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>
using namespace std;
#include<stdio.h>

class union_find  // union find.
{
    int* U;
public:
    union_find(int n){ U = new int [n + 1]; for(int i = 1; i <= n; i++) U[i] = i; }
    int root(int a){ if(U[a] == a){ return a; } U[a] = root(U[a]); return U[a]; }
    bool same(int a, int b){ return root(a) == root(b); }
    void join(int a, int b){ int x = root(a), y = root(b); U[x] = y; }
    ~union_find(){ delete [] U; }
};

struct edge{ int v1, v2; int weight; };  // edge.

edge init(){
    edge e;
    e.v1 = 0; e.v2 = 0; e.weight = -1;
    return e;
}

edge init(int a, int b, int w){
    edge e;
    e.v1 = a; e.v2 = b; e.weight = w;
    return e;
}

void swap_e(edge e1, edge e2)
{
    swap(e1.v1, e2.v1); swap(e1.v2, e2.v2); swap(e1.weight, e2.weight);
}

class edge_heap  // heap.
{
    edge* E;
    int edge_n;
public:
    edge_heap(int n){ E = new edge [n + 1]; edge_n = 0; }
    void min_heap(int i);
    void insert_heap(edge e);   // sorting.
    void insert_nl(edge e);     // no sorting.
    edge delete_min();
    bool isEmpty(){ return edge_n == 0; }
    int total_weight();
    ~edge_heap(){ delete [] E; }
};

void edge_heap::min_heap(int i)
{
    int l = 2 * i, r = 2 * i + 1;
    if(l > edge_n && r > edge_n) return;
    int minid = i;
    if(l <= edge_n){ if(E[l].weight < E[minid].weight) minid = l; }
    if(r <= edge_n){ if(E[r].weight < E[minid].weight) minid = r; }
    if(minid == i) return;
    swap(E[minid], E[i]);
    min_heap(minid);
}

void edge_heap::insert_heap(edge e)
{
    edge_n++;
    int w = e.weight;
    int i = edge_n;
    while(i > 1){
        if(w < E[i / 2].weight){
        E[i] = E[i / 2];
        }else{ break; }
        i /= 2;
    };
    E[i] = e;
}
void edge_heap::insert_nl(edge e){ edge_n++; E[edge_n] = e; }

edge edge_heap::delete_min()
{
    edge e = E[1];
    E[1] = E[edge_n]; edge_n--;
    min_heap(1);
    return e;
}

int edge_heap::total_weight()
{
    int tw = 0;
    for(int i = 1; i <= edge_n; i++) tw += E[i].weight;
    return tw;
}

int main()
{
    int n, i, j, w;
    scanf("%d", &n);
    edge_heap EH(n * (n + 1) / 2); // All Edge heap.
    edge e;

    for(i = 1; i <= n; i++){
        for(j = 1; j <= n; j++){
            scanf("%d", &w);
            if(j <= i || w < 0) continue;
            e = init(i, j, w);
            EH.insert_heap(e);
        }
    }

    edge_heap TH(n);   // Tree heap.

    union_find VG(n);  // vertex group.

    while(!EH.isEmpty()){
        e = EH.delete_min();
        if(!VG.same(e.v1, e.v2)){
            VG.join(e.v1, e.v2);
            TH.insert_nl(e);
        };
    };

    printf("%d\n", TH.total_weight());

    return 0;
}

C++: #include<iostream>
using namespace std;
static const int MAX=100;
static const int INF=(1<<21);
static const int WHI=0;
static const int GRA=1;
static const int BLA=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]=INF;
    p[i]=-1;
    color[i]=WHI;
  }
  d[0]=0;
  while(114514){
    minv=INF;
    u=-1;
    for(int i=0;i<n;++i){
      if(minv>d[i] && color[i]!=BLA){
	u=i;
	minv=d[i];
      }
    }
    if(u==-1) break;
    color[u]=BLA;
    for(int v=0;v<n;++v){
      if(color[v]!=BLA && m[u][v]!=INF){
	if(d[v]>m[u][v]){
	  d[v]=m[u][v];
	  p[v]=u;
	  color[v]=GRA;
	}
      }
    }
  }
  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)?INF:e;
    }
  }
  cout<<prim()<<endl;
  return 0;
}

C++: #include <iostream>
 using namespace std;
 static const int WHITE = 0;
 static const int GRAY = 1;
 static const int BLACK =2;
 static const int MAX = 100;
 static const int INFTY = (1<<21);
 
 int n,M[MAX][MAX];
 
 int prim(){
 	int u,miv;
 	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){
 		u=-1;
 		miv = INFTY;
 		for(int j=0;j<n;j++){
 			if(color[j]!=BLACK&&d[j]<miv){
 				miv = d[j];
 				u=j;
 			}
 		}
 		if(u==-1) break;
 		
 		color[u] = BLACK;
 		
 		for(int i=0;i<n;i++){
 			if(color[i]!=BLACK&&M[u][i]!=INFTY){
 				if(d[i]>M[u][i]){
 					d[i] = M[u][i];
 					p[i] = u;
 					color[i] = 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 <set>
using namespace std;
    int d[101];
    int G[101][101];
void krusta(int n){
    int color[101],l,m;
    for(int i=1;i<=n;i++){
        d[i]=1000000;
        color[i]=0;
    }
    d[1]=0;
    while(1){
        m=1000000;
        for(int i=1;i<=n;i++){
            if(color[i]!=1 && d[i]<m){
                m=d[i];
                l=i;
            }
        }
        color[l]=1;
        if(m==1000000){
            break;
        }
        for(int i=1;i<=n;i++){
            if(color[i]!=1 && G[l][i]!=-1 && G[l][i]<d[i]){
                d[i]=G[l][i];
            }
        }
    }
}
int main(){
    int n,m;
    cin>>n;
    for(int i=1;i<=n;i++){
        for(int j=1;j<=n;j++){
            cin>>m;
            G[i][j]=m;
        }
    }
    krusta(n);
    int sum=0;
    for(int i=1;i<=n;i++){
        sum=sum+d[i];
    }
    cout<<sum<<endl;
    return 0;
}