Go: package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
	"strings"
)

const (
	WHITE int = iota
	GRAY
	BLACK
)

type graph struct {
	G [][]int
	C []int
	D []int
	P []int
}

var INFTY = 1 << 32

func constructGraph() *graph {
	sc := bufio.NewScanner(os.Stdin)
	sc.Scan()
	n, _ := strconv.Atoi(sc.Text())

	g := make([][]int, n)
	c := make([]int, n)
	d := make([]int, n)
	p := make([]int, n)

	for i := 0; i < n; i++ {
		sc.Scan()
		fields := strings.Fields(sc.Text())
		g[i] = toIntSlice(fields)
	}

	return &graph{g, c, d, p}
}

func (g *graph) initMst() {
	for i := 0; i < len(g.G); i++ {
		g.C[i] = WHITE
		g.D[i] = INFTY
	}
	g.D[0] = 0
	g.P[0] = -1
}

func (g *graph) primMst() int {
	g.initMst()

	var u int
	for {
		mincost := INFTY
		for i := 0; i < len(g.G); i++ {
			if g.C[i] != BLACK && g.D[i] < mincost {
				mincost = g.D[i]
				u = i
			}
		}

		if mincost == INFTY {
			break
		}

		g.C[u] = BLACK

		for i := 0; i < len(g.G); i++ {
			if g.C[i] != BLACK && g.existEdge(u, i) {
				if g.G[u][i] < g.D[i] {
					g.D[i] = g.G[u][i]
					g.P[i] = u
				}
			}
		}
	}

	totalCost := 0
	for _, cost := range g.D {
		totalCost += cost
	}
	return totalCost
}

func (g *graph) existEdge(u, v int) bool {
	return g.G[u][v] != INFTY && g.G[v][u] != INFTY
}

func toIntSlice(s []string) []int {
	a := make([]int, len(s))
	for i, v := range s {
		num, _ := strconv.Atoi(v)
		if num == -1 {
			num = INFTY
		}
		a[i] = num
	}
	return a
}

func main() {
	g := constructGraph()
	fmt.Println(g.primMst())
}

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>
#include<vector>

struct Edge {
	int row, column, weight;
};

void divide_graph(bool[], int);

int n;
Edge edges[99]{};

int main()
{
	int in_tmp, min{}, matrix[100][100]{};

	std::cin >> n;
	for (int i = 0; i < n; ++i)
		for (int j = 0; j < n; ++j)
		{
			std::cin >> in_tmp;
			matrix[i][j] = in_tmp;
		}

	int rest = n - 1, pre_size = 0;
	std::vector<int> queue{ 0 };
	bool visited[100]{ true };
	for (int i = 1; rest; ++i)
	{
		int now_size = queue.size();
		for (int j = pre_size; j < now_size; ++j)
			for (int k = 0; k < n; ++k)
			{
				int tmp = matrix[queue[j]][k];
				if (tmp != -1 && !visited[k])
				{
					visited[k] = true;
					rest--;
					edges[queue.size() - 1] = { queue[j], k , tmp};
					queue.push_back(k);
				}
			}
		pre_size = now_size;
	}
	bool flag = 1;
	while (flag)
	{
		flag = 0;
		for (int i = 0; i < n - 1; ++i)
		{
			bool conne_row[100]{};
			divide_graph(conne_row, i);

			for (int j = 0; j < n; ++j)
			{
				if (!conne_row[j]) continue;
				for (int k = 0; k < n; ++k)
				{
					if (conne_row[k]) continue;
					if (matrix[j][k] == -1 || matrix[j][k] >= edges[i].weight) continue;

					edges[i] = { j, k, matrix[j][k] };
					flag = true;
				}
			}
		}
	}

	for (int i = 0; i < n - 1; ++i)
		min += edges[i].weight;

	std::cout << min << std::endl;
	
	return 0;
}

void divide_graph(bool conne_row[], int max_edge)
{
	conne_row[edges[max_edge].row] = true;

	std::vector<int> queue = { edges[max_edge].row };
	int pre_size = 0;

	bool visit_edge[100]{};
	visit_edge[max_edge] = true;

	while (pre_size != queue.size())
	{
		int now_size = queue.size();
		for (int j = pre_size; j < now_size; ++j)
		{
			for (int k = 0; k < n - 1; ++k)
			{
				if (visit_edge[k]) continue;
				if (edges[k].row == queue[j])
				{
					visit_edge[k] = true;
					conne_row[edges[k].column] = true;
					queue.push_back(edges[k].column);
				}
				else if (edges[k].column == queue[j])
				{
					visit_edge[k] = true;
					conne_row[edges[k].row] = true;
					queue.push_back(edges[k].row);
				}
			}
		}
		pre_size = now_size;
	}
	return;
}

C++: #include <queue>
#include <vector>
#include <iostream>
using namespace std;

// ------ Class ------ //
class Graph2 {
public:
	// ------ Variables ------ //
	static const long long INF = 1000000000000000000;
	int V, E; vector<vector<pair<int, long long> > > G;

	// ------ Constructors ------ //
	Graph2() : V(0), E(0), G(vector<vector<pair<int, long long> > >()) {}
	Graph2(int v) : V(v), E(0), G(vector<vector<pair<int, long long> > >(v)) {}
	Graph2(vector<vector<pair<int, long long> > > g) : V(g.size()), G(g) { for (int i = 0; i < g.size(); i++) E += g[i].size(); }

	// ------ Basic Functions ------ //
	int size() { return V; }
	void add1(int v1, int v2, long long w) { G[v1].push_back(make_pair(v2, w)); E++; }
	void add2(int v1, int v2, long long w) { add1(v1, v2, w); add1(v2, v1, w); }

	// ------ Operators ------ //
	bool operator==(const Graph2& g) const { return G == g.G; }
	bool operator!=(const Graph2& g) const { return G != g.G; }
	vector<pair<int, long long> > operator[](int x) { return G[x]; }

	// ------ Algorithms ------ //
	long long minspan() {
		vector<long long> d(V, INF);
		vector<bool> used(V, false);
		priority_queue<pair<long long, int> > que;
		for (int i = 0; i < V; i++) d[i] = INF;
		d[0] = 0; que.push(make_pair(0, 0));
		while (!que.empty()) {
			pair<long long, int> pa = que.top(); que.pop();
			int u = pa.second; used[u] = true;
			for (int i = 0; i < G[u].size(); i++) {
				int v = G[u][i].first;
				if (d[v] > G[u][i].second && !used[v]) {
					d[v] = G[u][i].second; que.push(make_pair(-d[v], v));
				}
			}
		}
		long long sum = 0;
		for (int i = 0; i < V; i++) sum += d[i];
		return sum;
	}
};

// ------ Main ------ //
int n, a;
int main() {
	cin >> n; Graph2 g(n);
	for(int i = 0; i < n; i++) {
		for(int j = 0; j < n; j++) {
			cin >> a;
			if(a != -1) g.add1(i, j, a);
		}
	}
	cout << g.minspan() << endl;
}