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>
#include <vector>
#include <cstdlib>
#include <utility>
#include <algorithm>

using namespace std;

vector<int> djset, szset;
vector<pair<int, pair<int, int>>> G;

int root(int x)
{
    int v = djset[x];
    while (v != djset[v]) v = djset[v];
    djset[x] = v;
    return v;
}

int main(void)
{
    int N;
    cin >> N;
    G.resize(N * N);
    int count = 0;
    int temp;
    for (int i = 0; i < N; i++)
    {
        for (int j = 0; j < N; j++)
        {
            cin >> temp;
            if (i == j || temp == -1) continue;
            G[count++] = make_pair(temp, make_pair(i, j));
        }
    }
    djset.resize(count);
    szset.resize(count, 1);
    G.erase(G.begin() + count, G.end());
    sort(G.begin(), G.end());
    for (int i = 0; i < count; i++) djset[i] = i;
    int v[2], r[2];
    int ans = 0;
    for(auto i : G)
    {
        v[0] = i.second.first;
        v[1] = i.second.second;
        for (int i = 0; i < 2; i++) r[i] = root(v[i]);
        if (r[0] == r[1]) continue;
        if (szset[r[1]] > szset[r[0]]) swap(r[0], r[1]);
        djset[r[1]] = r[0];
        szset[r[0]] += szset[r[1]];
        szset[r[1]] = 0;
        ans += i.first;
    }
    cout << ans << endl;
    return 0;
}