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 "bits/stdc++.h"

using namespace std;

using ll = long long;
using ld = long double;
using P = pair<string, int>;
constexpr ld EPS = 1e-12;
constexpr int INF = numeric_limits<int>::max() / 2;
constexpr int MOD = 1e9 + 7;

template <typename T>
void printv(const vector<T> &v)
{
    int sz = v.size();
    for (int i = 0; i < sz; i++)
    {
        cout << v[i] << " \n"[i == sz - 1];
    }
}

struct UnionFind
{
    vector<int> data;
    UnionFind(int size) : data(size, -1) {}
    bool unite(int x, int y)
    {
        x = root(x);
        y = root(y);
        if (x != y)
        {
            if (data[y] < data[x])
                swap(x, y);
            data[x] += data[y];
            data[y] = x;
        }
        return x != y;
    }
    bool find(int x, int y)
    {
        return root(x) == root(y);
    }
    int root(int x)
    {
        return data[x] < 0 ? x : data[x] = root(data[x]);
    }
    int size(int x)
    {
        return -data[root(x)];
    }
};

struct Edge
{
    int from, to, cost;
    Edge(int from, int to, int cost) : from(from), to(to), cost(cost) {}
};

bool operator<(const Edge &e, const Edge &f)
{
    return e.cost > f.cost;
}

int main()
{
    cin.tie(0);
    ios::sync_with_stdio(false);

    int n;
    cin >> n;
    priority_queue<Edge> es;
    int res = 0;
    UnionFind uf(n);
    for (int i = 0; i < n; i++)
    {
        for (int j = 0; j < n; j++)
        {
            int a;
            cin >> a;
            if (a == -1)
                continue;
            if (j <= i)
                continue;
            es.push(Edge{i, j, a});
        }
    }
    while (!es.empty())
    {
        Edge e = es.top();
        es.pop();
        if (uf.find(e.from, e.to))
            continue;
        res += e.cost;
        uf.unite(e.from, e.to);
    }
    cout << res << endl;
}

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;
}

C++: #include <iostream>
#include <algorithm>
#include <vector>
#include <deque>
#include <queue>
#include <list>
#include <stack>
#include <string>
#include <tuple>
#include <functional>
#include <numeric>
#include <cmath>
#include <iomanip>
#include <map>
#include <random>
#include <cstdlib>
#define INT_MAX 2147483647
#define INT_MIN -2147483646
#define Loop(i, n) for(int i = 0; i < (int)n; i++)
#define Loop1(i, n) for(int i = 1; i <= (int)n; i++)
#define Loopr(i, n) for(int i = (int)n - 1; i >= 0; i--)
#define Loopr1(i, n) for(int i = (int)n; i >= 1; i--)
using namespace std;
typedef long long int ll;
typedef vector<int> vi;
typedef vector<vector<int>> vvi;

static const int N = 105;

struct vertex {
	int id;
	int p;
	int d;
	bool operator<(const vertex & another) const {
		return (d != another.d ? d>another.d : id>another.id);
	}
};

int main() {
	vvi mx(N, vi(N, -1));
	int n;
	cin >> n;
	Loop(i, n) {
		Loop(j, n) {
			int a;
			cin >> a;
			mx[i][j] = a;
		}
	}
	
	vector<bool> color(N, 0);
	priority_queue<vertex> q;
	q.push({ 0,-1,0});
	int ret = 0;
	while(q.size() > 0) {
		vertex v = q.top();
		q.pop();
		if (color[v.id] == 1) continue;
		color[v.id] = 1;
		ret += v.d;
		Loop(j, n) {
			if (mx[v.id][j] != -1 && color[j] != 1){
				q.push({ j, v.id,mx[v.id][j] });
			}
		}
	}
	cout << ret << endl;
	return 0;
}

C++: #include <iostream>
#include <algorithm>
#include <numeric>
#include <vector>

using namespace std;

template <class T>
struct Edge {
    int from, to;
    T cost;
    Edge(int from, int to, T cost) : from(from), to(to), cost(cost){};

    bool operator<(const Edge<T>& e) const { return this->cost < e.cost; }
    bool operator>(const Edge<T>& e) const { return this->cost > e.cost; }
};

template <class T = int>
class Graph {
public:
    explicit Graph(int N = 0) : size(N) { path.resize(size); }
    void span(int u, int v, T cost = 1) { path[u].push_back(Edge<T>(u, v, cost)); }
    vector<Edge<T>> operator[](int v) const { return path[v]; }

    int size;
    vector<vector<Edge<T>>> path;
};

class UnionFind {
public:
    vector<int> par, num;

    explicit UnionFind(int N) {
        par.resize(N);
        iota(par.begin(), par.end(), 0);
        num.assign(N, 1);
    }

    int find(int v) {
        return (par[v] == v) ? v : (par[v] = find(par[v]));
    }

    void unite(int u, int v) {
        u = find(u), v = find(v);
        if (u == v) return;

        if (num[u] < num[v]) swap(u, v);
        num[u] += num[v];
        par[v] = u;
    }

    bool same(int u, int v) { return find(u) == find(v); }
    bool ispar(int v) { return v == find(v); }
    int size(int v) { return num[find(v)]; }
};

int main() {
    int N;
    cin >> N;
    vector<Edge<int>> edges;
    for (int u = 0; u < N; ++u) {
        for (int v = 0; v < N; ++v) {
            int c;
            cin >> c;
            if (u < v && c >= 0) edges.push_back(Edge<int>(u, v, c));
        }
    }
    sort(edges.begin(), edges.end());

    int ans = 0;
    UnionFind uf(N);
    for (const auto& e : edges) {
        if (uf.same(e.from, e.to)) continue;
        ans += e.cost;
        uf.unite(e.from, e.to);
    }

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