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 <bits/stdc++.h>
using namespace std;

typedef long long ll;


class Union_Find {
private:
    std::vector<int> par;
    std::vector<int> rank;

public:
    Union_Find(int n) { // ?????????: O(n)
	par = rank = std::vector<int>(n);
	for(int i=0; i<n; i++) {
	    par[i]  = i;
	    rank[0] = 0;
	}
    }

    int root(int x) { // ??¨??????????±???????: O(?±(n)) 
	if(par[x] == x) return x;
	return par[x] = root(par[x]); // ??????????????????????????????????????????
    }

    void unite(int x, int y) { // x,y????????????????????????: O(?±(n)) 
	x = root(x); y = root(y);
	if( x == y ) return;

	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) { //x,y??????????????????????????????????????????: O(?±(n)) 
	return root(x) == root(y);
    }
};

struct Edge {
    ll s,t;
    ll w;
    bool operator < (const Edge& tgt) const {
	return w > tgt.w;
    }
};

ll Kruskal(priority_queue<Edge>& edges, const int V) {

    ll res = 0;
    
    Union_Find uf(V);
    while( !edges.empty() ) {
	Edge e = edges.top(); edges.pop();
	if( !uf.same(e.s, e.t) ) {
	    uf.unite(e.s, e.t);
	    res += e.w;
	}
    }
    return res;
}


int main () {

    ll V;
    cin >> V;

    priority_queue<Edge> edges;
    
    for(int i=0; i<V; i++) {
	for(int j=0; j<V; j++) {
	    ll d;
	    cin >> d;
	    if( d != -1 )
		edges.push( Edge{i,j,d} );
	}	
    }

    ll res = Kruskal(edges,V);
    cout << res << endl;    
    return 0;
}

C++: #include <bits/stdc++.h>
#define REP(i,n) for (int i = 0; i <(n); ++i)
#define REP2(i,x,n) for (int i = x; i <(n); ++i)
#define ALL(v) v.begin(), v.end()
#define RALL(v) v.rbegin(), v.rend()
using namespace std;
using ll = long long;
using P = pair<int,int>;
static const double PI = acos(-1);
static const int INF = 1e9+7;

/*
  最小全域木

  重み付きグラフの最小全域木の辺の重みの総和を計算する．
  最小全域木とは？：グラフで，すべての頂点に到達できるものの中で，　辺の重みが最小のもの
  プリムのアルゴリズム

*/

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];
  //d: MSTに属する頂点とそれ以外の頂点の中で，重みが最小の辺の重み
  //p: p[v]  MSTにおける頂点vの親
  //color 訪問状態

  //初期化
  for(int i = 0; i < n; i++){
    d[i] = INFTY;
    p[i] = -1;
    color[i] = WHITE;
  }

  d[0] = 0;

  while( true ){
    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;

  //入力
  REP(i,n)REP(j,n){
    int e;
    cin >> e;
    if(e == -1) M[i][j] = INFTY;
    else M[i][j] = e;
  }

  cout << prim() << endl;

  return 0;
}

C++: #include<iostream>
#include<algorithm>
#define MAX 120
#define INFTY (1 << 21)
#define WHITE -1
#define GRAY 0
#define BLACK 1

using namespace std;

int A[MAX], M[MAX][MAX];
int n;
int color[MAX], d[MAX], p[MAX];

void prim(){
    int u, mini;
    
    for(int i=0; i<n; i++){
        color[i] = WHITE;
        d[i] = INFTY;
        p[i] = -1;
    }//すべて初期化する
    
    d[0] = 0;
    
    while(1){
        mini = INFTY;
        
        for(int i=0; i<n; i++){
            if( color[i] != BLACK && d[i] < mini){
                mini = d[i];
                u=i;
            }
        }
        
        if(mini == INFTY){
            break;
        }
        
        color[u] = BLACK;
        
        for(int v=0; v<n; v++){
            if(color[v] != BLACK && M[u][v] != -1){
                if(M[u][v] < d[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 = sum + M[i][p[i]];
        }
    }
    cout << sum << endl;
}

int main(){
    
    cin >> n;
    
    for(int i=0; i<n; i++){
        for(int j=0; j<n; j++){
            M[i][j] = -1;
        }
    }
    
    for(int i=0; i<n; i++){
        for(int j=0; j<n; j++){
            cin >> M[i][j];
        }
    }
    
    prim();
    
    /*for(int i=0; i<n; i++){
        for(int j=0; j<n; j++){
            cout << M[i][j] << " ";
        }
        cout << endl;
        
    }*/
    
    return 0;
}

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

typedef int Weight;
struct Edge{
	int src,dst;
	Weight weight;
};

bool asc(const Edge &e,const Edge &f){
	return e.weight < f.weight;
}

vector<Edge> Edges;
vector<int> L(100005,-1);

int check(Edge &e,int n){
	int j;
	if (L[e.src]!=L[e.dst]){
		int t = L[e.dst];
		for (j=0;j<n;j++){
			if (L[j]==t){
				L[j]=L[e.src];
			}
		}
		return 1;
	}else{
		return 0;
	}
}

int main(){
	int n,m,i,j,w;
	cin >> n ;
	
	for (i=0;i<n;i++){
		for(j=0;j<n;j++){
			cin >> w;
			if ((w!=-1)&&(i>j)){
				Edge edge={i,j,w};
				Edges.push_back(edge);
			}
		}
	}
	
	sort(Edges.begin(),Edges.end(),asc);
	
	m = Edges.size();
	
	for (j=0;j<n;j++){
		L[j]=j;
	}
	
	int TotalWeight=0;
	int count=0;
	int k;
	for (i=0;i<m;i++){
		if (count==n-1){break;}
		if (check(Edges[i],n)){
			TotalWeight+=Edges[i].weight;
			count++;
		}
	}
	cout << TotalWeight;
	return 0;
}

C++: #include <bits/stdc++.h>
#define INF 1000000000
#define MAX_N 100
using namespace std;
typedef pair<int, int> P;
struct edge {
	int to, cost;
	edge(int to_, int cost_) : to(to_), cost(cost_) {}
};

int n;
vector<edge> G[MAX_N];

int Prim() {
	int res = 0;
	vector<bool> used(n, false);
	priority_queue<P, vector<P>, greater<P>> que;
	que.push(P(0, 0));
	while (!que.empty()) {
		P p = que.top(); que.pop();
		int v = p.second;
		if (used[v]) continue;
		used[v] = true;
		res += p.first;
		for (int i = 0; i < G[v].size(); i++) {
			edge e = G[v][i];
			que.push(P(e.cost, e.to));
		}
	}
	return res;
}

int main()
{
	cin >> n;
	for (int i = 0, c; i < n; i++) {
		for (int j = 0; j < n; j++) {
			cin >> c;
			if (c != -1)
				G[i].push_back(edge(j, c));
		}
	}
	cout << Prim() << endl;
	return 0;
}