Go: package main

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

const BIG_COST = 3000

const (
	ST_NOT_YET = iota
	ST_JOINED
)

func main() {
	sc := bufio.NewScanner(os.Stdin)
	sc.Split(bufio.ScanWords)

	nNodes := scanInt(sc)
	costs := make([][]int, nNodes)

	for r := 0; r < nNodes; r++ {
		costs[r] = make([]int, nNodes)
		for c := 0; c < nNodes; c++ {
			costs[r][c] = scanInt(sc)
		}
	}

	states := make([]int, nNodes)
	minCosts := make([]int, nNodes)

	for i := 1; i < nNodes; i++ {
		minCosts[i] = BIG_COST
	}

	for {
		var u int
		minCost := BIG_COST

		for i := 0; i < nNodes; i++ {
			if states[i] != ST_JOINED && minCosts[i] < minCost {
				minCost = minCosts[i]
				u = i
			}
		}

		// 全てのノードが MST に JOIN できたので終了。
		if minCost == BIG_COST {
			break
		}

		states[u] = ST_JOINED

		for v := 0; v < nNodes; v++ {
			cost := costs[u][v]

			// MST内の u と MST外の v との辺の重みをそれぞれ minCosts[v] として
			// 保存しておく。このうち辺の重みが最小となる v が次のループで MST に JOIN する。
			if states[v] != ST_JOINED && cost != -1 && cost < minCosts[v] {
				minCosts[v] = cost
			}
		}
	}

	sum := 0
	for _, cost := range minCosts {
		sum += cost
	}
	fmt.Println(sum)
}

func scanInt(sc *bufio.Scanner) int {
	sc.Scan()
	v, _ := strconv.Atoi(sc.Text())
	return v
}

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;
}

C++: #include<iostream>
#include<queue>
#include<algorithm>
using namespace std;
typedef pair<int,int> P;
typedef pair<int,P> T;

class uft {//union-find tree
public:
    int parent[101];
    int size[101];
    int root(int x);
    bool same(int x,int y);
    void unite(int x,int y);
};
int uft::root(int x){
    if(parent[x]==x){
        return x;
    }
    return parent[x] = root(parent[x]);
}
bool uft::same(int x,int y){
    if(root(x)==root(y)){
        return true;
    }else{
        return false;
    }
}
void uft::unite(int x,int y){
    x=root(x);
    y=root(y);
    if(x == y) return;
    parent[x] = y;
    size[y] = size[y]+size[x];
}

int solv(priority_queue<T, vector<T>, greater<T> > q,int N){
    uft u;
    for(int i=0;i<N;i++){
        u.parent[i]=i;
        u.size[i]=1;
    }
    int ret=0;
    while(u.size[u.root(0)] != N && !q.empty()){
        T edge = q.top(); q.pop();
        if(u.root((edge.second).first) != u.root((edge.second).second)){
            u.unite((edge.second).first,(edge.second).second);
            ret += edge.first;
        }
    }
    return ret;
}

int main(){
    int N;
    priority_queue<T, vector<T>, greater<T> > q;
    cin>>N;
    for(int i=0;i<N;i++){
        for(int j=0;j<N;j++){
            int a;
            cin>>a;
            if(a!=-1 && i<j){
                T d = T(a,P(i,j));
                q.push(d);
            }
        }
    }
    int ans=solv(q,N);
    cout<<ans<<endl;
    return 0;
}

C++: #include<iostream>


using namespace std;
int n;
int M[100][100];

int prim() {
	int u;
	int color[100];
	int d[100];
	int p[100];
	int mincost;
	for (int i = 0;i < n;i++) {
		color[i] = 0;
		d[i] =2001;
		p[i] = -1;
	}
	d[0] = 0;
	

	while (true) {
		mincost = 2001;
		for (int i = 0;i < n;i++) {
			if (color[i] != 2 && d[i] < mincost) {
				mincost = d[i];
				u = i;
			}
		}
		if (mincost== 2001) {
			break;
		}

		color[u] = 2;
		for (int v = 0;v < n;v++) {
			if (color[v] != 2 && M[u][v] != -1) {
				if (M[u][v] < d[v]) {
					d[v] = M[u][v];
					p[v] = u;
					color[v] = 1;
				}
			}
		}
	}
	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;
		}
	}
	cout << prim() << endl;

	return 0;
}