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>
using namespace std;
#define INFTY (1 << 30)

enum Color {
	WHITE,
	GRAY,
	BLACK,
};

void Prim(int verValWeight[][100], int verNum) {
	int distance[100];
	Color color[100];
	int parent[100];
	for (int i = 0; i < verNum; i++) {
		color[i] = WHITE;
		distance[i] = INFTY;
		parent[i] = -1;
	}
	distance[0] = 0;

	while (true) {
		int min_cost = INFTY;
		int subScript;
		for (int i = 0; i < verNum; i++) {
			if (color[i] != BLACK && distance[i] < min_cost) {
				min_cost = distance[i];
				subScript = i;
			}

			if (min_cost == INFTY)
				break; //end

			color[subScript] = BLACK;

			for (int i = 0; i < verNum; i++) {
				if (color[i] != BLACK && verValWeight[subScript][i] != INFTY) {
					if (verValWeight[subScript][i] < distance[i]) {
						distance[i] = verValWeight[subScript][i];
						parent[i] = subScript;
						color[i] = GRAY;
					}
				}
			}
		}
	}

	int weight_sum = 0;
	for (int i = 0; i < verNum; i++)
		if (parent[i] != -1)
			weight_sum += verValWeight[i][parent[i]];
	cout << weight_sum << endl;

}

int main() {
	int verNum;
	int verValWeight[100][100];
	int parent[100];

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

	Prim(verValWeight, verNum);

	return 0;
}

C++: #include <iostream>
//#include <string>
#include <algorithm>
using namespace std;
//#define int long long
//#define _DEBUG
class UnionFindTree{
    private:
    int P[10010];
    public:
    UnionFindTree(int N){
        for(int i=0; i<=N; i++) P[i] = i;
    }
    int root(int a){
        if (P[a] == a) return a;
        return (P[a] = root(P[a]));
    }
    bool is_same_set(int a, int b){
        return root(a) == root (b);
    }
    void unite(int a, int b){
        P[root(a)] = root(b);
    }
};
struct weight_edge{// (weight, (vert_a,vert_b))
    int w;
    int a;
    int b;
};
int n, k;
weight_edge we[10007];
bool com_we(weight_edge a, weight_edge b){
    return ((a.w == b.w)? a.a < b.a: a.w < b.w);
}
int solve(){
    if(n==0)return 0;
    sort(we, we+k, com_we);
    //for(int i=0; i<k; i++)printf("%d,%d,%d\n",we[i].w,we[i].a,we[i].b);
    auto uft = UnionFindTree(n);
    int i=0, r=0, l=0;
    while(l<n-1){
        if(!uft.is_same_set(we[i].a, we[i].b)){
            uft.unite(we[i].a, we[i].b);
            r += we[i].w;
            l++;
            //printf("%d;\tw:%d,\t(%d,%d)\n",l,we[i].w,we[i].a,we[i].b);
        }
        i++;
    }
    return r;
}
void get(){
    k = 0;
    for(int i=0; i<n; i++){
        for(int j=0; j<n; j++){
            int t;
            cin >> t;
            if(i<j && t>=0){
                we[k].w = t;
                we[k].a = i; we[k].b = j;
                k++;
            }
            //cout << t<<":"<<i<<","<<j<<"\t";
        }
        //cout << endl;
    }
    //cout << "k"<<k<<endl;
}
signed main(){    
    while(cin >> n){
        get();
        cout << solve() <<  endl;
    }
}

C++: #define _CRT_SECURE_NO_WARNINGS
#define _USE_MATH_DEFINES
//#include "MyMath.h"
//#include "MyDisjointset.h"
#include <iostream>
#include <vector>
#include <algorithm>
#include <queue>
#include <functional>
#include <stdio.h>
using namespace std;
typedef pair<int, int> P;
const int INF = 2000000000;
class Disjointset {
public:
	vector<int> rank, p;
	Disjointset(int n);
	int find(int x);
	bool same(int x, int y);
	void unite(int x, int y);
};

Disjointset::Disjointset(int n) {
	rank.resize(n, 0);
	for (int i = 0; i < n; i++)
		p.push_back(i);
}
int Disjointset::find(int x) {
	if (x != p[x])
		return p[x] = find(p[x]);
	else return x;
}
bool Disjointset::same(int x, int y) {
	return find(x) == find(y);
}
void Disjointset::unite(int x, int y) {
	int px = find(x);
	int py = find(y);
	if (rank[px] > rank[py]) {
		p[py] = px;
	}
	else {
		p[px] = py;
		if (rank[px] == rank[py]) rank[py]++;
	}
}
struct edge {
	int from, to, cost;
	bool operator < (const edge &e) const {
		return cost > e.cost;
	}
};
int main() {
	int n; cin >> n;
	priority_queue <edge> q;
	for (int i = 0; i < n; i++)for (int j = 0; j < n; j++) {
		int c; cin >> c;
		if (c != -1) { edge e; e.from = i; e.to = j; e.cost = c; q.push(e); }
	}
	Disjointset D(n);
	int res = 0;
	while (!q.empty()) {
		edge e = q.top(); q.pop();
		int f = e.from, t = e.to, c = e.cost;
		if (D.same(f, t)) continue;
		res += c;
		D.unite(f, t);
	}
	cout << res << endl;
	return 0;
}