Go: package main

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

var scanner = bufio.NewScanner(os.Stdin)

func nextString() string {
	scanner.Scan()
	return scanner.Text()
}

func nextInt() int {
	n, err := strconv.Atoi(nextString())
	if err != nil {
		fmt.Printf("strconv.Atoi failed: %v\n", err)
	}
	return n
}

type ElementKey int
type Node struct {
	key    ElementKey
	parent *Node
	rank   int
}

func newNode(key ElementKey) *Node {
	node := Node{}
	node.key = key
	node.parent = &node
	node.rank = 0

	return &node
}

func union(x, y *Node) {
	xSet := findSet(x)
	ySet := findSet(y)
	link(xSet, ySet)
}

func link(x, y *Node) {
	if x.rank > y.rank {
		y.parent = x
	} else {
		x.parent = y

		if x.rank == y.rank {
			y.rank++
		}
	}
}

func findSet(x *Node) *Node {
	if x != x.parent {
		x.parent = findSet(x.parent)
	}
	return x.parent
}

func newDisjointSets(nSets int) *DisjointSets {
	sets := DisjointSets{}
	sets.keyToNode = map[ElementKey]*Node{}
	for i := 0; i < nSets; i++ {
		key := ElementKey(i)
		sets.keyToNode[key] = newNode(key)
	}
	return &sets
}

type DisjointSets struct {
	keyToNode map[ElementKey]*Node
}

func (sets *DisjointSets) union(x, y ElementKey) {
	xNode := sets.keyToNode[x]
	yNode := sets.keyToNode[y]
	union(xNode, yNode)
}

func (sets DisjointSets) findSet(x ElementKey) *Node {
	xNode := sets.keyToNode[x]
	return findSet(xNode)
}

type Edge struct {
	i, j   int
	weight int
}

func main() {
	scanner.Split(bufio.ScanWords)

	n := nextInt()
	sets := newDisjointSets(n)
	edges := []Edge{}
	a := make([][]int, n)
	for i := 0; i < n; i++ {
		a[i] = make([]int, n)
		for j := 0; j < n; j++ {
			a[i][j] = nextInt()
			if a[i][j] >= 0 {
				e := Edge{
					i:      i,
					j:      j,
					weight: a[i][j],
				}
				edges = append(edges, e)
			}
		}
	}
	sort.Slice(edges, func(i, j int) bool { return edges[i].weight < edges[j].weight })
	totalWeight := 0
	for _, edge := range edges {
		i := ElementKey(edge.i)
		j := ElementKey(edge.j)
		if sets.findSet(i) != sets.findSet(j) {
			totalWeight += edge.weight
			sets.union(i, j)
		}
	}
	fmt.Println(totalWeight)
}

C++: #include<iostream>

using namespace std;
#define GRAY 1
#define BLACK 2
#define WHITE 0
#define INF  2000000
int n, mincost, u;
int M[100][100];

//プリム法
void prim(){
 int d[100],p[100],color[100];
 for(int i=0;i<n;i++){
   d[i]=INF;
   p[i]=-1;
   color[i]=WHITE;
 }
 d[0]=0;

 while(1){
   mincost = INF;
   u=-1;
   for(int i=0;i<n;i++){
     if(color[i]!= BLACK && d[i]< mincost){
       mincost = d[i];
       u = i;
     }
   }
   if(u== -1){
     break;
   }
   color[u] = BLACK;

   for(int v=0;v<n;v++){
     if(color[v]!=BLACK &&  M[u][v]!=INF){
       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]];
 }
 cout<<sum<<endl;
}


int main(){
  cin>>n;
  for(int i=0;i< n ; i++){
   for(int j=0;j<n;j++){
      int in;
      cin >> in ;
      if(in == -1){
        M[i][j]= INF;
      }
      else M[i][j] = in;
   }
  }
prim();

  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 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({ 0,0 });
    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, mx[v.id][j] });
            }
        }
    }
    cout << ret << endl;
    return 0;
}

C++: #include<iostream>
using namespace std;
static const int N = 100;
static const int INFTY = (1<<21);
static const int WHITE = 0;
static const int GRAY = 1;
static const int BLACK = 2;

int color[N];
int M[N][N];
int n,cnt;

void prim(){
	int d[N],p[N];
	for(int i=0;i<n;i++){
		color[i] = WHITE;
		d[i] = INFTY;
	}
	d[0]=0;p[0]=-1;
	int u;
	
	while(1){
		int mincost=INFTY;
		for(int i=0;i<n;i++){
			if(color[i] != BLACK && d[i] < mincost){
				mincost = d[i];
				u = i;
			}
		}
		if(mincost == INFTY)break;
		cnt += mincost;
		color[u] = BLACK;
		for(int i=0;i<n;i++){
			if(color[i] != BLACK && M[u][i] != INFTY){
				if(d[i] > M[u][i]){
					d[i] = M[u][i];
					p[i] = u;
					color[i] = GRAY;
				}
			}
		}
	}
}

int main(){
	cnt = 0;
	int d;
	cin >> n;
	for(int i=0;i<n;i++){
		for(int j=0;j<n;j++){
			cin >> d;
			M[i][j] = (d!=-1?d:INFTY);
		}
	}
	prim();
	cout << cnt << endl;
	return 0;
}

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

int x[10000][10000];
int MIN[10000];
int MINX=100000000;

int main() {
	memset(MIN, 127, sizeof(MIN));
	int n, a, sum = 0;
	cin >> n;
	for (int i = 0; i < n; i++) {
		for (int j = 0; j < n; j++) {
			cin >> a;
			x[i][j] = a;
		}
	}
	for (int h = 0; h < n; h++) {
		memset(MIN, 127, sizeof(MIN));
		MIN[h] = 0;
		sum = 0;
		for (int i = 0; i < n; i++) {
			for (int j = 0; j < n; j++) {
				for (int k = 0; k < n; k++) {
					if (x[j][k] != -1) {
						MIN[k] = min(MIN[k], MIN[j] + x[j][k]);
					}
				}
			}
		}
		for (int i = 0; i < n; i++) {
			sum += MIN[i];
		}
		MINX = min(sum, MINX);
	}
	
	cout << MINX * 3 / 4 << endl;
	return 0;
}