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 <bits/stdc++.h>
#define rep(i,a) for(i=0;i<a;i++)
using namespace std;
static const int W = 0;
static const int G = 1;
static const int B = 2;

void prim(int n, int &total,vector<vector<int> > &M) {
    int i,u,min;
    vector<int> d(n);
    vector<int> p(n);
    vector<int> color(n);

    rep(i,n){
        d[i] = INT_MAX;
        p[i] = -1;
        color[i] = W;
    }
    d[0] = 0;
    while(true){
        min = INT_MAX;
        u = -1;
        rep(i,n){
            if(min > d[i] && color[i] != B) {
                u = i;
                min = d[i];
            }
        }
        if(u==-1) break;
        color[u] = B;
        rep(i,n){
            if(color[i] != B && M[u][i] != INT_MAX){
                if(d[i] > M[u][i]) {
                    d[i] = M[u][i];
                    p[i] = u;
                    color[i] = G;
                }
            }
        }
    }
    rep(i,n){
        if(p[i] != -1) total += M[i][p[i]];
    }
}

int main(void) {
    int i,j,n,total = 0;
    vector<vector<int> > M;
    cin >> n;
    M.resize(n);

    rep(i,n){
        M[i].resize(n);
    }
    rep(i,n){
        rep(j,n){
            cin >> M[i][j];
            if(M[i][j] == -1) M[i][j] = INT_MAX;
        }
    }
    prim(n,total,M);
    cout << total << 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; // u??????
    int d[MAX], p[MAX], color[MAX]; // d:??¢?´¢??????????±?, p:MST??????????????????v??????

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

    d[0] = 0; // MST?????¢?´¢????§?????????????????????°???

    while (1){
	    minv = INFTY;
	    u = -1;
	    for(int i = 0; i < n; i++){
		    // ??¢?´¢????±?????????§??????????°??????????????????§??????????????¢?´¢???????????????????????????	
		    if(minv > d[i] && color[i] != BLACK){
			    u = i; // MST?????????????????????????±???????
			    minv = d[i];
		    }
	    }
	    if(u == -1) break; //??¨??????????±??????¢?´¢????????¨???????????¢?´¢??????????????? 
	    color[u] = BLACK; // ??¢?´¢???????????????
	    for(int v = 0; v < n; v++){
		   //?????¢?´¢????????§?????????u??¨?????????????????£???????????????
		   //  ?¬??????¢?´¢????±???¨????????????????????????????????????
		   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]]; // MST???????????????????????¨???
    }

    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 WHITE=0;
static const int BLACK=2;
static const int GRAY=1;
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(void){
    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 <algorithm>
#include <climits>
#include <vector>
#include <stack>
#include <queue>
#include <set>
#include <map>
using namespace std;

int n;
vector<int> g[101];
vector<int> d(101);
vector<int> p(101);
vector<int> vis(101);

void prim(){
    for (int i = 0; i < n; ++i) {
        vis[i] = 0;
        d[i] = INT_MAX;
        p[i] = -1;
    }

    d[0] = 0;
    while(true) {
        int mincost = INT_MAX;
        int next = -1;
        for (int i = 0; i < n; ++i) {
            if(vis[i] == 0 && d[i] < mincost) {
                mincost = d[i];
                next = i;
            }
        }
        if(next == -1) break;
        vis[next] = 1;

        for (int j = 0; j < n; ++j) {
            if(g[next][j] < d[j] && vis[j] != 1 && g[next][j] != -1) {
                p[j] = next;
                d[j] = g[next][j];
            }
        }
    }
}

int main() {
    cin >> n;
    for (int i = 0; i < n; ++i) {
        for (int j = 0; j < n; ++j) {
            int v;
            cin >> v;
            g[i].push_back(v);
        }
    }

    prim();

    long cst=0;
    for (int k = 0; k < n; ++k) {
        cst += d[k];
    }
    cout << cst << endl;

    return 0;
}