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>
using namespace std;

struct UnionFind{
  int n;
  vector<int> r, p;
  UnionFind() {}
  UnionFind(int sz): n(sz), r(sz, 1), p(sz, 0){ iota(p.begin(), p.end(), 0); }
  int find(int x) {
    return (x == p[x] ? x : p[x] = find(p[x]));
  }
  bool same(int x, int y) {
    return find(x) == find(y);
  }
  void unite(int x, int y){
    x = find(x); y = find(y);
    if ( x == y ) return;
    if ( r[x] < r[y] ) swap(x, y);
    r[x] += r[y];
    p[y] = x;
  }
};

struct Edge {
  int cost, to, from;
  bool operator<(const Edge& e) const {
    return cost < e.cost;    
  }  
};

int main() {
  int n;
  cin >> n;

  vector<Edge> G;
  for ( int i = 0; i < n; i++ ) {
    for ( int j = 0; j < n; j++ ) {
      int cost;
      cin >> cost;
      if ( j > i && cost >= 0 ) G.push_back({cost, i, j});      
    }
  }

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

  UnionFind uf(n);
  int cnt = 0;
  int sum = 0;
  for ( Edge e: G ) {
    int cost = e.cost, to = e.to, from = e.from;
    if ( uf.same(to, from) ) continue;
    sum += cost;
    if ( ++cnt >= n-1 ) break;
    uf.unite(to, from);    
  }

  cout << sum << endl;
  
  return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;
#define int long long

struct edge{ int u,v,cost; };
bool comp(const edge& e1,const edge& e2){
  return e1.cost < e2.cost;
}

vector<edge> es;
int V,E; //頂点数、辺数
const int N = 1e5+10;

struct UF{
  const int N = 1e5+10;
  vector<int> par; //親
  vector<int> depth; //木の深さ

  void init(){
    par.resize(N);
    depth.resize(N);
    for(int i = 0; i < N; i++){
      par[i] = i;
      depth[i] = 0;
    }
  }

  //木の根を求める
  int root(int x){
    if(par[x] == x){
      return x;
    }
    else {
      return par[x] = root(par[x]);
    }
  }

  //xとyの属する集合を併合
  void unite(int x,int y){
    x = root(x);
    y = root(y);
    if(x == y) return;

    if(depth[x] < depth[y]) par[x] = y;
    else {
      par[y] = x;
      if(depth[x] == depth[y]) depth[x]++;
    }
  }
}uf;

int kruskal(){
  sort(es.begin(),es.end(),comp);
  uf.init();
  int min_cost = 0;
  for(int i = 0; i < es.size(); i++){
    edge e = es[i];
    if(e.cost == -1) continue;
    if(uf.root(e.u) != uf.root(e.v)){
      min_cost += e.cost;
      uf.unite(e.u,e.v);
    }
  }
  return min_cost;
}

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

  cin >> V;
  for(int i = 0; i < V; i++){
    for(int j = 0; j < V; j++){
      int cost; cin >> cost;
      es.push_back({i,j,cost});
    }
  }
  cout << kruskal() << endl;

  return 0;
}

C++: #include <stdio.h>
#include <iostream>
#include <utility>
#include <algorithm>
#include <math.h>
#include <vector>
using namespace std;

class edge
{
  public:
    int i, j, weigt;
    bool operator()(edge e1, edge e2)
    {
        return e1.weigt < e2.weigt;
    }
};
edge e[10000];
int main()
{
    int n;
    cin >> n;
    int b[n][n] = {};
    int k = 0;
    int tmp = 0;
    int sum = 0;
    for (int i = 0; i < n; i++)
    {
        for (int j = 0; j < n; j++)
        {
            cin >> tmp;
            if (j < i + 1 || tmp < 0)
                continue;
            e[k].i = i;
            e[k].j = j;
            e[k].weigt = tmp;
            k++;
        }
    }
    sort(e, e + k, edge());
    for (int i = 0; i < k; i++)
    {
        if (b[e[i].i][e[i].j] == 0)
        {
            sum += e[i].weigt;
            b[e[i].i][e[i].j] = 1;
            b[e[i].j][e[i].i] = 1;
            for (int j = 0; j < n; j++)
            {
                if (b[e[i].i][j] || b[e[i].j][j])
                    b[e[i].i][j] = b[e[i].j][j] = b[j][e[i].i] = b[j][e[i].j] = 1;
            }
        }
    }
    cout << sum << "\n";
    return 0;
}

C++: #define _CRT_SECURE_NO_WARNINGS
#include <cstdio>
#include <cstdlib>
#include <string>
#include <iostream>
#include <climits>
#include <list>
#include <queue>

using namespace std;

static const int INFTY = 2001;

int main() {
	int Dist[101];
	int AdjacenceyList[100][100];
	int AdjacenceyWeightList[100][100];
	int Degree[100];
	int i, j;
	int N;
	int W;
	int Deg;
	int MinTotalWeight = 0;
	int u, v;
	int PrevSecondSmallestVertex;
	int SecondSmallestVertex;
	int MinVertex;
	bool inT[100];

	cin >> N;

	for (i = 0; i < N; i++) {
		inT[i] = false;
		Dist[i] = INFTY;
		Deg = -1;
		for (j = 0; j < N; j++) {
			cin >> W;
			if(W != -1){
				AdjacenceyList[i][++Deg] = j;
				AdjacenceyWeightList[i][Deg] = W;
			}
		}
		Degree[i] = Deg + 1;
	}
	Dist[100] = INFTY;

	u = 0;
	inT[u] = true;
	Dist[u] = 0;
	PrevSecondSmallestVertex = 100;
	for (i = 1; i <= N - 1; i++) {
		MinVertex = 100;
		for (j = 0; j < Degree[u]; j++) {
			v = AdjacenceyList[u][j];
			if (inT[v] == false) {
				W = AdjacenceyWeightList[u][j];
				if (W < Dist[v]) {
					Dist[v] = W;
				}
				if (Dist[v] < Dist[MinVertex]) {
					SecondSmallestVertex = MinVertex;
					MinVertex = v;
				}
				else if (Dist[v] < Dist[SecondSmallestVertex]) {
					SecondSmallestVertex = v;
				}
			}
		}

		//DEBUG
		cout << "MinVerTex = " << MinVertex << " SecondSmallestVertex = " << SecondSmallestVertex << endl;

		if (Dist[PrevSecondSmallestVertex] < Dist[MinVertex]) {
			SecondSmallestVertex = MinVertex;
			MinVertex = PrevSecondSmallestVertex;
			PrevSecondSmallestVertex = SecondSmallestVertex;
		}
		else if (Dist[SecondSmallestVertex] < Dist[PrevSecondSmallestVertex]) {
			PrevSecondSmallestVertex = SecondSmallestVertex;
		}
		inT[MinVertex] = true;
		u = MinVertex;
		MinTotalWeight += Dist[MinVertex];

		//DEBUG
		cout << "MinVertex = " << MinVertex << " PrevSecondSmallestVertex = " << PrevSecondSmallestVertex << endl;
		cout << "inT: ";
		for (int k = 0; k < N; k++) {
			cout << inT[k] << " ";
		}
		cout << endl;
		printf("MinTotalWeight = %d\n", MinTotalWeight);
	}
	//printf("%d\n", MinTotalWeight);

	return 0;
}