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;
#define _GLIBCXX_DEBUG

vector<int> P;
int n;

void init() {
  for (int i = 0; i < n; i++)
    P[i] = i;
}
int root(int c) {
  if (P[c] == c)
    return c;
  return (P[c] = root(P[c]));
}
bool is_same_set(int a, int b) { return root(a) == root(b); }
void unite(int a, int b) { P[root(a)] = root(b); }

int main() {
  cin >> n;
  P = vector<int>(n);
  init();
  vector<tuple<int, int, int>> edges; //<weight, start, end>
  for (int i = 0; i < n; i++) {
    for (int j = 0; j < n; j++) {
      int a;
      cin >> a;
      if (i > j && a != -1)
        edges.push_back(make_tuple(a, i, j));
    }
  }
  sort(edges.begin(), edges.end());

  int ans = 0;
  for (tuple<int, int, int> edge : edges) {
    int w = get<0>(edge), s = get<1>(edge), e = get<2>(edge);
    if (!is_same_set(s, e)) {
      unite(s, e);
      ans += w;
    }
  }
  cout << ans << endl;
}

C++: #include<bits/stdc++.h>
#include"UnionFind.hpp"
using namespace std;

class UnionFind{
    int N;
    vector<int> par;
    vector<int> rank;
    public:
    UnionFind(int N):N(N),par(N),rank(N,0){iota(par.begin(),par.end(),0);}
    int find(int x);
    void unite(int x,int y);
    bool same(int x,int y){return find(x)==find(y);}
};

int UnionFind::find(int x){
    if(par[x]==x) return x;
    return par[x]=find(par[x]);
}
void UnionFind::unite(int x,int y){
    x=find(x),y=find(y);
    if(x==y) return;
    if(rank[x]>=rank[y]){
        if(rank[x]==rank[y]) rank[x]++;
        par[y]=x;
    }else{
        par[x]=y;
    }
}

using Weight=long long;
struct Edge{
    int from,to;
    Weight weight;
};
bool operator<(const Edge& lhs,const Edge& rhs){
    return lhs.weight<rhs.weight;
}

using Edges=vector<Edge>;
using Graph=vector<Edges>;


Edges MinimumSpanningTree(int n,Edges edges){
    Edges res;
    UnionFind u(n);
    for(int i=0;i<edges.size();i++){
        if(!u.same(edges[i].from,edges[i].to)){
            res.push_back(edges[i]);
            u.unite(edges[i].from,edges[i].to);
        }
    }
    return res;
}

Edges MinimumSpanningTree(const Graph& g){
    Edges edges;
    for(int i=0;i<g.size();i++){
        copy(g[i].begin(),g[i].end(),back_inserter(edges));
    }
    auto res=MinimumSpanningTree(g.size(),edges);
    return res;
}

int main(){
    int n;
    cin>>n;
    Graph g(n);
    for(int i=0;i<n;i++){
        for(int j=0;j<n;j++){
            Weight c;
            cin>>c;
            if(i==j) continue;
            g[i].push_back({i,j,c});
        }
    }
    auto mst=MinimumSpanningTree(g);
    cout<<accumulate(mst.begin(),mst.end(),Weight(0),[](const Weight& lhs,const Edge& rhs){return lhs+rhs.weight;})<<endl;
    return 0;
}

C++: #include <algorithm>
#include <iostream>
#include <vector>
#include <math.h>
#include <set>
#include <map>
#include <string>
#include <stack>
#include <queue>
#include <iomanip>
#include <numeric>
#define _USE_MATH_DEFINES
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
double pi = 3.141592653589793;
ll mod = 1000000007;
int intmax = 2147483647;
int intmin = -2147483648;
ll llmax = 9223372036854775807;
ll llmin = -9223372036854775808;

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 same(int x, int y) {
        return root(x) == root(y);
    }

    int root(int x) {
        return (data[x] < 0) ? x : data[x] = root(data[x]);
    }

    int rank(int x) {
        return -data[root(x)];
    }
};

struct edge {
    int from, to, cost;
};

bool asc (const edge &e, const edge &f) {
    return (e.cost != f.cost) ? e.cost < f.cost : (e.from != f.from ? e.from < f.from : e.to < f.to);
}

int mat[200][200];

int main() {
    int N;
    cin >> N;
    vector<edge> G;
    for (int i = 0; i < N; i++) {
        for (int j = 0; j < N; j++) {
            cin >> mat[i][j];
        }
    }
    for (int i = 0; i < N; i++) {
        for (int j = i + 1; j < N; j++) {
            if (mat[i][j] != -1) {
                edge e;
                e.from = i;
                e.to = j;
                e.cost = mat[i][j];
                G.push_back(e);
            }
        }
    }
    sort(G.begin(), G.end(), asc);
    int ans = 0;
    int num = 0;
    unionFind u(N);
    for (int i = 0; i < G.size(); i++) {
        if (u.root(G[i].from) != u.root(G[i].to)) {
            u.unite(G[i].from, G[i].to);
            ans += G[i].cost;
            num++;
        }
        if (num == N - 1) {
            break;
        }
    }
    cout << ans << endl;
}