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++: #if 0

#endif
#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <assert.h>
#include <complex>
#include <memory>
#define REP(i,n) for(int i=0;i<n;i++)
using namespace std;
typedef long long ll;
class UnionFindTree {
    ll n;
    ll P[100000];
public:
    void init(ll n) {
        for (ll i = 0; i <= n; ++i) P[i] = i;
    };
    ll root(ll node) {
        if (P[node] == node) return node; // aはroot
        return (P[node] = root(P[node])); // aの親のrootを求め，aの親とする(圧縮)
    }
    bool is_group(ll node_a, ll node_b) {
        return root(node_a) == root(node_b);
    }
    void unite(ll node_a, ll node_b) {
        P[root(node_a)] = root(node_b);
    }
};
ll a[10000][10000];
vector<pair<ll, pair<ll, ll> > > edge;
int main(void){
    cin.tie(0);
    ios::sync_with_stdio(false);

    unique_ptr<UnionFindTree> tree(new UnionFindTree);
    tree->init(10000);
    ll N;
    cin >> N;
    for (ll i = 0; i < N; ++i) {
        for (ll j = 0; j < N; ++j) {
            cin >> a[i][j];
            if (i >= j || a[i][j] == -1) continue;
            edge.push_back(make_pair(a[i][j], make_pair(i, j)));
        }
    }
    sort(edge.begin(), edge.end());

    ll cost_sum = 0;
    for (ll i = 0; i < edge.size(); ++i) {
        if (tree->is_group(edge[i].second.first, edge[i].second.second)) continue;
        tree->unite(edge[i].second.first, edge[i].second.second);
        cost_sum += edge[i].first;
    }

    cout << cost_sum << endl;

    return 0;
}

C++: #include <stdio.h>
#include <math.h>
#include <vector>

using namespace std;

int main(){

	int n,tmp,sum=0,tmp_min;
	scanf("%d",&n);
	int table[n][n],check[n],num_of_group=0,parent,child;
	vector<int> Group;

	for(int i=0;i<n;i++){
		for(int k=0;k<n;k++){
			scanf("%d",&tmp);
			table[i][k]=tmp;
		}
		check[i]=0;
	}

	Group.push_back(0);
	check[0]=1;
	num_of_group = 1;
	while(num_of_group<n){
		tmp_min=2001;

		for(int i=0;i<num_of_group;i++){
			for(int k=0;k<n;k++){
				if(table[Group[i]][k]!=-1&&check[k]==0&&table[Group[i]][k]<tmp_min){
					tmp_min = table[Group[i]][k];
					parent=Group[i];
					child = k;
				}
			}
		}
		Group.push_back(child);
		sum+=table[parent][child];
		check[child]=1;
		num_of_group++;
	}

	printf("%d\n",sum);

    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<iostream>
#define rep(i,n)for(int i=0;i<n;i++)
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];

    rep(i,n)
    {
        d[i]=INFTY;
        p[i]=-1;
        color[i]=WHITE;
    }

    d[0]=0;

    while (true)
    {
        minv=INFTY;
        u=-1;
        rep(i,n)
        {
           if (minv>d[i] && color[i]!=BLACK)
           {
               minv=d[i];
               u=i;
           }
        }

        if(u==-1)break;
        color[u]=BLACK;
        rep(v,n)
        {
            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;
    rep(i,n)
    {
        if(p[i]!=-1)sum+=M[i][p[i]];
    }
    return sum;
}

int main()
{
    cin>>n;
    rep(i,n)
    {
        rep(j,n)
        {
            int e;
            cin>>e;
            M[i][j]=(e==-1) ? INFTY:e;
        }
    }

    cout<<prim()<<endl;

    return 0;
}