Go: package main

import (
	"bufio"
	"fmt"
	"io"
	"os"
	"strconv"
)

const (
	MAX   = 100
	INFTY = 1 << 21
	NIL   = -1
)

const (
	WHITE = iota
	GRAY
	BLACK
)

type Graph [MAX][MAX]int

func (g *Graph) prim(n int) int {
	var (
		u, minv     = 0, 0
		d, p, color [MAX]int
	)

	for i := 0; i < n; i++ {
		d[i] = INFTY
		p[i] = -1
		color[i] = WHITE
	}
	d[0] = 0

	for {
		minv = INFTY
		u = -1
		for 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 v := 0; v < n; v++ {
			if color[v] != BLACK && g[u][v] != INFTY {
				if d[v] > g[u][v] {
					d[v] = g[u][v]
					p[v] = u
					color[v] = GRAY
				}
			}
		}
	}

	sum := 0
	for i := 0; i < n; i++ {
		if p[i] != -1 {
			sum += g[i][p[i]]
		}
	}

	return sum
}

func answer(reader io.Reader) string {
	sc := bufio.NewScanner(reader)
	sc.Split(bufio.ScanWords)

	sc.Scan()
	n, _ := strconv.Atoi(sc.Text())

	var g Graph
	for i := 0; i < n; i++ {
		for j := 0; j < n; j++ {
			sc.Scan()
			e, _ := strconv.Atoi(sc.Text())
			if e == -1 {
				g[i][j] = INFTY
				continue
			}
			g[i][j] = e
		}
	}

	return fmt.Sprint(g.prim(n))
}

func main() {
	fmt.Println(answer(os.Stdin))
}

C++: #include<iostream>
#include<algorithm>
#include<vector>
#include<functional>
using namespace std;

#if 0
所用時間:

感想:

#endif

int N, buff, M = 0, SUM = 0, A[101][101], cost[5000], dpt[5000], dst[5000], P[100001]; //A[Y][X]
vector<pair<int, int> > edges;

void init(int N){ //木構造の初期化
  for(int i = 0; i <= N; ++i){
    P[i] = i;
  }
}

int root(int a){ //根の参照
  if(P[a] == a) return a;
  return (P[a] = root(P[a]));
}

bool is_same_set(int a, int b){ //同じ根か?
  return root(a) == root(b);
}

void unite(int a, int b){
  P[root(a)] = root(b); //aのrootをbの子にする
}

void preprocess(){//辺の前処理
  for(int r = 0; r < N; ++r){
    for(int c = r+1; c < N; ++c){
      if (A[r][c] > -1) {
        cost[M] = A[r][c];
        dpt[M] = r;
        dst[M] = c;
        M++;
      }
    }
  }

  //for(int i = 0; i < M; ++i) cout << cost[i] << endl;

  for(int i = 0; i < M; ++i){
    edges.push_back(make_pair(cost[i], i));
  }
  // for(int i = 0; i < M; ++i) cout << edges[i].first << " " << edges[i].second << endl;

  sort(edges.begin(), edges.end());

  // for(int i = 0; i < M; ++i) cout << edges[i].first << " " << edges[i].second << endl;

}

void solve(){
  int ans = 0;
  preprocess();

  init(N);

  for(int i = 0; i < M; ++i){
    int index = edges[i].second;
    if(is_same_set(dpt[index], dst[index])) continue;
    else {
      unite(dpt[index], dst[index]);
      ans += cost[index];
    }
  }
  cout << ans << endl;
}

int main(){

cin >> N;

for(int i = 0; i < N; ++i){
  for(int j = 0; j < N; ++j){
    cin >> buff;
    A[i][j] = buff;
    }
  }

  //行列の表示
  // cout << "Matrix:" << endl;
  // for(int row=0; row < N; ++row){
  //   for(int col = 0; col < N; ++col){
  //     cout << A[row][col];
  //   }
  //   cout << endl;
  // }

  solve();
}

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;
}