Go: package main

import (
	"bufio"
	"fmt"
	"log"
	"math"
	"os"
	"strconv"
	"strings"
)

type color int

const (
	white color = iota // 未訪問
	mst                // MSTとして確定
)

// 頂点
type node struct {
	matrix   []int // 隣接行列
	minToMst int   // MSTへの重み最小値
	parent   int   // 最小値の相手
	color    color
}

func main() {
	scanner := bufio.NewScanner(os.Stdin)
	n := eGetInt(scanner)

	nodes := make([]node, n)
	for i := 0; i < n; i++ {
		nodes[i].matrix = eAtoi((strings.Fields(eGetline(scanner))))
		nodes[i].parent = -1
		nodes[i].minToMst = math.MaxInt16
		nodes[i].color = white
	}

	// 初回はnode[0]がMSTに入るように,minToMstを設定
	nodes[0].minToMst = 0
	for { // 全部チェックするまで

		// どの頂点を新たにMSTに取り込むか
		min := math.MaxInt16
		u := -1
		for i := 0; i < n; i++ {
			if nodes[i].color == mst {
				continue
			}
			if nodes[i].minToMst < min {
				u = i
				min = nodes[i].minToMst
			}
		}

		if u == -1 {
			break
		}
		// MSTに追加する頂点決定
		nodes[u].color = mst

		// MSTに属していない頂点それぞれについて,MSTとの最小の重みを更新
		for i := 0; i < n; i++ {
			if nodes[i].color == mst {
				continue
			}
			if nodes[i].matrix[u] == -1 {
				// 接してない
				continue
			}
			if nodes[i].matrix[u] < nodes[i].minToMst {
				// node[u]との重みが最小
				nodes[i].minToMst = nodes[i].matrix[u]
				nodes[i].parent = u
			}
		}
	}

	sum := 0
	for _, v := range nodes {
		// node[0]はparentが入っていない
		if v.parent != -1 {
			sum += v.matrix[v.parent]
		}
	}
	fmt.Println(sum)
}

func eGetInt(scanner *bufio.Scanner) int {
	t := eAtoi(strings.Fields(eGetline(scanner)))
	return t[0]
}

func eGetline(p *bufio.Scanner) string {
	if !p.Scan() {
		if err := p.Err(); err != nil {
			log.Fatal(err.Error())
		}
		return ""
	}
	return p.Text()
}

func eAtoi(s []string) []int {
	ints := make([]int, len(s))
	var err error
	for i, e := range s {
		ints[i], err = strconv.Atoi(e)
		if err != nil {
			log.Fatal(err.Error())
		}
	}
	return ints
}

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;
}