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<bits/stdc++.h>
//cout<<setprecision(1)<<fixed<<endl;
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
const int MAX_N=100000;
#define eps 1e-7
//#define Kongxiangzhouye

static const int MAX=100;
static const int INF=(1<<21);
static const int WHITE=0;
static const int GRAY=1;
static const int BLACK=2;

int begin[10000];
int to[10000];
int next[10000];
int weight[10000];
int e,n;
void add(int x,int y,int z){
	to[++e]=y;
	next[e]=begin[x];
	begin[x]=e;
	weight[e]=z;
}

struct p{
	int edge;
	int weight;
	bool operator < (const p& u)const{
		return weight>u.weight;
	}
};
int createtree(){
	priority_queue<p> q;
	int flag[100];
	memset(flag,0,sizeof(flag));
	int te=begin[1];
	while(to[te]!=-1){
		if(flag[to[te]]==0)
			q.push((p){te,weight[te]});
		te=next[te];
	}
	flag[1]=1;
	int ans=0;
	while(1){
		int flag2=1;
		for(int i=1;i<=n;i++){
			if(flag[i]==0){
				flag2=0;
				break;
			}
		}
		if(flag2==1)
			break;
		while(flag[to[q.top().edge]]==1)
			q.pop();
		p now=q.top();
		q.pop();
		ans=ans+now.weight;
		flag[to[now.edge]]=1;
		int te=begin[to[now.edge]];
		while(te!=-1){
			if(flag[to[te]]==0)
				q.push((p){te,weight[te]});
			te=next[te];
		}
	}
	return ans;
}
int main(){
	ios::sync_with_stdio(false);
	#ifdef Kongxiangzhouye
		freopen("in.txt","r",stdin);
		freopen("out.txt","w",stdout);
	#endif
	while(cin>>n){
		memset(begin,-1,sizeof(begin));
		memset(next,-1,sizeof(next));
		memset(to,-1,sizeof(to));
		e=0;
		for(int i=1;i<=n;i++){
			for(int j=1;j<=n;j++){
				int te;
				cin>>te;
				if(te!=-1)
					add(i,j,te);
			}
		}
		cout<<createtree()<<endl;
	}

	return 0;
}

C++: #include <iostream>
#include <iomanip>
#include <algorithm>
#include <cfloat>
#include <climits>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <functional>
#include <sstream>
#include <string>
#include <utility>
#include <map>
#include <memory>
#include <set>
#include <vector>
#include <deque>
#include <list>
#include <stack>
#include <queue>
// #include <array>

using namespace std;

class MSTree
{
public:
  int solve();
};

int MSTree::solve()
{
  int n;
  cin >> n;
  
  vector<vector<int>> adj(n, vector<int>(n));
  list<int> t, vt;

  for ( auto i = 0; i < n; ++i )
    for ( auto j = 0; j < n; ++j )
      cin >> adj[i][j];

  t.push_back( 0 );

  for ( auto i = 1; i < n; ++i )
    vt.push_back( i );

  int w = 0;

  while ( vt.size() )
    {
      int mw = 4000;
      list<int>::iterator itm;
      
      for ( auto ita = t.begin(); ita != t.end(); ++ita )
        for ( auto itb = vt.begin(); itb != vt.end(); ++itb )
          {
            if ( adj[*ita][*itb] >= 0 && mw > adj[*ita][*itb] )
              {
                itm = itb;
                mw = adj[*ita][*itb];
              }
          }
      
      w += mw;
      t.push_back( *itm );
      vt.erase( itm );
    }

  cout << w << endl;
    
  return 0;
}

int main()
{
  MSTree m;

  m.solve();

  return 0;
}

C++: #include<iostream>
#include<algorithm>
#include<queue>
#include<utility>
#include<vector>

using namespace std;



int main() {
	int n,tmp;
	cin >> n;
	pair<int, pair<int, int>>* sorted_weight = new pair<int, pair<int, int>>[n * n];
	int* equiv = new int[n];


	int cnt_equiv = 0;
	int cnt_edge = 0;
	int cost = 0;
	
	for (int i = 0;i < n;i++) {
		for (int j = 0;j < n;j++) {
			cin >> tmp;
			sorted_weight[i * n + j] = make_pair(tmp, make_pair(i, j));
		}
		equiv[i] = 0;
	}
	sort(sorted_weight, sorted_weight + n * n);
	for (int i = upper_bound(sorted_weight, sorted_weight + n * n, make_pair(-1, make_pair(n, n))) - sorted_weight;cnt_edge<n-1;i++) {
		if (equiv[sorted_weight[i].second.second] == 0 || equiv[sorted_weight[i].second.first] == 0) {
			cnt_edge++;
			cost += sorted_weight[i].first;
			if (equiv[sorted_weight[i].second.second] == 0 && equiv[sorted_weight[i].second.first] == 0) {
				cnt_equiv++;
				equiv[sorted_weight[i].second.second] = equiv[sorted_weight[i].second.first] = cnt_equiv;
			}
			else if (equiv[sorted_weight[i].second.second] == 0) {
				equiv[sorted_weight[i].second.second] = equiv[sorted_weight[i].second.first];
			}
			else {
				equiv[sorted_weight[i].second.first] = equiv[sorted_weight[i].second.second];
			}
		}
		else if (equiv[sorted_weight[i].second.second] != equiv[sorted_weight[i].second.first]) {
			cnt_edge++;
			cost += sorted_weight[i].first;
			tmp = equiv[sorted_weight[i].second.second];
			for (int j = 0;j < n;j++) {
				if (equiv[j] == tmp)equiv[j] = equiv[sorted_weight[i].second.first];
			}
		}
	}
	
	cout << cost << endl;
}

C++: #include <stdio.h>
#define INF 100000000
#define BLACK 0
#define WHITE 1
#define N 101
 
int n,a[N][N];

int prim(void)
{
  int u,v,i,mincost,d[N],pi[N],total = 0,color[N];
 
  for(u = 1;u <= n;u++){
    d[u] = INF;
    pi[u] = -1;
    color[u] = WHITE;
  }
 
  d[1] = 0;
     
  while(1){
    mincost = INF;
    for(i = 1;i <= n;i++){
      if(color[i] != BLACK && d[i] < mincost){
	mincost = d[i];
	u = i;
      }
    }
     
    if(mincost == INF) break;
    
    color[u] = BLACK;
     
    for(v = 1;v <= n;v++){
      if(color[v] != BLACK && a[u][v] < d[v]){
	pi[v] = u;
	d[v] = a[u][v];
      }
    }
    color[v] = BLACK;
  }
 
  for(i = 1;i <= n; i++){
    if(pi[i] != -1) total += a[i][pi[i]];
  }
 
  return total;
}

 
int main()
{
  int i,j;
 
  scanf("%d",&n);
 
  for(i = 1;i <= n;i++){
    for(j = 1;j <= n;j++){
      scanf("%d",&a[i][j]);
      if(a[i][j] == -1) a[i][j] = INF;
    }
  }
 
   printf("%d\n",prim());
    
   return 0;
 
}