans = adj[v][i].weight;
                vertex = u;
            }
        }
    }

    ans = ans == INT_MAX ? -1 : ans;
    return make_pair(ans, vertex);
}

int GetMinVertex() {
    int v = -1;
    int min = INT_MAX;
    for (int i = 0; i < V.size(); ++i) {
        pair<int, int> p = GetMinWeight(V[i]);
        int weight = p.first;
        if (weight != -1 && weight <= min) {
            min = weight;
            v = p.second;
        }
    }

    ans += min;
    return v;
}

int Prime() {
    V.push_back(1);
    isMarked[1] = true;

    while (V.size() < n) {
        int v = GetMinVertex();
        //cout << v << endl;
        V.push_back(v);
        isMarked[v] = true;
    }
}

int main() {
    fill(isMarked, isMarked + kSize, false);
    cin >> n;
    for (int i = 1; i <= n; ++i) {
        for (int j = 1; j <= n; ++j) {
            int weight;
            cin >> weight;
            if (weight != -1) {
                adj[i].emplace_back(j, weight);
            }
        }
    }

    Prime();
    cout << ans << endl;
}

C++: #include<stdio.h>
//#include<cstdlib>
// 0origin

#define MAX 100
#define INF 2001
#define WHITE 0
#define BLACK 1

int n;

// calculate the minimum cost
int PrimMST(int a[MAX][MAX]){ //refered to http://www.deqnotes.net/acmicpc/prim/#a1
    int v,i,j = 0, color[MAX], mincost, sumcost = 0, connected;
    
    for (i = 0; i < n; i++) {
        color[i] = WHITE;
    }
    
    color[0] = BLACK;
    while (j < n-1){
        
        mincost = INF;
        connected = INF;
        for (v = 0; v < n; v++) {
            if(color[v] == WHITE){
                continue;
            }
            for (i = 0; i < n; i++) {
                //printf("color[%d] : %s, cost %d to %d : %d\n", i, color[i] == 1 ? "BLACK" : "WHITE", v, i, a[v][i]);
                if (color[i] == WHITE && mincost > a[v][i]) {
                    mincost = a[v][i];
                    connected = i;
                    //printf("  min is rewritten : %d\n", mincost);
                }
            }
        
        }
        //printf("\nfin:%d\n\n", mincost);
        sumcost += mincost;
        color[v] = BLACK;
        if (connected != INF) {
            color[connected] = BLACK;
        }
        
        j++;
        
    }
    return sumcost;
}

int main(){
    int i, j, a[MAX][MAX];
    
    scanf("%d", &n);
    
    //printf("\n\n\n");
    for(i = 0; i < n; i++){
        for(j = 0; j < n; j++){
            scanf("%d", &a[i][j]);
            if(a[i][j] == -1){
                a[i][j] = INF;
            }
            //printf(" %4d", a[i][j] != INF ? a[i][j] : -1);
        }
        //printf("\n");
    }
    
    printf("%d\n", PrimMST(a));
    
    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;
}