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;
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>
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 <bits/stdc++.h>
#include <cmath>

using namespace std;
typedef long long ll;
//typedef unsigned long long ll;

#define rep(i, n) for (int i = 0; i < (n); ++i)
//#define rep(i, n) for (ll i = 0; i < (n); ++i)
//#define sz(x) ll(x.size())
//typedef pair<ll, int> P;
//typedef pair<ll, ll> P;
const double INF = 1e10;
//const ll INF = LONG_LONG_MAX / 10;
//const ll INF = 1e15;
const ll MINF = LONG_LONG_MIN;
//const int INF = INT_MAX / 10;
#define cmin(x, y) x = min(x, y)
#define cmax(x, y) x = max(x, y)

bool contain(set<int> &s, int a) { return s.find(a) != s.end(); }

//ifstream myfile("C:\\Users\\riku\\Downloads\\0_00.txt");
//ofstream outfile("log.txt");
//outfile << setw(6) << setfill('0') << prefecture << setw(6) << setfill('0') << rank << endl;
// std::cout << std::bitset<8>(9);

typedef pair<double, double> P;

void print_line(vector<int> &line) {
    if (line.size() == 0ll) {
        cout << endl;
        return;
    }
    for (ll i = 0; i < line.size(); i++) {
        cout << line[i];
        if (i == line.size() - 1) cout << endl;
        else cout << ' ';
    }
}


typedef priority_queue<long long, vector<long long>, greater<long long>> PQ_ASK;
const int mod = 1000000007;

class UnionFind {
public:
    // 親の番号を格納する。親だった場合-size
    vector<int> parents;

    UnionFind(int n) {
        parents = vector<int>(n, -1);
    }

    // aがどのグループに属しているか
    int root(int a) {
        if (parents[a] < 0) {
            return a;
        }
        return parents[a] = root(parents[a]);
    }

    int size(int a) {
        return -parents[root(a)];
    }

    // aとbをくっつける
    bool connect(int a, int b) {
        int ra = root(a);
        int rb = root(b);
        if (ra == rb) {
            return false;
        }
        // 大きいほうにA
        if (size(ra) < size(rb)) {
            swap(ra, rb);
        }
        parents[ra] += parents[rb];
        parents[rb] = ra;
        return true;
    }

};

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

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

    vector<Road> road;

    rep(i, n) rep(j, n) {
            int cost;
            cin >> cost;
            if (cost == -1) continue;
            road.push_back({i, j, cost});
        }

    sort(road.begin(), road.end(), [](Road &r, Road &s) { return r.cost < s.cost; });

    UnionFind uf(n);
    ll cost = 0;
    for (Road &r : road) {
        if (uf.root(r.from) == uf.root(r.to)) continue;
        uf.connect(r.from, r.to);
        cost += r.cost;
    }
    cout << cost << endl;
}