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<iomanip>
#include<limits>
#include<thread>
#include<utility>
#include<iostream>
#include<string>
#include<algorithm>
#include<set>
#include<map>
#include<vector>
#include<stack>
#include<queue>
#include<cmath>
#include<numeric>
#include<cassert>
#include<random>
#include<chrono>
using namespace std;
typedef unsigned long long int ull;
typedef long long int ll;
typedef pair<ll,ll> pll;
typedef pair<int,int> pi;
typedef pair<double,double> pd;
typedef pair<double,ll> pdl;
#define F first
#define S second
#define MK make_pair
const ll E=1e18+7;
const ll MOD=1000000007;

vector<ll> uft;

ll search(ll where){
    if(uft[where]==where){return where;}
    else{return uft[where]=search(uft[where]);}
}

void add(ll a,ll b){
    uft[search(a)]=search(b);
}


int main(){
    ll n;
    cin>>n;
    priority_queue<vector<ll>,vector<vector<ll>>,greater<vector<ll>>> E;
    for(int i=0;i<n;i++){
        for(int t=0;t<n;t++){
            ll a;
            cin>>a;
            if(a!=-1){
                E.push({a,i,t});
            }
        }
    }
    uft.resize(n);
    for(int i=0;i<n;i++){
        uft[i]=i;
    }
    ll cost=0;
    while(!E.empty()){
        vector<ll> e=E.top();
        E.pop();
        if(search(e[1])!=search(e[2])){
            cost+=e[0];
            add(e[1],e[2]);
        }
    }
    cout<<cost<<endl;
    
    
    return 0;
}

C++: #include <iostream>
using namespace std;
static const int INFTY = (1 << 21);
int n,M[100][100];

int prim(){
  int u,v,d[100],p[100],color[100],mincost;
  for (int i = 0; i < n; i++) {
    color[i] = 0;
    d[i] = INFTY;
    p[i] = -1;
  }
  d[0] = 0;

  while (1) {
    mincost = INFTY;
    u = -1;
    for (int i = 0; i < n; i++) {
      if (color[i] != 2 && d[i] < mincost) {
        u = i;
        mincost = d[i];
      }
    }
    if (u == -1) break;
    color[u] = 2;
    for (v = 0; v < n; v++) {
      if (color[v] != 2 && M[u][v] != INFTY) {
        if (M[u][v] < d[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 t;
  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<bits/stdc++.h>
using namespace std;
int main(){
	int mapp[101][101];
	int d[101],vis[101];
	int ans=0,n,u;
	cin>>n;
	for (int i=0; i<n; i++)
	    for (int j=0; j<n; j++)
	        cin>>mapp[i][j];
	memset(vis,0,sizeof(vis));
	memset(d,0x1f1f1f1f,sizeof(d));
	d[0]=0; 
	while (true){
		  int minn=0x1f1f1f1f;
		  for (int i=0; i<n; i++)
		  	  if (!vis[i] && d[i]<minn){
		  	  	 minn=d[i]; u=i;
			  }
		  vis[u]=1;
		  if (minn==0x1f1f1f1f)
		     break;
		  for (int i=0; i<n; i++)
		      if (!vis[i] && mapp[u][i]!=-1 && mapp[u][i]<d[i]){
		      	 d[i]=mapp[u][i];
			  }
	}
	for (int i=0; i<n; i++)
		ans+=d[i];
	cout<<ans<<endl;
	return 0;
}

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 <set>
#include <map>
#include <list>
#include <queue>
#include <stack>
#include <cmath>
#include <ctime>
#include <cstdio>
#include <vector>
#include <string>
#include <bitset>
#include <cctype>
#include <cstdlib>
#include <cstring>
#include <utility>
#include <numeric>
#include <complex>
#include <sstream>
#include <fstream>
#include <iomanip>
#include <cassert>
#include <iostream>
#include <iterator>
#include <algorithm>
using namespace std;
using ll = long long;
using VL=vector<ll>;

#define REP(i, n) for(int i = 0;i < n;i++)
#define REPR(i, n) for(int i = n;i >= 0;i--)
#define FOR(i, m, n) for(int i = m;i < n;i++)
const long long INF =  999999999;

typedef pair<int,int> P;

int cost[2010][2010];
int mincost[2020];
bool used[2020];
int V;

int main(){
  int n,res=0;
  cin>>n;
  REP(i,n){
    REP(j,n){
      cin>>cost[i][j];
    }
  }
  REP(i,n){
    mincost[i]=INF;
    used[i]=false;
  }
  mincost[0]=0;

  while(true){
    int v=-1;

    REP(u,n){
      if(!used[u] &&(v==-1 || mincost[u]<mincost[v]))v=u;
    }

    if(v==-1)break;
    used[v]=true;
    res += mincost[v];

    REP(u,n){
      if(cost[v][u]==-1)continue;
      mincost[u]=min(mincost[u],cost[v][u]);
    }
  }
  cout<<res<<endl;

}