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;
static const int mx=107;
static const int inf=(1<<21);

int n,m[mx][mx];

int prim()
{
	int u,minv;
	int d[mx],p[mx],color[mx];
	
	for(int i=0;i<n;i++)
	{
		d[i]=inf;
		p[i]=-1;
		color[i]=0;
	}
	d[0]=0;
	
	while (1) {
		minv=inf;
		u=-1;
		for(int i=0;i<n;i++)
		{
			if(minv>d[i]&&color[i]!=2)
			{
				u=i;
				minv=d[i];
			}
		}
		if(u==-1)break;
		color[u]=2;
		for(int v=0;v<n;v++)
		{
			if(color[v]!=2&&m[u][v]!=inf)
			{
				if(d[v]>m[u][v])
				{
					d[v]=m[u][v];
					p[v]=u;
					color[v]=1;
				}
			}
		}
	}
	int sum=0;
	for(int i=0;i<n;i++)
	{
		if(p[i]!=-1)sum+=m[i][p[i]];
	}
	return sum;
}
int main(int argc, char *argv[]) {
	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)?inf:e;
		}
	}
	cout<<prim()<<endl;
	return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
#define rep(i,n)for(int i=0;i<(n);i++)
using ll = long long;
using P = pair<int, int>;

const int INF = 1001001001;
vector<vector<int>> cost;
vector<int> mincost;
vector<bool> used;
int V;

int prim(){
    mincost.assign(V,INF);
    used.assign(V,0);
    mincost[0] = 0;
    int res = 0;
    while(true){
        int v = -1;
        for(int u = 0; u < V; ++u){
            if(!used[u] && (v == -1 || mincost[u] < mincost[v])) v = u;
        }
        if(v == -1) break;//completed
        used[v] = 1;
        res += mincost[v];
        for(int u = 0; u < V; ++u){
            if(cost[v][u] != -1)mincost[u] = min(mincost[u], cost[v][u]);//確定した頂点から行ける頂点について、最小コストを更新
        }
    }
    return res;
}


int main(){
    cin >> V;
    cost.assign(V,vector<int>(V));
    rep(i,V)rep(j,V) cin >> cost[i][j];
    int ans = prim();
    cout << ans << endl;
}

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

using namespace std;

class UnionFindTree {
private:
    vector<int32_t> parent, rank;

    int32_t find(int32_t x) {
        if (parent[x] == x) {
            return x;
        } else {
            int32_t p = find(parent[x]);
            parent[x] = p;
            return p;
        }
    }

public:
    UnionFindTree(int32_t n) {
        parent.resize(n);
        rank.resize(n);

        for (int32_t i = 0; i < n; i++) {
            parent[i] = i;
            rank[i] = 0;
        }
    }

    void unite(int32_t x, int32_t y) {
        x = find(x);
        y = find(y);

        if (x == y) return;

        if (rank[x] < rank[y]) {
            parent[x] = y;
        } else {
            parent[y] = x;
            if (rank[x] == rank[y]) rank[x]++;
        }
    }

    bool same(int32_t x, int32_t y) {
        return find(x) == find(y);
    }
};

int main() {
    int32_t n;
    cin >> n;

    UnionFindTree uf(n);
    priority_queue<tuple<int32_t, int32_t, int32_t>, vector<tuple<int32_t, int32_t, int32_t>>, greater<tuple<int32_t, int32_t, int32_t>>> E;

    for (int32_t i = 0; i < n; i++) {
        for (int32_t j = 0; j < n; j++) {
            int32_t w;
            cin >> w;

            if (w > -1) {
                E.push(make_tuple(w, i, j));
            }
        }
    }

    int32_t sum = 0;
    while (!E.empty()) {
        int32_t w, u, v;
        tie(w, u, v) = E.top();
        E.pop();
        if (!uf.same(u, v)) {
            sum += w;
            uf.unite(u, v);
        }
    }

    cout << sum << endl;

    return 0;
}