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++: #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<<sum<<endl;
}

C++: #include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define repr(i, n) for (int i = (int)(n); i >= 0; i--)
#define REP(i, m, n) for (int i = (int)(m); i <= (int)(n); i++)
#define REPR(i, m, n) for (int i = (int)(m); i >= (int)(n); i--)
#define all(v) v.begin(), v.end()
typedef long long ll;

using Graph = vector<vector<int> >;

int n, sum;
vector<bool> seen;
priority_queue<pair<int, int> > pq;

void dfs(const Graph &G, int v, int p){
  seen[v] = true;

  rep(u, n){
    if(u == p) continue;
    if(G[v][u] == -1) continue;
    pq.push(make_pair((-1)*G[v][u], u));
  }

  while(!pq.empty()){
    pair<int, int> tmp;
    tmp = pq.top();
    pq.pop();
    if(seen[tmp.second]) continue;
    sum += (-1)*tmp.first;
    int nv = tmp.second;
    p = v;
    dfs(G, nv, p);
  }

  return;
}

int main(){
  cin >> n;
  Graph G(n, vector<int>(n));
  rep(i, n) rep(j, n) cin >> G[i][j];

  seen.assign(n, false);
  sum = 0;
  dfs(G, 0, -1);

  cout << sum << endl;

  return 0;
}

C++: #include <iostream>
#include <utility>
#include <string>
#include <algorithm>
#include <set>
#include <deque>
#include <vector>
#define MAX 150
#define rep(i,p) for(int i=0;i<p;i++)
#define rep1(i,p) for(int i=1;i<=p;i++)
#define pb push_back
using namespace std;
int n,temp,a,b;
int A[MAX];
int B[MAX];
int W[MAX];
int V[MAX];
pair<int,int> edge[MAX];
int main(){
  cin >> n;
  rep(i,n) 
    rep(j,n) {
      int nnn = i*n + j;
      cin >> W[nnn];
      A[nnn] = i;
      B[nnn] = j;
      edge[nnn].first = W[nnn];
      edge[nnn].second = nnn;
  }
  // sort by lenght of edge
  sort(edge, edge+n*n);
  // printing
  // rep(i,n*n) {
  //   cout << A[edge[i].second] << " " << B[edge[i].second]
  //        << " " << edge[i].first << endl;;
  // }
  int sum = 0;
  rep(i,n) V[i] = i;
  rep(i,n*n) {
    if (edge[i].first == -1) continue;
    temp = edge[i].second;
    // a , b : vertex
    a = A[temp]; 
    b = B[temp];
    if (V[a] != V[b]) {
      // ?????????????????????
      sum += edge[i].first;
      if (V[a] != a && V[b] == b){
        V[b] = V[a];
      } else if (V[a] == a && V[b] != b) {
        V[a] = V[b];
      } else if (V[a] == a && V[b] == b) {
        V[b] = V[a];
      }
    } else {

    }
  }
  cout << sum << endl;
  return 0;
}

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

#define MAX_SIZE 110
#define INF (1<<30)
#define NOT_VISITED 0
#define VISITED 1
#define COMPLETE 2

int n, G[MAX_SIZE][MAX_SIZE];

int prim() {
    int d[MAX_SIZE], parent[MAX_SIZE], status[MAX_SIZE];
    for (int i = 0; i < n; i++) {
        d[i] = INF;
        parent[i] = -1;
        status[i] = NOT_VISITED;
    }
    d[0] = 0;

    int u, min_cost;
    while (1) {
        u = -1;
        min_cost = INF;
        for (int i = 0; i < n; i++) {
            if (min_cost > d[i] && status[i] != COMPLETE) {
                u = i;
                min_cost = d[i];
            }
        }
        if (u == -1) break;
        status[u] = COMPLETE;
        for (int i = 0; i < n; i++) {
            if (status[i] == COMPLETE || G[u][i] == INF) continue;
            if (d[i] > G[u][i]) {
                d[i] = G[u][i];
                parent[i] = u;
                status[i] = VISITED;
            }
        }
    }
    
    int sum = 0;
    for (int i = 0; i < n; i++) {
        if (parent[i] != -1) sum += G[i][parent[i]];
    }

    return sum;
}

int main() {
    cin >> n;

    int a;
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < n; j++) {
            cin >> a;
            if (a == -1) G[i][j] = INF;
            else G[i][j] = a;
        }
    }

    cout << prim() << endl;

    return 0;
}

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

const int N = 100;
int n;
int A[N][N];
priority_queue< pair<int, int> > Q;

int main()
{
	cin >> n;
	for (int i=0; i<n; i++)
	{
		for (int j=0; j<n; j++)
		{
			cin >> A[i][j];
		}
	}

	int mcost = -INT_MAX;

	for (int i=0; i<n; i++)
	{
		bool visit[N] = {0};

		Q.push(pair<int, int>(0,i));

		int cost = 0;

		while (Q.size())
		{
			pair<int, int> q = Q.top();
			Q.pop();
			int k = q.second;
			if (visit[k]) continue;
			cost += q.first;
			visit[k]=true;
			for(int j=0;j<n;j++)
			{
				int c = A[k][j];
				if (c <= 0) continue;
				if (visit[j]) continue;
				Q.push(pair<int, int>(-c,j));
			}
		}

		if (mcost < cost) mcost = cost;
	}

	cout << -mcost << endl;
		
	return 0;
}