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++: //By Vlgd
#include<bits/stdc++.h>
#define INF 20000000
using namespace std;
const int N=100;
int G[N][N],n;
int vis[N]={0},dis[N]={0};
void solve(){
	int ans=0;
	for(int i=0;i<n;i++) dis[i]=INF;
	dis[0]=0;
	while(1){
		int mv=INF,u=-1;
		for(int i=0;i<n;i++){
			if(mv>dis[i]&&!vis[i]){
				mv=dis[i];
				u=i;
			}
		}
		if(u==-1) break;
		vis[u]=1;
		ans+=dis[u];

		for(int v=0;v<n;v++){
			if(!vis[v]&&G[u][v]!=-1){
				if (dis[v]>G[u][v])	dis[v]=G[u][v];
			}
		}
	}
	printf("%d\n",ans);	
}
main(){
	scanf("%d",&n);
	for (int i=0;i<n;i++) for (int j=0;j<n;j++) scanf("%d",&G[i][j]);
	solve();
}

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++: #include<iostream>
#include<algorithm>
using namespace std;
#define MAX_E 100005
#define MAX_V 10005

struct edge{int u, v, cost;};

bool comp(const edge& e1, const edge& e2){
	return e1.cost < e2.cost;
}
int V, E;
edge es[MAX_E];
int par[MAX_V];
int _rank[MAX_V];

void init(int x){
	for(int i = 0; i < x; i++){
		par[i] = i;
		_rank[i] = 0;
	}
}

int find(int x){
	return par[x] == x ? x : par[x] = find(par[x]);
}

void unite(int x, int y){
	x = find(x);
	y = find(y);
	
	if(_rank[x] < _rank[y]){
		par[x] = y;
	} else{
		par[y] = x;
		if(_rank[x] == _rank[y]) _rank[x]++;
	}
}

bool same(int x, int y){
	return find(x) == find(y);
}

int kruskal(){
	sort(es, es + E, comp);
	init(V);
	int res = 0, count = 0;
	for(int i = 0; i < E; i++){
		edge e = es[i];
		if(!same(e.u, e.v)){
			unite(e.u,e.v);
			res += e.cost;
			count++;
		}
	}
	return res;
}

int main(){
	int s, t, w, x, y = 0;
	
	cin>>V;
	
	for(int i = 1; i <= V; i++){
		for(int j = 1; j <= V; j++){
			cin>>x;
			if(x != -1 && i <= j){
				es[y].u = i;
				es[y].v = j;
				es[y++].cost = x;
			}
		}
	}
	
	E = y;
	
	cout<<kruskal()<<endl;
	
	return 0;
	
}

C++: #include <iostream>
using namespace std;
static const int MAX = 100, INFTY = (1 << 21), WHITE = 0, GRAY = 1, BLACK = 2;

int N, M[MAX][MAX];

int prim() {
	int u, minv;
	int D[MAX], P[MAX], C[MAX];

	for (int i = 0; i < N; i++) {
		D[i] = INFTY;
		P[i] = -1;
		C[i] = WHITE;
	}

	D[0] = 0;
	while (true) {
		minv = INFTY;
		u = -1;
		for (int i = 0; i < N; i++) {
			if (minv > D[i] && C[i] != BLACK) {
				u = i;
				minv = D[i];
			}
		}
		if (u == -1) break;
		C[u] = BLACK;
		for (int v = 0; v < N; v++) {
			if (C[v] != BLACK && M[u][v] != INFTY) {
				if (D[v] > M[u][v]) {
					D[v] = M[u][v];
					P[v] = u;
					C[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;
}

C++: #include <iostream>
#include <vector>
#include <algorithm>
#include <numeric>
#include <queue>
#include <stack>
#include <map>
#define FOR(i,b,e) for(int i=b;i<e;i++)
#define REP(i,n) FOR(i,0,n)
#define INFTY 1000000
using namespace std;

const int N = 100;
int n;
int D[N], M[N][N];
bool V[N];

int prim() {
  int nn, minD, cost;

  REP(i, n) D[i] = INFTY;
  D[0] = 0;

  while (true) {
    minD = INFTY;
    REP(i, n) {
      if (!V[i] && D[i] < minD) {
        minD = D[i];
        nn = i;
      }
    }
    if (minD == INFTY) break;

    cost += D[nn];
    V[nn] = true;
    REP(i, n) {
      if (!V[i] && M[nn][i] != -1 && M[nn][i] < D[i]) {
        D[i] = M[nn][i];
      }
    }
  }

  return cost;
}

int main() {
	cin.tie(0);
 	ios::sync_with_stdio(false);

  cin >> n;
  REP(i, n) {
    REP(j, n) {
      cin >> M[i][j];
    }
  }

  cout << prim() << endl;
}

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;
    for(int i = 0; i < n; i++){
        for(int j = 0; j < n; j++){
            int in;
            cin >> in;
            if(in == -1) continue;
            es.push_back({i, j, in});
        }
    }
    m = es.size();
    cout << kruskal() << endl;
}