Go: package main

import (
	"fmt"
	"sort"
)

type Edge struct {
	I int
	J int
	A int
}

func main() {
	var n int
	fmt.Scan(&n)

	g := make([][]int, n)
	edges := make([]Edge, 0, n*(n-1)/2)
	parents := make([]int, n)
	for i := 0; i < n; i++ {
		g[i] = make([]int, n)
		for j := 0; j < n; j++ {
			fmt.Scan(&g[i][j])
			if g[i][j] >= 0 {
				edges = append(edges, Edge{i, j, g[i][j]})
			}
		}
		parents[i] = i
	}
	sort.SliceStable(edges, func(i, j int) bool { return edges[i].A >= edges[j].A })

	sum := 0
	num := 0
	for len(edges) > 0 && num < n-1 {
		edge := edges[len(edges)-1]
		edges = edges[:len(edges)-1]

		if same(edge.I, edge.J, &parents) {
			continue
		}
		union(edge.I, edge.J, &parents)
		sum += edge.A
		num += 1
	}
	fmt.Println(sum)
}



func root(id int, parents *[]int) int {
	if id == (*parents)[id] {
		return id
	}
	return root((*parents)[id], parents)
}

func union(i, j int, parents *[]int) {
	(*parents)[root(j, parents)] = (*parents)[root(i, parents)]
}

func same(i, j int, parents *[]int) bool {
	return root(i, parents) == root(j, parents)
}

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++: #if 0
????????£????????????1.5h
??¢?????????????????§??????????????¢??°????????£????????????sum????¨??????¨????????????????????£????????§?????????
???????????¢??????????????§????????????????????????????????????"-1"???????????????????????????????????§???????????§????????¨???????°???????????????????
#endif

#include <iostream>
#include <algorithm>
#include <utility>
using namespace std;

int P[10000+10];

void init(int N){
	for(int i=0;i<=N;++i) P[i]=i;
}
int root(int a){
	if(P[a]==a)return a;
	else return(P[a]=root(P[a]));
}
bool is_same_set(int a,int b){
	return root(a)==root(b);
}
void unite(int a,int b){
	P[root(a)]=root(b);
}
int main(){
	int N;
	cin>>N;
	int A[N][N];
	
	for(int i=0; i<N;i++){
		for(int j=0;j<N;j++){
			cin>>A[j][i]; //???????????\???
		}
	}

	int M = 0; 
	int cost[N*N],src[N*N],dst[N*N];
	for (int r=0; r<N;r++)
		for (int c=0; c<r;c++) //r<c???????§?????????????
			if (A[r][c] > -1) {
				cost[M] = A[r][c];
				src[M] = r;
				dst[M] = c;
				M += 1;
			}
	pair<int,int> edges[2000+10];
	for(int i=0;i<M;i++)
		edges[i]=make_pair(cost[i],i); //first???????????????second???ID
	sort(edges,edges+M);
	
	//for(int i=0;i<M;++i)
		//cout<<edges[i].first<<endl;

	init(N);
	int sum=0;
	for(int i=0;i<M;i++){
		if(is_same_set(src[edges[i].second],dst[edges[i].second])==0){
			unite(src[edges[i].second],dst[edges[i].second]);
			sum+=edges[i].first;
			cout << edges[i].first <<endl;
		}
	}
		cout<<sum<<endl;
}

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

static const int MAX = 100;
static const int INFTY = (1 << 21);

int n,M[MAX][MAX];

int prim() {
	int u,minv;
	int color[MAX] = {}, d[MAX], p[MAX];
	for (int i = 0;i < n;i++) {
		d[i] = INFTY;
		p[i] = -1;
	}
	d[0] = 0;
	while (1) {
		minv = INFTY;
		u = -1;
		for (int i = 0;i < n;i++) {
			if (minv > d[i] && color[i] != 2) {
				u = i;
				minv = d[i];
			}
		}
		if (u == -1) {
			break;
		}
		color[u] = 2;
		for (int v = 0;v < n;v++) {
			if (color[v] != 2 && M[u][v] != INFTY) {
				if (d[v] > M[u][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;
	int e;
	for (int i = 0;i < n;i++) {
		for (int j = 0;j < n;j++) {
			cin >> e;
			if (e == -1) {
				M[i][j] = INFTY;
			}
			else {
				M[i][j] = e;
			}
			
		}
	}
	cout << prim() << endl;
	return 0;

}

C++: #include <bits/stdc++.h>
using namespace std;
typedef pair<int, int> P;
const int INF = 1e9;
const int MAX_V = 1000;

int cost[MAX_V][MAX_V];
bool used[MAX_V];
int n;

int prim(){
    int res = 0;
    priority_queue<P, vector<P>, greater<P> > q;
    memset(used, false, sizeof(used));
    q.push(P(0, 0));
    while(!q.empty()){
        P p = q.top(); q.pop();
        int v = p.second;
        int d = p.first;
        if(used[v]) continue;
        used[v] = true;
        res += d;
        for(int i = 0; i < n; i++){
            q.push(P(cost[v][i], i));
        }
    }
    return res;
}

int main(){
    cin >> n;
    for(int i = 0; i < n; i++){
        for(int j = 0; j < n; j++){
            cin >> cost[i][j];
            if(cost[i][j] == -1) cost[i][j] = INF;
        }
    }
    cout << prim() << endl;
}

/* verified
https://onlinejudge.u-aizu.ac.jp/#/courses/lesson/1/ALDS1/12/ALDS1_12_A
*/

C++: #include <bits/stdc++.h>
using namespace std;

struct UF{
   vector<int> par;
   vector<int> sz;
   UF(){}
   UF(int n){
       par.resize(n);
       sz.resize(n, 1);
       for(int i = 0; i < n; i++) par[i] = i;
   }
   int find(int x){
       if(x == par[x]) return x;
       return par[x] = find(par[x]);
   }
   void unite(int x, int y){
       x = find(x); y = find(y);
       if(x == y) return;
       if(sz[x] < sz[y]) swap(x, y);
       sz[x] += sz[y];
       par[y] = x;
   }
   bool same(int x, int y){
       return find(x) == find(y);
   }
};

struct edge{
    int u, v, cost;
    bool operator<(const edge &e) const{
        return cost < e.cost;
    }
};

int n, m;
vector<edge> es; 

int kruskal(){
    int res = 0;    
    UF uf(n);
    sort(es.begin(), es.end());
    for(int i = 0; i < m; i++){
        edge e = es[i];
        if(!uf.same(e.u, e.v)){
            uf.unite(e.u, e.v);
            res += e.cost;
        }
    }
    return res;
}

int main(){
    cin >> n >> m;
    for(int i = 0; i < m; i++){
        int u, v, w;
        cin >> u >> v >> w;
        //u--; v--;
        es.push_back({u, v, w});
    }
    cout << kruskal() << endl;
}

/* verified
https://onlinejudge.u-aizu.ac.jp/#/courses/library/5/GRL/2/GRL_2_A
*/