Go: package main

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

//util
var sc = bufio.NewScanner(os.Stdin)

func nextStr() string {
	sc.Scan()
	return sc.Text()
}

func nextInt() int {
	sc.Scan()
	i, e := strconv.Atoi(sc.Text())
	if e != nil {
		panic(e)
	}
	return i
}

func printList(list []int) {
	n := len(list)
	buf := bufio.NewWriter(os.Stdout)
	for _, v := range list[0 : n-1] {
		buf.Write([]byte(fmt.Sprintf("%d ", v)))
	}
	buf.Write([]byte(fmt.Sprintf("%d\n", list[n-1])))
	buf.Flush()
}

func max(a, b int) int {
	if a > b {
		return a
	} else {
		return b
	}
}

func min(a, b int) int {
	if a > b {
		return b
	} else {
		return a
	}
}

//solve
const (
	MAX   = 100
	INFTY = (1 << 21)
	WHITE = 0
	GRAY  = 1
	BLACK = 2
)

//V: グラフの頂点全体の集合
//T: MSTに属する頂点の集合
var (
	n     int
	M     [MAX][MAX]int //重みを記録する隣接行列
	color [MAX]int
	p     [MAX]int //MSTにおける頂点の親を記録
	d     [MAX]int //Tに属する頂点と、V−Tに属する頂点を繋ぐ辺の中で、重みが最小のものを記録
)

func prim() int {
	var u, minv int
	//初期化
	for i := 0; i < n; i++ {
		d[i] = INFTY
		p[i] = -1
		color[i] = WHITE
	}

	d[0] = 0

	for {
		//頂点uを決めていく
		//つまり、Tに属する頂点と、V−Tに属する頂点を繋ぐ辺の中で、重みが最小のものを決める
		minv = INFTY
		u = -1
		for i := 0; i < n; i++ {
			//未訪問かつ、辺の重みが最小となるuを選ぶ
			if minv > d[i] && color[i] != BLACK {
				u = i
				minv = d[i]
			}
		}

		if u == -1 {
			break
		}

		color[u] = BLACK
		for v := 0; v < n; v++ {
			//未訪問かつ、辺が存在するとき
			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
				}
			}
		}
	}

	sum := 0
	for i := 0; i < n; i++ {
		if p[i] != -1 {
			sum += M[i][p[i]]
		}
	}
	return sum
}

func main() {
	sc.Split(bufio.ScanWords)
	n = nextInt()

	for i := 0; i < n; i++ {

		for j := 0; j < n; j++ {
			tmp := nextInt()
			if tmp == -1 {
				tmp = INFTY
			}
			M[i][j] = tmp
		}

	}

	fmt.Println(prim())

}

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

void init(int n, vector<int>& v){
  v.resize(n);
  for(int i = 0; i < n; i++){
    v[i] = i;
  }
}

int root(int a, vector<int>& v){
  if(v[a] == a) return a;
  return (v[a] = root(v[a], v));
}

bool same(int a, int b, vector<int>& v){
  return root(a,v) == root(b,v);
}

void unite(int a, int b, vector<int>& v){
  v[root(a,v)] = root(b,v);
}


int main(){
  int n;
  cin >> n;
  vector<vector<int> > a(n, vector<int>(n, -1));
  for(int i = 0; i < n; i++)
    for(int j = 0; j < n; j++)
      cin >> a[i][j];

  vector<int> cost, src, dst;
  for(int i = 0; i < n; i++){
    for(int j = i+1; j < n; j++){
      if(a[i][j] > -1){
        cost.push_back(a[i][j]);
        src.push_back(i);
        dst.push_back(j);
      }
    }
  }
  vector<pair<int, int> > edges(cost.size());

  for(int i = 0; i < cost.size(); i++){
    // cout << "cost" << cost[i] << endl;
    // cout << "i" << i << endl;
    edges[i] = make_pair(cost[i], i);
    // cout << edges[i].first << " : " << edges[i].second << endl;
  }

  sort(edges.begin(),edges.end());

  // for(int i = 0; i < cost.size(); i++)
    // cout << edges[i].first << " : " << edges[i].second << endl;

  int sum = 0;
  vector<int> units;
  init(n, units);
  for(int i = 0; i < edges.size(); i++){
    int idx = edges[i].second;
    int s = src[idx], d = dst[idx];
    // cout << s << "," << d << endl;
    if(same(s, d, units))
      continue;
    unite(s, d, units);

    sum += cost[idx];
  }

  cout << sum << endl;
}

C++: #include <iostream>
#include <algorithm>
#include <string>
#include <stack>
#include <cstdlib>
#include <queue>
#include <vector>
#include <set>
#include <stdio.h>
#include <cstdio>
using namespace std;
int p[100001],s[100001],t[10001],cost[10001];
int root(int a) {
	if (p[a] == a) return a;
	return (p[a] = root(p[a]));
}
void unite(int a,int b){
	p[root(a)] = root(b);
}
bool same(int a ,int b){
	if (root(a) == root(b)) return 1;
	else return 0;
}
bool rcheck(int a){
	if (root(a) == a) return 1;
	else return 0;
}
pair <int,int> path[1001];
int main() {
	int n,coun = 0;
	cin >> n;
	for (int i = 0; i < n; i++){
		p[i] = i;
		for (int j = 0;j<n;j++) {
			s[n*i+j] = i; t[n*i+j]=j; 
			cin >> cost[n*i+j];
		}
	}
	for (int i = 0;i < n*n;i++) {
		path[i].first = cost[i];
		path[i].second = i;
	}
	sort(path,path+n*n);
	for (int i = 0; i < n*n;i++) {
		if (path[i].first > 0) {
			if(same(s[path[i].second],t[path[i].second]) == 0) {
				unite(s[path[i].second],t[path[i].second]);
				coun += path[i].first;
			}
		}
	}
	cout << coun << endl;
}

C++: #include <cstdio>
#include <iostream>
#include <vector>
#include <queue>
using namespace std;
int n;
int a[100][100];
int used[100];
int mincost[100];
const int INF=1000000000;
int prim(){
  for(int i=0;i<n;i++){
    used[i]=0;
    mincost[i]=INF;
  }
  mincost[0]=0;
  int res=0;
  while(1){
    int v=-1;
    for(int u=0;u<n;u++){
      if((v==-1 || mincost[v]>mincost[u]) && !used[u])v=u;
    }
    if(v==-1)break;
    used[v]=1;
    res+=mincost[v];
    for(int u=0;u<n;u++){
      mincost[u]=min(mincost[u],a[v][u]);
    }
  }
  return res;
}
int main(){
  scanf("%d",&n);
  for(int i=0;i<n;i++){
    for(int j=0;j<n;j++){
      scanf("%d",&a[i][j]);
      if(a[i][j]==-1)a[i][j]=INF;
    }
  }
  printf("%d\n",prim());
  return 0;
}

C++: #include <cstdio>
#include <iostream>
#include <algorithm>
#include <cstring>
#include <cstdlib>
#include <queue>
#include <stack>
using namespace std;
const int maxx=110;
const int infinity=(1<<30);

int A[maxx][maxx];
int d[maxx],p[maxx];//d[i]里面存的是V-T中的点，要加入到T中，选取的最小的权值；
					//d[i]=u;表示的是要把V-T中的点i加入到T中所需要的代价是u；
					//p[v]=u;表示的是V-T中的点v，要和T中的点u连接，即u是v的父节点；
bool flag[maxx];	//用flag来标记这个节点是不是在MST中，true是在T里，false是在V-T里；

int main (){
		int n;
		cin>>n;
		for(int i =1;i<=n;i++){
			for(int j=1;j<=n;j++){
				int e;cin>>e;
				A[i][j]=(e==-1)?infinity:e;
			}
		}

		for(int i=0;i<maxx;i++) {
			flag[i]=false;
			d[i]=infinity;
			p[i]=-1;
		}
		int sum=0,minv,minj;
		flag[1]=true;
		while(1){
			minv=infinity;
			minj=-1;
			for(int i=1;i<=n;i++){
				if(flag[i]==true){
					for(int j=1;j<=n;j++){
						if(A[i][j]!=-1 && A[i][j]<minv && flag[j]==false) {
							minv=A[i][j];
							minj=j;
						} 
					}
				}
			}
//			cout<<minv<<endl;
//			cout<<minj<<endl;
			if(minv==infinity || minj==-1) break;
			flag[minj]=true;
			sum+=minv;
		}
		
		cout<<sum<<endl;
		return 0;
		
}

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

static const int MAX =100;
static const int INF=(1<<21);
static const int WHITE =0;
static const int GRAY =1;
static const int BRACK =2;

int n,A[MAX][MAX];

int prim(){
  int x,minv,d[MAX],p[MAX],c[MAX];

  for(int i=0;i<n;i++){
    d[i]=INF;
    p[i]=-1;
    c[i]=WHITE;
  }

  d[0]=0;
  while(1){
    minv=INF;
    x=-1;
    for(int i=0;i<n;i++){
      if(minv>d[i] && c[i]!=BRACK){
	x=i;
	minv=d[i];
      }
    }
    if(x==-1)break;
    c[x]=BRACK;
    for(int v=0;v<n;v++){
      if(c[v]!=BRACK && A[x][v]!=INF){
	if(d[v]>A[x][v]){
	  d[v]=A[x][v];
	  p[v]=x;
	  c[v]=GRAY;
	}
      }
    }
  }
  int sum=0;
  for(int i=0;i<n;i++){
    if(p[i]!=-1)sum+=A[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;
      A[i][j]=(e==-1)?INF:e;
    }
  }

  cout<<prim()<<endl;
  return 0;
}